diff --git a/.gitignore b/.gitignore
index 7591ef3..37fc2f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 # ide
 .spyderproject
+.spyproject
 .project
 .pydevproject
 .settings/
diff --git a/.travis.yml b/.travis.yml
index 93e5175..966ca73 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,7 @@
 language: python
+python:
+    - "2.7"
+    - "3.6"
 cache: pip
 sudo: required
 before_install:
@@ -18,7 +21,7 @@ deploy:
     on:
       tags: true
   - provider: pypi
-    user: bwanamarko
+    user: perfspwr
     password: $PYPI_PASSWORD
     distributions: "sdist bdist_wheel" # Your distributions here
     skip_upload_docs: true
diff --git a/pvmismatch/contrib/gen_coeffs/__init__.py b/pvmismatch/contrib/gen_coeffs/__init__.py
index 027f7b3..ea9e616 100644
--- a/pvmismatch/contrib/gen_coeffs/__init__.py
+++ b/pvmismatch/contrib/gen_coeffs/__init__.py
@@ -6,7 +6,7 @@
 import numpy as np
 from scipy import optimize
 from pvmismatch.contrib.gen_coeffs import diode, two_diode
-from pvmismatch.pvmismatch_lib.pvcell import ISAT1_T0, ISAT2, RS, RSH
+from pvmismatch.pvmismatch_lib.pvcell import ISAT1_T0, ISAT2_T0, RS, RSH
 
 # IEC 61853 test matrix
 TC_C = [15.0, 25.0, 50.0, 75.0]
@@ -37,7 +37,7 @@ def gen_two_diode(isc, voc, imp, vmp, nseries, nparallel,
     vmp_cell = vmp / nseries
     if x0 is None:
         isat1 = ISAT1_T0  # [A]
-        isat2 = ISAT2
+        isat2 = ISAT2_T0
         rs = RS  # [ohms]
         rsh = RSH  # [ohms]
     else:
diff --git a/pvmismatch/contrib/gen_coeffs/example.py b/pvmismatch/contrib/gen_coeffs/example.py
index 9f0e514..aa24659 100644
--- a/pvmismatch/contrib/gen_coeffs/example.py
+++ b/pvmismatch/contrib/gen_coeffs/example.py
@@ -23,16 +23,16 @@
 iec61853 = gen_coeffs.gen_iec_61853_from_sapm(gen_coeffs.PVMODULES[PROD_NAME])
 iec61853['i_mp'] = iec61853['p_mp'] / iec61853['v_mp']
 #isc0, alpha_isc = gen_coeffs.gen_sapm(iec61853)
-x, sol = gen_coeffs.gen_two_diode(ISC0, VOC0, IMP0, VMP0, NS, NP, T0)
-#x, sol = gen_coeffs.gen_two_diode(
-#    iec61853['i_sc'], iec61853['v_oc'], iec61853['i_mp'],
-#    iec61853['v_mp'], NS, NP, tc=TC, method='lm',
-#    x0=(2.25e-11, 1.5e-6, 0.004, 10.0)
-#)
+#x, sol = gen_coeffs.gen_two_diode(ISC0, VOC0, IMP0, VMP0, NS, NP, T0)
+x, sol = gen_coeffs.gen_two_diode(
+    iec61853['i_sc'], iec61853['v_oc'], iec61853['i_mp'],
+    iec61853['v_mp'], NS, NP, tc=TC, method='lm',
+    x0=(2.25e-11, 1.5e-6, 0.004, 10.0)
+)
 isat1, isat2, rs, rsh = x
 
 pvc = pvcell.PVcell(
-    Rs=rs, Rsh=rsh, Isat1_T0=isat1, Isat2=isat2,
+    Rs=rs, Rsh=rsh, Isat1_T0=isat1, Isat2_T0=isat2,
     Isc0_T0=ISC0/NP, alpha_Isc=AISC
 )
 f1 = plt.figure()
diff --git a/pvmismatch/contrib/gen_coeffs/tests/test_diode.py b/pvmismatch/contrib/gen_coeffs/tests/test_diode.py
index 4f1e6fe..eea9933 100644
--- a/pvmismatch/contrib/gen_coeffs/tests/test_diode.py
+++ b/pvmismatch/contrib/gen_coeffs/tests/test_diode.py
@@ -3,7 +3,7 @@
 """
 
 from pvmismatch.pvmismatch_lib.pvcell import (
-    RS as RS_2, RSH as RSH_2, ISAT1_T0 as ISAT1_2, ISAT2 as ISAT2_2
+    RS as RS_2, RSH as RSH_2, ISAT1_T0 as ISAT1_2, ISAT2_T0 as ISAT2_2
 )
 from pvmismatch.contrib.gen_coeffs import diode
 from pvmismatch.contrib.gen_coeffs import PVMODULES
diff --git a/pvmismatch/contrib/gen_coeffs/tests/test_two_diode.py b/pvmismatch/contrib/gen_coeffs/tests/test_two_diode.py
index af18d26..8b02393 100644
--- a/pvmismatch/contrib/gen_coeffs/tests/test_two_diode.py
+++ b/pvmismatch/contrib/gen_coeffs/tests/test_two_diode.py
@@ -56,22 +56,22 @@ def test_didv_dpdv_frsh():
                  'rsh': RSH_2, 'ic': IC, 'vc': VC, 'vt': VT}
     fdidv_test, jdidv_test = fdidv(**test_data)
     expected_data = {
-        'isat1': ISAT1_2, 'isat2': ISAT2_2, 'rs': RS_2, 'rsh': RSH_2,
-        'ic(vc)': IC, 'vc': VC, 'vd': VD_2, 'vt': VT
+        isat1: ISAT1_2, isat2: ISAT2_2, rs: RS_2, rsh: RSH_2,
+        ic: IC, vc: VC, 'vd': VD_2, vt: VT
     }
-    didv_simple = didv.subs('vc + ic(vc) * rs', 'vd')
+    didv_simple = didv.subs(vc + ic * rs, 'vd')
     fdidv_expected = np.float(didv_simple.evalf(subs=expected_data))
     LOGGER.debug('fdidv test: %g, expected: %g', fdidv_test, fdidv_expected)
     assert np.isclose(fdidv_test, fdidv_expected)
     # jacobian
-    d_didv_isat1 = didv.diff(isat1).subs('vc + ic(vc) * rs', 'vd')
-    d_didv_isat2 = didv.diff(isat2).subs('vc + ic(vc) * rs', 'vd')
-    d_didv_rs = didv.diff(rs).subs('vc + ic(vc) * rs', 'vd')
-    d_didv_rsh = didv.diff(rsh).subs('vc + ic(vc) * rs', 'vd')
-    d_didv_ic = didv.diff(ic).subs('vc + ic(vc) * rs', 'vd')
-    d_didv_vc = didv.diff(vc).subs('vc + ic(vc) * rs', 'vd')
+    d_didv_isat1 = didv.diff(isat1).subs(vc + ic * rs, 'vd')
+    d_didv_isat2 = didv.diff(isat2).subs(vc + ic * rs, 'vd')
+    d_didv_rs = didv.diff(rs).subs(vc + ic * rs, 'vd')
+    d_didv_rsh = didv.diff(rsh).subs(vc + ic * rs, 'vd')
+    d_didv_ic = didv.diff(ic).subs(vc + ic * rs, 'vd')
+    d_didv_vc = didv.diff(vc).subs(vc + ic * rs, 'vd')
     # update expected test data with calculated derivative
-    expected_data['Derivative(ic(vc), vc)'] = fdidv_expected
+    expected_data[di_dv] = fdidv_expected
     jdidv_expected = np.array([
         d_didv_isat1.evalf(subs=expected_data),
         d_didv_isat2.evalf(subs=expected_data),
@@ -88,17 +88,17 @@ def test_didv_dpdv_frsh():
     dpdv = didv * vc + ic
     # test fdpdv
     fdpdv_test, jdpdv_test = fdpdv(**test_data)
-    dpdv_simple = dpdv.subs('vc + ic(vc) * rs', 'vd')
+    dpdv_simple = dpdv.subs(vc + ic * rs, 'vd')
     fdpdv_expected = np.float(dpdv_simple.evalf(subs=expected_data))
     LOGGER.debug('fdpdv test: %g, expected: %g', fdpdv_test, fdpdv_expected)
     assert np.isclose(fdpdv_test, fdpdv_expected)
     # jacobian
-    d_dpdv_isat1 = dpdv.diff(isat1).subs('vc + ic(vc) * rs', 'vd')
-    d_dpdv_isat2 = dpdv.diff(isat2).subs('vc + ic(vc) * rs', 'vd')
-    d_dpdv_rs = dpdv.diff(rs).subs('vc + ic(vc) * rs', 'vd')
-    d_dpdv_rsh = dpdv.diff(rsh).subs('vc + ic(vc) * rs', 'vd')
-    d_dpdv_ic = dpdv.diff(ic).subs('vc + ic(vc) * rs', 'vd')
-    d_dpdv_vc = dpdv.diff(vc).subs('vc + ic(vc) * rs', 'vd')
+    d_dpdv_isat1 = dpdv.diff(isat1).subs(vc + ic * rs, 'vd')
+    d_dpdv_isat2 = dpdv.diff(isat2).subs(vc + ic * rs, 'vd')
+    d_dpdv_rs = dpdv.diff(rs).subs(vc + ic * rs, 'vd')
+    d_dpdv_rsh = dpdv.diff(rsh).subs(vc + ic * rs, 'vd')
+    d_dpdv_ic = dpdv.diff(ic).subs(vc + ic * rs, 'vd')
+    d_dpdv_vc = dpdv.diff(vc).subs(vc + ic * rs, 'vd')
     jdpdv_expected = np.array([
         d_dpdv_isat1.evalf(subs=expected_data),
         d_dpdv_isat2.evalf(subs=expected_data),
@@ -117,23 +117,23 @@ def test_didv_dpdv_frsh():
     del test_data['ic'], test_data['vc']  # remove Ic, Vc
     test_data['isc'] = ISC0  # add Isc
     frsh_test, jfrsh_test = fjrsh(**test_data)
-    frsh_simple = frsh.subs('vc + ic(vc) * rs', 'vd')
+    frsh_simple = frsh.subs(vc + ic * rs, 'vd')
     # update expected test data with calculated derivative
-    expected_data['ic(vc)'] = ISC0
-    expected_data['vc'] = 0
+    expected_data[ic] = ISC0
+    expected_data[vc] = 0
     expected_data['vd'] = ISC0 * RS_2
     didv_isc = np.float(didv_simple.evalf(subs=expected_data))
-    expected_data['Derivative(ic(vc), vc)'] = didv_isc
+    expected_data[di_dv] = didv_isc
     frsh_expected = np.float(frsh_simple.evalf(subs=expected_data))
     LOGGER.debug('frsh test: %r, expected: %r', frsh_test, frsh_expected)
     assert np.isclose(frsh_test, frsh_expected)
     # jacobian
-    dfrsh_isat1 = frsh.diff(isat1).subs('vc + ic(vc) * rs', 'vd')
-    dfrsh_isat2 = frsh.diff(isat2).subs('vc + ic(vc) * rs', 'vd')
-    dfrsh_rs = frsh.diff(rs).subs('vc + ic(vc) * rs', 'vd')
-    dfrsh_rsh = frsh.diff(rsh).subs('vc + ic(vc) * rs', 'vd')
-    dfrsh_ic = frsh.diff(ic).subs('vc + ic(vc) * rs', 'vd')
-    dfrsh_vc = frsh.diff(vc).subs('vc + ic(vc) * rs', 'vd')
+    dfrsh_isat1 = frsh.diff(isat1).subs(vc + ic * rs, 'vd')
+    dfrsh_isat2 = frsh.diff(isat2).subs(vc + ic * rs, 'vd')
+    dfrsh_rs = frsh.diff(rs).subs(vc + ic * rs, 'vd')
+    dfrsh_rsh = frsh.diff(rsh).subs(vc + ic * rs, 'vd')
+    dfrsh_ic = frsh.diff(ic).subs(vc + ic * rs, 'vd')
+    dfrsh_vc = frsh.diff(vc).subs(vc + ic * rs, 'vd')
     jfrsh_expected = np.array([
         dfrsh_isat1.evalf(subs=expected_data),
         dfrsh_isat2.evalf(subs=expected_data),
diff --git a/pvmismatch/pvmismatch_lib/pvcell.py b/pvmismatch/pvmismatch_lib/pvcell.py
index 28f0566..22b94f9 100644
--- a/pvmismatch/pvmismatch_lib/pvcell.py
+++ b/pvmismatch/pvmismatch_lib/pvcell.py
@@ -5,6 +5,8 @@
 object which is used by modules, strings and systems.
 """
 
+from __future__ import absolute_import
+from future.utils import iteritems
 from pvmismatch.pvmismatch_lib.pvconstants import PVconstants
 import numpy as np
 from matplotlib import pyplot as plt
@@ -14,7 +16,7 @@
 RS = 0.004267236774264931  # [ohm] series resistance
 RSH = 10.01226369025448  # [ohm] shunt resistance
 ISAT1_T0 = 2.286188161253440E-11  # [A] diode one saturation current
-ISAT2 = 1.117455042372326E-6  # [A] diode two saturation current
+ISAT2_T0 = 1.117455042372326E-6  # [A] diode two saturation current
 ISC0_T0 = 6.3056  # [A] reference short circuit current
 TCELL = 298.15  # [K] cell temperature
 ARBD = 1.036748445065697E-4  # reverse breakdown coefficient 1
@@ -45,7 +47,10 @@ class PVcell(object):
     :param pvconst: configuration constants object
     :type pvconst: :class:`~pvmismatch.pvmismatch_lib.pvconstants.PVconstants`
     """
-    def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2=ISAT2,
+
+    _calc_now = False  #: if True ``calcCells()`` is called in ``__setattr__``
+
+    def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2_T0=ISAT2_T0,
                  Isc0_T0=ISC0_T0, aRBD=ARBD, bRBD=BRBD, VRBD=VRBD_,
                  nRBD=NRBD, Eg=EG, alpha_Isc=ALPHA_ISC,
                  Tcell=TCELL, Ee=1., pvconst=PVconstants()):
@@ -53,7 +58,7 @@ def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2=ISAT2,
         self.Rs = Rs  #: [ohm] series resistance
         self.Rsh = Rsh  #: [ohm] shunt resistance
         self.Isat1_T0 = Isat1_T0  #: [A] diode one sat. current at T0
-        self.Isat2 = Isat2  #: [A] diode two saturation current
+        self.Isat2_T0 = Isat2_T0  #: [A] diode two saturation current
         self.Isc0_T0 = Isc0_T0  #: [A] short circuit current at T0
         self.aRBD = aRBD  #: reverse breakdown coefficient 1
         self.bRBD = bRBD  #: reverse breakdown coefficient 2
@@ -67,6 +72,8 @@ def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2=ISAT2,
         self.Icell = None  #: cell currents on IV curve [A]
         self.Vcell = None  #: cell voltages on IV curve [V]
         self.Pcell = None  #: cell power on IV curve [W]
+        # set calculation flag
+        self._calc_now = True  # overwrites the class attribute
 
     def __str__(self):
         fmt = '<PVcell(Ee=%g[suns], Tcell=%g[K], Isc=%g[A], Voc=%g[V])>'
@@ -76,27 +83,28 @@ def __repr__(self):
         return str(self)
 
     def __setattr__(self, key, value):
+        # check for floats
         try:
             value = np.float64(value)
         except (TypeError, ValueError):
-            pass
+            pass  # fail silently if not float, eg: pvconst or _calc_now
         super(PVcell, self).__setattr__(key, value)
-        # after all attributes have been initialized, recalculate IV curve
-        # every time __setattr__() is called
-        if hasattr(self, 'pvconst'):
+        # recalculate IV curve
+        if self._calc_now:
             Icell, Vcell, Pcell = self.calcCell()
-            super(PVcell, self).__setattr__('Icell', Icell)
-            super(PVcell, self).__setattr__('Vcell', Vcell)
-            super(PVcell, self).__setattr__('Pcell', Pcell)
+            self.__dict__.update(Icell=Icell, Vcell=Vcell, Pcell=Pcell)
 
     def update(self, **kwargs):
         """
         Update user-defined constants.
         """
-        # TODO: use __dict__.update(), check for floats and update IV curve
-        # self.__dict__.update(kwargs)
-        for k, v in kwargs.iteritems():
+        # turn off calculation flag until all attributes are updated
+        self._calc_now = False
+        # don't use __dict__.update() instead use setattr() to go through
+        # custom __setattr__() so that numbers are cast to floats
+        for k, v in iteritems(kwargs):
             setattr(self, k, v)
+        self._calc_now = True  # recalculate
 
     @property
     def Vt(self):
@@ -134,6 +142,18 @@ def Isat1(self):
         )
         return self.Isat1_T0 * _Tstar * _expTstar  # [A] Isat1(Tcell)
 
+    @property
+    def Isat2(self):
+        """
+        Diode two saturation current at Tcell in amps.
+        """
+        _Tstar = self.Tcell ** 3. / self.pvconst.T0 ** 3.  # scaled temperature
+        _inv_delta_T = 1. / self.pvconst.T0 - 1. / self.Tcell  # [1/K]
+        _expTstar = np.exp(
+            self.Eg * self.pvconst.q / (2.0 * self.pvconst.k) * _inv_delta_T
+        )
+        return self.Isat2_T0 * _Tstar * _expTstar  # [A] Isat2(Tcell)
+    
     @property
     def Isc0(self):
         """
@@ -274,4 +294,4 @@ def plot(self):
         plt.xlim(0, self.Voc)
         plt.ylim(0, (self.Isc + 1) * self.Voc)
         plt.grid()
-        return cell_plot
\ No newline at end of file
+        return cell_plot
diff --git a/pvmismatch/pvmismatch_lib/pvconstants.py b/pvmismatch/pvmismatch_lib/pvconstants.py
index f9d2d03..5c17c8b 100644
--- a/pvmismatch/pvmismatch_lib/pvconstants.py
+++ b/pvmismatch/pvmismatch_lib/pvconstants.py
@@ -74,23 +74,39 @@ class PVconstants(object):
     T0 = 298.15  #: [K] reference temperature
 
     def __init__(self, npts=NPTS):
-        # set number of points in IV curve(s)
-        self.npts = npts  #: number of points in IV curves
-        # point spacing from 0 to 1, used for Vcell, Vmod, Vsys and Istring
-        # decrease point spacing as voltage approaches Voc by using logspace
-        pts = (11. - np.logspace(np.log10(11.), 0., self.npts)) / 10.
-        pts[0] = 0.  # first point must be exactly zero
-        self.pts = pts.reshape((self.npts, 1))
+        self._npts = None
+        self.pts = None
         """array of points with decreasing spacing from 0 to 1"""
-        negpts = (11. - np.logspace(np.log10(11. - 1. / float(self.npts)),
-                                    0., self.npts)) / 10.
-        negpts = negpts.reshape((self.npts, 1))
-        self.Imod_negpts = 1 + 1. / float(self.npts) / 10. - negpts
+        self.Imod_negpts = None
         """array of points with decreasing spacing from 1 to just less than but
         not including zero"""
-        self.negpts = np.flipud(negpts)  # reverse the order
+        self.negpts = None
         """array of points with increasing spacing from 1 to just less than but
         not including zero"""
+        self.Imod_pts = None
+        """array of points with increasing spacing from 0 to 1"""
+        # call property setter
+        self.npts = npts  #: number of points in IV curves
+
+    @property
+    def npts(self):
+        """number of points in IV curves"""
+        return self._npts
+
+    @npts.setter
+    def npts(self, npts):
+        # set number of points in IV curve(s)
+        self._npts = npts  # number of points in IV curves
+        # point spacing from 0 to 1, used for Vcell, Vmod, Vsys and Istring
+        # decrease point spacing as voltage approaches Voc by using logspace
+        pts = (11. - np.logspace(np.log10(11.), 0., self._npts)) / 10.
+        pts[0] = 0.  # first point must be exactly zero
+        self.pts = pts.reshape((self._npts, 1))
+        negpts = (11. - np.logspace(np.log10(11. - 1. / float(self._npts)),
+                                    0., self._npts)) / 10.
+        negpts = negpts.reshape((self._npts, 1))
+        self.Imod_negpts = 1 + 1. / float(self._npts) / 10. - negpts
+        self.negpts = np.flipud(negpts)  # reverse the order
         # shift and concatenate pvconst.negpts and pvconst.pts
         # so that tight spacing is around MPP and RBD
         self.Imod_pts = 1 - np.flipud(self.pts)
diff --git a/pvmismatch/pvmismatch_lib/pvmodule.py b/pvmismatch/pvmismatch_lib/pvmodule.py
index 926cf46..8613985 100644
--- a/pvmismatch/pvmismatch_lib/pvmodule.py
+++ b/pvmismatch/pvmismatch_lib/pvmodule.py
@@ -3,6 +3,10 @@
 This module defines the :class:`~pvmismatch.pvmismatch_lib.pvmodule.PVmodule`.
 """
 
+from __future__ import absolute_import
+from past.builtins import xrange, range
+from builtins import zip
+from six import itervalues
 import numpy as np
 from copy import copy
 from matplotlib import pyplot as plt
@@ -53,6 +57,7 @@ def standard_cellpos_pat(nrows, ncols_per_substr):
         cellpos.append(newsubstr)
     return cellpos
 
+
 # standard cell positions presets
 STD24 = standard_cellpos_pat(1, [1] * 24)
 STD72 = standard_cellpos_pat(12, [2, 2, 2])
@@ -118,6 +123,7 @@ def crosstied_cellpos_pat(nrows_per_substrs, ncols, partial=False):
 Substrings have 27, 28 and 27 rows of cells per diode
 """
 
+
 def combine_parallel_circuits(IVprev_cols, pvconst):
     """
     Combine crosstied circuits in a substring
@@ -147,6 +153,7 @@ def combine_parallel_circuits(IVprev_cols, pvconst):
         Irows, Vrows, Isc_rows.mean(), Imax_rows.max()
     )
 
+
 class PVmodule(object):
     """
     A Class for PV modules.
@@ -154,28 +161,42 @@ class PVmodule(object):
     :param cell_pos: cell position pattern
     :type cell_pos: dict
     :param pvcells: list of :class:`~pvmismatch.pvmismatch_lib.pvcell.PVcell`
-    :type pvcells: list
+    :type pvcells: list, :class:`~pvmismatch.pvmismatch_lib.pvcell.PVcell`
     :param pvconst: An object with common parameters and constants.
     :type pvconst: :class:`~pvmismatch.pvmismatch_lib.pvconstants.PVconstants`
     :param Vbypass: bypass diode trigger voltage [V]
     :param cellArea: cell area [cm^2]
     """
-    def __init__(self, cell_pos=STD96, pvcells=None, pvconst=PVconstants(),
+    def __init__(self, cell_pos=STD96, pvcells=None, pvconst=None,
                  Vbypass=VBYPASS, cellArea=CELLAREA):
         # TODO: check cell position pattern
         self.cell_pos = cell_pos  #: cell position pattern dictionary
         self.numberCells = sum([len(c) for s in self.cell_pos for c in s])
         """number of cells in the module"""
+        # is pvcells a list?
+        try:
+            pvc0 = pvcells[0]
+        except TypeError:
+            # is pvcells an object?
+            try:
+                pvconst = pvcells.pvconst
+            except AttributeError:
+                #  try to use the pvconst arg or create one if none
+                if not pvconst:
+                    pvconst = PVconstants()
+                # create pvcell
+                pvcells = PVcell(pvconst=pvconst)
+            # expand pvcells to list
+            pvcells = [pvcells] * self.numberCells
+        else:
+            pvconst = pvc0.pvconst
+            for p in pvcells:
+                if p.pvconst is not pvconst:
+                    raise Exception('PVconstant must be the same for all cells')
         self.pvconst = pvconst  #: configuration constants
         self.Vbypass = Vbypass  #: [V] trigger voltage of bypass diode
         self.cellArea = cellArea  #: [cm^2] cell area
-        if pvcells is None:
-            # faster to use copy instead of making each object in a for-loop
-            # use copy instead of deepcopy to keep same pvconst for all objects
-            # PVcell.calcCell() creates new np.ndarray if attributes change
-            pvcells = PVcell(pvconst=self.pvconst)
-        if isinstance(pvcells, PVcell):
-            pvcells = [pvcells] * self.numberCells
+        # check cell position pattern matches list of cells
         if len(pvcells) != self.numberCells:
             # TODO: use pvexception
             raise Exception(
@@ -244,7 +265,7 @@ def setSuns(self, Ee, cells=None):
                     else:
                         new_pvcells[cell_id] = old_pvcells[pvcell]
                 self.pvcells = new_pvcells
-                pvcell_set = old_pvcells.itervalues()
+                pvcell_set = itervalues(old_pvcells)
                 for pvc in pvcell_set:
                     pvc.Ee = Ee
             elif np.size(Ee) == self.numberCells:
@@ -289,8 +310,10 @@ def setSuns(self, Ee, cells=None):
                 raise Exception("Input irradiance value (Ee) for each cell!")
         self.Imod, self.Vmod, self.Pmod, self.Isubstr, self.Vsubstr = self.calcMod()
 
-# TODO setTemps is a nearly identical copy of setSuns. The DRY principle says that we should not be copying code.
-# TODO Replace both setSuns() and setTemps() with a single method for updating cell parameters that works for all params
+    # TODO setTemps is a nearly identical copy of setSuns. The DRY principle
+    # says that we should not be copying code.
+    # TODO Replace both setSuns() and setTemps() with a single method for
+    # updating cell parameters that works for all params
 
     def setTemps(self, Tc, cells=None):
         """
@@ -314,7 +337,7 @@ def setTemps(self, Tc, cells=None):
                     else:
                         new_pvcells[cell_id] = old_pvcells[pvcell]
                 self.pvcells = new_pvcells
-                pvcell_set = old_pvcells.itervalues()
+                pvcell_set = itervalues(old_pvcells)
                 for pvc in pvcell_set:
                     pvc.Tcell = Tc
             elif np.size(Tc) == self.numberCells:
diff --git a/pvmismatch/pvmismatch_lib/pvstring.py b/pvmismatch/pvmismatch_lib/pvstring.py
index e0693a0..86a27f0 100644
--- a/pvmismatch/pvmismatch_lib/pvstring.py
+++ b/pvmismatch/pvmismatch_lib/pvstring.py
@@ -4,6 +4,9 @@
 class.
 """
 
+from __future__ import absolute_import
+from past.builtins import range
+from future.utils import iteritems
 import numpy as np
 from copy import copy
 from matplotlib import pyplot as plt
@@ -23,23 +26,32 @@ class PVstring(object):
     :param pvconst: a configuration constants object
     """
     def __init__(self, numberMods=NUMBERMODS, pvmods=None,
-                 pvconst=PVconstants()):
-        self.pvconst = pvconst
-        self.numberMods = numberMods
-        if pvmods is None:
-            # use deepcopy instead of making each object in for-loop, 2x faster
-            pvmods = PVmodule(pvconst=self.pvconst)
-        if isinstance(pvmods, PVmodule):
-            pvmods = [pvmods] * self.numberMods
-            # reset pvconsts in all pvcells and pvmodules
+                 pvconst=None):
+        # is pvmods a list?
+        try:
+            pvmod0 = pvmods[0]
+        except TypeError:
+            # is pvmods an object?
+            try:
+                pvconst = pvmods.pvcons
+            except AttributeError:
+                #  try to use the pvconst arg or create one if none
+                if not pvconst:
+                    pvconst = PVconstants()
+                # create pvmod
+                pvmods = PVmodule(pvconst=pvconst)
+            # expand pvmods to list
+            pvmods = [pvmods] * numberMods
+        else:
+            pvconst = pvmod0.pvconst
+            numberMods = len(pvmods)
             for p in pvmods:
-                for c in p.pvcells:
-                    c.pvconst = self.pvconst
-                p.pvconst = self.pvconst
-        if len(pvmods) != self.numberMods:
-            # TODO: use pvmismatch exceptions
-            raise Exception("Number of modules doesn't match.")
-        self.pvmods = pvmods
+                if p.pvconst is not pvconst:
+                    raise Exception('pvconst must be the same for all modules')
+        self.pvconst = pvconst  #: ``PVconstants`` used in  ``PVstring``
+        self.numberMods = numberMods  #: number of module in string
+        self.pvmods = pvmods  #: list of ``PVModule`` in ``PVstring``
+        # calculate string
         self.Istring, self.Vstring, self.Pstring = self.calcString()
 
     # TODO: use __getattr__ to check for updates to pvcells
@@ -101,7 +113,7 @@ def setSuns(self, Ee):
         else:
             self.pvmods = copy(self.pvmods)  # copy list first
             try:
-                for pvmod, cell_Ee in Ee.iteritems():
+                for pvmod, cell_Ee in iteritems(Ee):
                     pvmod = int(pvmod)
                     self.pvmods[pvmod] = copy(self.pvmods[pvmod])
                     if hasattr(cell_Ee, 'keys'):
@@ -164,7 +176,7 @@ def setTemps(self, Tc):
         else:
             self.pvmods = copy(self.pvmods)  # copy list first
             try:
-                for pvmod, cell_Tc in Tc.iteritems():
+                for pvmod, cell_Tc in iteritems(Tc):
                     pvmod = int(pvmod)
                     self.pvmods[pvmod] = copy(self.pvmods[pvmod])
                     if hasattr(cell_Tc, 'keys'):
diff --git a/pvmismatch/pvmismatch_lib/pvsystem.py b/pvmismatch/pvmismatch_lib/pvsystem.py
index 0c9a68d..02f4e3b 100644
--- a/pvmismatch/pvmismatch_lib/pvsystem.py
+++ b/pvmismatch/pvmismatch_lib/pvsystem.py
@@ -4,6 +4,9 @@
 class.
 """
 
+from __future__ import absolute_import
+from past.builtins import basestring
+from future.utils import iteritems
 import numpy as np
 from copy import copy
 from matplotlib import pyplot as plt
@@ -24,21 +27,42 @@ class PVsystem(object):
     :param numberMods: number of modules per string
     :param pvmods: list of modules, a ``PVmodule`` object or None
     """
-    def __init__(self, pvconst=PVconstants(), numberStrs=NUMBERSTRS,
+    def __init__(self, pvconst=None, numberStrs=NUMBERSTRS,
                  pvstrs=None, numberMods=NUMBERMODS, pvmods=None):
-        self.pvconst = pvconst
-        self.numberStrs = numberStrs
-        self.numberMods = numberMods
-        if pvstrs is None:
-            pvstrs = PVstring(numberMods=self.numberMods, pvmods=pvmods,
-                              pvconst=self.pvconst)
-        # use deep copy instead of making each object in a for-loop
-        if isinstance(pvstrs, PVstring):
-            pvstrs = [pvstrs] * self.numberStrs
-        if len(pvstrs) != self.numberStrs:
-            # TODO: use pvmismatch excecptions
-            raise Exception("Number of strings don't match.")
-        self.pvstrs = pvstrs
+        # is pvstrs a list?
+        try:
+            pvstr0 = pvstrs[0]
+        except TypeError:
+            # is pvstrs a PVstring object?
+            try:
+                pvconst = pvstrs.pvconst
+            except AttributeError:
+                # try to use the pvconst arg or create one if none
+                if not pvconst:
+                    pvconst = PVconstants()
+                # create a pvstring
+                pvstrs = PVstring(numberMods=numberMods, pvmods=pvmods,
+                                  pvconst=pvconst)
+            # expand pvstrs to list
+            pvstrs = [pvstrs] * numberStrs
+            numberMods = [numberMods] * numberStrs
+        else:
+            pvconst = pvstr0.pvconst
+            numberStrs = len(pvstrs)
+            numberMods = []
+            for p in pvstrs:
+                if p.pvconst is not pvconst:
+                    raise Exception('pvconst must be the same for all strings')
+                numberMods.append(len(p.pvmods))
+        self.pvconst = pvconst  #: ``PVconstants`` used in ``PVsystem``
+        self.numberStrs = numberStrs  #: number strings in the system
+        self.numberMods = numberMods  #: list of number of modules per string
+        self.pvstrs = pvstrs  #: list of ``PVstring`` in system
+        # calculate pvsystem
+        self.update()
+
+    def update(self):
+        """Update system calculations."""
         self.Isys, self.Vsys, self.Psys = self.calcSystem()
         (self.Imp, self.Vmp, self.Pmp,
          self.Isc, self.Voc, self.FF, self.eff) = self.calcMPP_IscVocFFeff()
@@ -117,13 +141,12 @@ def setSuns(self, Ee):
             for pvstr in self.pvstrs:
                 pvstr.setSuns(Ee)
         else:
-            for pvstr, pvmod_Ee in Ee.iteritems():
+            for pvstr, pvmod_Ee in iteritems(Ee):
                 pvstr = int(pvstr)
                 self.pvstrs[pvstr] = copy(self.pvstrs[pvstr])
                 self.pvstrs[pvstr].setSuns(pvmod_Ee)
-        self.Isys, self.Vsys, self.Psys = self.calcSystem()
-        (self.Imp, self.Vmp, self.Pmp,
-         self.Isc, self.Voc, self.FF, self.eff) = self.calcMPP_IscVocFFeff()
+        # calculate pvsystem
+        self.update()
 
     def setTemps(self, Tc):
         """
@@ -155,34 +178,26 @@ def setTemps(self, Tc):
             for pvstr in self.pvstrs:
                 pvstr.setTemps(Tc)
         else:
-            for pvstr, pvmod_Tc in Tc.iteritems():
+            for pvstr, pvmod_Tc in iteritems(Tc):
                 pvstr = int(pvstr)
                 self.pvstrs[pvstr] = copy(self.pvstrs[pvstr])
                 self.pvstrs[pvstr].setTemps(pvmod_Tc)
-        self.Isys, self.Vsys, self.Psys = self.calcSystem()
-        (self.Imp, self.Vmp, self.Pmp,
-         self.Isc, self.Voc, self.FF, self.eff) = self.calcMPP_IscVocFFeff()
+        # calculate pvsystem
+        self.update()
 
     def plotSys(self, sysPlot=None):
         """
         Plot system I-V curves.
-        Arguments sysPlot : matplotlib.figure.Figure
-        Returns sysPlot : matplotlib.figure.Figure
+
+        :param sysPlot: integer, string, or existing figure
+        :returns: new figure
         """
-        # create new figure if sysPlot is None
-        # or make the specified sysPlot current and clear it
-        if not sysPlot:
-            sysPlot = plt.figure()
-        elif isinstance(sysPlot, (int, basestring)):
+        # create new figure if sysPlot or make the specified sysPlot current
+        # and clear it
+        try:
+            sysPlot.clear()
+        except (AttributeError, SyntaxError):
             sysPlot = plt.figure(sysPlot)
-        else:
-            try:
-                sysPlot = plt.figure(sysPlot.number)
-            except TypeError as e:
-                print '%s is not a figure.' % sysPlot
-                print 'Sorry, "plotSys" takes a "int", "str" or "Figure".'
-                raise e
-        sysPlot.clear()
         plt.subplot(2, 1, 1)
         plt.plot(self.Vsys, self.Isys)
         plt.xlim(0, self.Voc * 1.1)
diff --git a/pvmismatch/tests/__init__.py b/pvmismatch/tests/__init__.py
index 98d0635..6cdf796 100644
--- a/pvmismatch/tests/__init__.py
+++ b/pvmismatch/tests/__init__.py
@@ -5,5 +5,4 @@
 from nose.tools import ok_
 
 def test_parallel_cals_is_gone():
-    from pvmismatch import *
-
+    from pvmismatch import pvconstants, pvcell, pvmodule, pvstring, pvsystem
diff --git a/pvmismatch/tests/calc_series_test_iv.dat b/pvmismatch/tests/calc_series_test_iv.dat
index feef269..8b87aac 100644
--- a/pvmismatch/tests/calc_series_test_iv.dat
+++ b/pvmismatch/tests/calc_series_test_iv.dat
@@ -1,2 +1,2 @@
-2.669944976071763776e+01 2.613032759910023373e+01 2.557469007107548720e+01 2.503221767521580787e+01 2.450259848027790710e+01 2.398552794583688552e+01 2.348070874717004486e+01 2.298785060428993887e+01 2.250667011502820003e+01 2.203689059207428969e+01 2.157824190387539076e+01 2.113046031930595703e+01 2.069328835601767125e+01 2.026647463238250069e+01 1.984977372294382647e+01 1.944294601729244576e+01 1.904575758228635252e+01 1.865798002753500739e+01 1.827939037407083234e+01 1.790977092613237431e+01 1.754890914598532703e+01 1.719659753170961380e+01 1.685263349788209908e+01 1.651681925908636117e+01 1.618896171618258606e+01 1.586887234527211632e+01 1.555636708929292666e+01 1.525126625218346632e+01 1.495339439555430872e+01 1.466258023780793351e+01 1.437865655564878153e+01 1.410146008792689898e+01 1.383083144175985879e+01 1.356661500087907513e+01 1.330865883614763234e+01 1.305681461819839129e+01 1.281093753214193676e+01 1.257088619429546661e+01 1.233652257088466975e+01 1.210771189867184106e+01 1.188432260746466440e+01 1.166622624446097589e+01 1.145329740038615718e+01 1.124541363738058308e+01 1.104245541859565449e+01 1.084430603945801863e+01 1.065085156056234794e+01 1.046198074215418572e+01 1.027758498016509314e+01 1.009755824376340883e+01 9.921797014384612368e+00 9.750200226206345633e+00 9.582669208033740560e+00 9.419107626561764590e+00 9.259421430981848999e+00 9.103518798900990916e+00 8.951310083542285412e+00 8.802707762196401831e+00 8.657626385894511500e+00 8.515982530273577211e+00 8.377694747605900716e+00 8.242683519965208205e+00 8.110871213502454324e+00 7.982182033804972932e+00 7.856541982313340000e+00 7.733878813770909133e+00 7.614121994681481098e+00 7.497202662751325519e+00 7.383053587292115694e+00 7.271609130562099566e+00 7.162805210023236668e+00 7.056579261492607280e+00 6.952870203166881424e+00 6.851618400499221551e+00 6.752765631908337340e+00 6.656255055300074552e+00 6.562031175382167625e+00 6.470039811753497183e+00 6.380228067749364129e+00 6.292544300025006088e+00 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diff --git a/pvmismatch/tests/test_pvcell.py b/pvmismatch/tests/test_pvcell.py
index ce395a6..bed2465 100644
--- a/pvmismatch/tests/test_pvcell.py
+++ b/pvmismatch/tests/test_pvcell.py
@@ -69,6 +69,37 @@ def test_pvcell_calc_rbd():
     pvc2 = PVcell(bRBD=-0.056)
     ok_(isinstance(pvc2, PVcell))
 
-        
+
+def test_pvcell_calc_now_flag():
+    """
+    Test ``_calc_now`` turns off recalc in ``__setattr__``.
+    """
+    pvc = PVcell()
+    itest, vtest, ptest = pvc.Icell, pvc.Vcell, pvc.Pcell
+    pvc._calc_now = False
+    pvc.Rs = 0.001
+    assert np.allclose(itest, pvc.Icell)
+    assert np.allclose(vtest, pvc.Vcell)
+    assert np.allclose(ptest, pvc.Pcell)
+    icell, vcell, pcell = pvc.calcCell()
+    pvc._calc_now = True
+    assert np.allclose(icell, pvc.Icell)
+    assert np.allclose(vcell, pvc.Vcell)
+    assert np.allclose(pcell, pvc.Pcell)
+
+
+def test_update():
+    pvc = PVcell()
+    Rs = pvc.Rs
+    itest = pvc.Icell[170]
+    pvc.update(Rs=0.001)
+    assert np.isclose(pvc.Icell[170], 5.79691674)
+    pvc._calc_now = False
+    pvc.Rs = Rs
+    pvc.update()  # resets _calc_now to True
+    assert np.isclose(pvc.Icell[170], itest)
+    assert pvc._calc_now
+
+
 if __name__ == "__main__":
     test_calc_series()
diff --git a/pvmismatch/tests/test_pvconstants.py b/pvmismatch/tests/test_pvconstants.py
new file mode 100644
index 0000000..1cf535c
--- /dev/null
+++ b/pvmismatch/tests/test_pvconstants.py
@@ -0,0 +1,21 @@
+from pvmismatch import *
+
+
+def test_pvconst_npts_setter():
+    """Test pvconst property and setter methods"""
+    pvconst = pvconstants.PVconstants()
+    assert pvconst.npts == pvconstants.NPTS
+    assert len(pvconst.pts) == pvconst.npts
+    assert pvconst.pts[0] == 0
+    assert pvconst.pts[-1] == 1
+    assert len(pvconst.negpts) == pvconst.npts
+    assert pvconst.negpts[0] == 1
+    assert pvconst.negpts[-1] > 0
+    pvconst.npts = 1001
+    assert pvconst.npts == 1001
+    assert len(pvconst.pts) == pvconst.npts
+    assert pvconst.pts[0] == 0
+    assert pvconst.pts[-1] == 1
+    assert len(pvconst.negpts) == pvconst.npts
+    assert pvconst.negpts[0] == 1
+    assert pvconst.negpts[-1] > 0
diff --git a/pvmismatch/tests/test_pvmodule.py b/pvmismatch/tests/test_pvmodule.py
index 4b91a68..060d33e 100644
--- a/pvmismatch/tests/test_pvmodule.py
+++ b/pvmismatch/tests/test_pvmodule.py
@@ -4,6 +4,7 @@
 
 from nose.tools import ok_
 from pvmismatch.pvmismatch_lib.pvmodule import PVmodule, TCT492, PCT492
+from pvmismatch.pvmismatch_lib.pvcell import PVcell
 import numpy as np
 from copy import copy
 
@@ -40,6 +41,30 @@ def test_calc_pct_bridges():
     pvmod = PVmodule(cell_pos=pct492_bridges)
     return pvmod
 
+
+def check_same_pvconst_and_lengths(pvmod):
+    assert len(pvmod.pvcells) == 96
+    for p in pvmod.pvcells:
+        assert p.pvconst is pvmod.pvconst
+
+
+def test_pvmodule_with_pvcells_list():
+    pvcells = [PVcell()] * 96
+    pvmod = PVmodule(pvcells=pvcells)
+    check_same_pvconst_and_lengths(pvmod)
+
+
+def test_pvmodule_with_pvcells_obj():
+    pvcells = PVcell()
+    pvmod = PVmodule(pvcells=pvcells)
+    check_same_pvconst_and_lengths(pvmod)
+
+
+def test_pvmodule_with_no_pvcells():
+    pvmod = PVmodule()
+    check_same_pvconst_and_lengths(pvmod)
+
+
 if __name__ == "__main__":
     test_calc_mod()
     test_calc_tct_mod()
diff --git a/pvmismatch/tests/test_pvstring.py b/pvmismatch/tests/test_pvstring.py
new file mode 100644
index 0000000..3a47aa9
--- /dev/null
+++ b/pvmismatch/tests/test_pvstring.py
@@ -0,0 +1,24 @@
+from pvmismatch import *
+
+
+def check_same_pvconst_and_lengths(pvstr):
+    assert len(pvstr.pvmods) == pvstring.NUMBERMODS
+    for p in pvstr.pvmods:
+        assert p.pvconst is pvstr.pvconst
+
+
+def test_pvstring_with_pvmods_list():
+    pvmods = [pvmodule.PVmodule()] * pvstring.NUMBERMODS
+    pvstr = pvstring.PVstring(pvmods=pvmods)
+    check_same_pvconst_and_lengths(pvstr)
+
+
+def test_pvstring_with_pvmods_obj():
+    pvmods = pvmodule.PVmodule()
+    pvstr = pvstring.PVstring(pvmods=pvmods)
+    check_same_pvconst_and_lengths(pvstr)
+
+
+def test_pvstring_with_no_pvmods():
+    pvstr = pvstring.PVstring()
+    check_same_pvconst_and_lengths(pvstr)
diff --git a/pvmismatch/tests/test_pvsystem.py b/pvmismatch/tests/test_pvsystem.py
new file mode 100644
index 0000000..99549bd
--- /dev/null
+++ b/pvmismatch/tests/test_pvsystem.py
@@ -0,0 +1,25 @@
+from pvmismatch import *
+
+
+def check_same_pvconst_and_lengths(pvsys):
+    assert len(pvsys.pvstrs) == pvsystem.NUMBERSTRS
+    for n, p in enumerate(pvsys.pvstrs):
+        assert p.pvconst is pvsys.pvconst
+        assert len(p.pvmods) == pvsys.numberMods[n]
+
+
+def test_pvsystem_with_pvstrs_list():
+    pvstrs = [pvstring.PVstring()] * pvsystem.NUMBERSTRS
+    pvsys = pvsystem.PVsystem(pvstrs=pvstrs)
+    check_same_pvconst_and_lengths(pvsys)
+
+
+def test_pvsystem_with_pvstrs_obj():
+    pvstrs = pvstring.PVstring()
+    pvsys = pvsystem.PVsystem(pvstrs=pvstrs)
+    check_same_pvconst_and_lengths(pvsys)
+
+
+def test_pvsystem_with_no_pvstrs():
+    pvsys = pvsystem.PVsystem()
+    check_same_pvconst_and_lengths(pvsys)
diff --git a/pvmismatch/tests/test_settemps.py b/pvmismatch/tests/test_settemps.py
index 6798f9e..ade40d9 100644
--- a/pvmismatch/tests/test_settemps.py
+++ b/pvmismatch/tests/test_settemps.py
@@ -7,6 +7,7 @@
 import numpy as np
 from nose.tools import ok_
 from pvmismatch.pvmismatch_lib.pvsystem import PVsystem
+from pvmismatch.pvmismatch_lib.pvcell import PVcell
 import logging
 
 logging.basicConfig(level=logging.DEBUG)
@@ -150,3 +151,27 @@ def test_settemp():
     assert pvsys.pvstrs[2].pvmods[4].pvcells[5].Tcell == 310
     assert pvsys.pvstrs[2].pvmods[4].pvcells[3] == pvsys.pvstrs[2].pvmods[4].pvcells[5]
     assert pvsys.pvstrs[1].pvmods[0] == pvsys.pvstrs[1].pvmods[2]
+
+def test_settemp_cell():
+    """
+    Test setTemp method for a wide range of temperatures. 
+    Test added after implementing Isat2 as Isat2(Tcell)
+    """
+    pvc = PVcell()
+    Pmp_arr = []
+    Vmp_arr = []
+    Voc_arr = []
+    Isc_arr = []
+    
+    temps = [-85, -60, -40, -25, 0, 25, 40, 60, 85]
+    for t in temps:
+        pvc.Tcell = float(t) + 273.15
+        Pmp_arr.append(pvc.Pcell.max())
+        Vmp_arr.append(pvc.Vcell[pvc.Pcell.argmax()])
+        Voc_arr.append(pvc.calcVcell(0))
+        Isc_arr.append(pvc.Isc)
+        
+    assert(np.all(np.gradient(np.squeeze(Vmp_arr)) < 0))
+    assert(np.all(np.gradient(np.squeeze(Voc_arr)) < 0))
+    assert(np.all(np.gradient(Isc_arr) > 0))
+        
diff --git a/requirements.txt b/requirements.txt
index 8dd2fbc..27bf2ba 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,9 +1,11 @@
-nose
-numpy
-scipy
-matplotlib
-sphinx
-dulwich
-sympy
-pvlib
-pytest
+nose==1.3.7
+numpy==1.13.3
+scipy==1.0.0
+matplotlib==2.1.0
+Sphinx==1.6.3
+dulwich==0.18.6
+sympy==1.1.1
+pvlib==0.5.1
+pytest==3.2.1
+future==0.16.0
+six==1.11.0
diff --git a/setup.cfg b/setup.cfg
new file mode 100644
index 0000000..2a9acf1
--- /dev/null
+++ b/setup.cfg
@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal = 1
diff --git a/setup.py b/setup.py
index 8ad8d15..35fb9ee 100644
--- a/setup.py
+++ b/setup.py
@@ -14,22 +14,56 @@
 except IOError:
     pass
 
-setup(name=__name__,
-      version=__version__,
-      description='PV Mismatch Calculator',
-      long_description=README,
-      author=__author__,
-      author_email=__email__,
-      url=__url__,
-      license='BSD 3-clause',
-      packages=['pvmismatch', 'pvmismatch.pvmismatch_lib',
-                'pvmismatch.pvmismatch_tk', 'pvmismatch.contrib',
-                'pvmismatch.contrib.gen_coeffs'],
-      requires=['numpy (>=1.8)', 'matplotlib (>=1.3)', 'scipy (>=0.12.0)'],
-      scripts=['pv_tk.py'],
-      package_data={'pvmismatch':
-                        ['pvmismatch_json/messagetext.English.json',
-                         'pvmismatch_json/validationConstants.json',
-                         'res/logo.png', 'res/logo_invert.png',
-                         'res/logo_bg.png', 'docs/conf.py', 'docs/*.rst',
-                         'docs/Makefile', 'docs/make.bat']})
+INSTALL_REQUIRES = [
+    'numpy>=1.13.3', 'matplotlib>=2.1.0', 'scipy>=1.0.0', 'future>=0.16.0',
+    'six>=1.11.0'
+]
+
+TESTS_REQUIRES = [
+    'nose>=1.3.7', 'pytest>=3.2.1', 'sympy>=1.1.1', 'pvlib>=0.5.1'
+]
+
+CLASSIFIERS = [
+    'Development Status :: 4 - Beta',
+    'License :: OSI Approved :: BSD License',
+    'Operating System :: OS Independent',
+    'Intended Audience :: Science/Research',
+    'Programming Language :: Python',
+    'Programming Language :: Python :: 2',
+    'Programming Language :: Python :: 2.7',
+    'Programming Language :: Python :: 3',
+    'Programming Language :: Python :: 3.3',
+    'Programming Language :: Python :: 3.4',
+    'Programming Language :: Python :: 3.5',
+    'Programming Language :: Python :: 3.6',
+    'Topic :: Scientific/Engineering',
+]
+
+setup(
+    name=__name__,
+    version=__version__,
+    description='PV Mismatch Calculator',
+    long_description=README,
+    author=__author__,
+    author_email=__email__,
+    url=__url__,
+    license='BSD 3-clause',
+    packages=[
+        'pvmismatch', 'pvmismatch.pvmismatch_lib',
+        'pvmismatch.pvmismatch_tk', 'pvmismatch.contrib',
+        'pvmismatch.contrib.gen_coeffs'
+    ],
+    install_requires=INSTALL_REQUIRES,
+    tests_require=TESTS_REQUIRES,
+    scripts=['pv_tk.py'],
+    package_data={
+        'pvmismatch': [
+            'pvmismatch_json/messagetext.English.json',
+            'pvmismatch_json/validationConstants.json',
+            'res/logo.png', 'res/logo_invert.png',
+            'res/logo_bg.png', 'docs/conf.py', 'docs/*.rst',
+            'docs/Makefile', 'docs/make.bat'
+        ]
+    },
+    classifiers=CLASSIFIERS
+)