implement voltage, current, power scaling function and method

docs/sphinx/source/whatsnew/v0.3.3.txt

@@ -10,6 +10,8 @@ Enhancements
 ~~~~~~~~~~~~
 
 * Adds the Erbs model. (:issue:`2`)
+* Adds the ``scale_voltage_current_power`` function and ``PVSystem`` method
+  to support simple array modeling. (:issue:`159`)
 
 
 Bug fixes

pvlib/modelchain.py

@@ -342,6 +342,8 @@ def run_model(self, times, irradiance=None, weather=None):
                                    self.airmass['airmass_absolute'],
                                    self.aoi)
 
+        self.dc = self.system.scale_voltage_current_power(self.dc)
+
         self.ac = self.system.snlinverter(self.dc['v_mp'], self.dc['p_mp'])
 
         return self

pvlib/pvsystem.py

@@ -35,6 +35,15 @@ class PVSystem(object):
     See the :py:class:`LocalizedPVSystem` class for an object model that
     describes an installed PV system.
 
+    The class supports basic system topologies consisting of:
+
+        * `N` total modules arranged in series
+          (`series_modules=N`, `parallel_modules=1`).
+        * `M` total modules arranged in parallel
+          (`series_modules=1`, `parallel_modules=M`).
+        * `NxM` total modules arranged in `M` strings of `N` modules each
+          (`series_modules=N`, `parallel_modules=M`).
+
     The class is complementary to the module-level functions.
 
     The attributes should generally be things that don't change about
@@ -69,6 +78,12 @@ class PVSystem(object):
     module_parameters : None, dict or Series
         Module parameters as defined by the SAPM, CEC, or other.
 
+    series_modules: int or float
+        See system topology discussion above.
+
+    parallel_modules: int or float
+        See system topology discussion above.
+
     inverter : None, string
         The model name of the inverters.
         May be used to look up the inverter_parameters dictionary
@@ -95,7 +110,7 @@ def __init__(self,
                  surface_tilt=0, surface_azimuth=180,
                  albedo=None, surface_type=None,
                  module=None, module_parameters=None,
-                 series_modules=None, parallel_modules=None,
+                 series_modules=1, parallel_modules=1,
                  inverter=None, inverter_parameters=None,
                  racking_model='open_rack_cell_glassback',
                  **kwargs):
@@ -363,6 +378,27 @@ def snlinverter(self, v_dc, p_dc):
         """
         return snlinverter(self.inverter_parameters, v_dc, p_dc)
 
+    def scale_voltage_current_power(self, data):
+        """
+        Scales the voltage, current, and power of the DataFrames
+        returned by :py:func:`singlediode` and :py:func:`sapm`
+        by `self.series_modules` and `self.parallel_modules`.
+
+        Parameters
+        ----------
+        data: DataFrame
+            Must contain columns `'v_mp', 'v_oc', 'i_mp' ,'i_x', 'i_xx',
+            'i_sc', 'p_mp'`.
+
+        Returns
+        -------
+        scaled_data: DataFrame
+            A scaled copy of the input data.
+        """
+
+        return scale_voltage_current_power(data, voltage=self.series_modules,
+                                           current=self.parallel_modules)
+
     def localize(self, location=None, latitude=None, longitude=None,
                  **kwargs):
         """Creates a LocalizedPVSystem object using this object
@@ -1668,3 +1704,37 @@ def snlinverter(inverter, v_dc, p_dc):
         ac_power = ac_power.ix[0]
 
     return ac_power
+
+
+def scale_voltage_current_power(data, voltage=1, current=1):
+    """
+    Scales the voltage, current, and power of the DataFrames
+    returned by :py:func:`singlediode` and :py:func:`sapm`.
+
+    Parameters
+    ----------
+    data: DataFrame
+        Must contain columns `'v_mp', 'v_oc', 'i_mp' ,'i_x', 'i_xx',
+        'i_sc', 'p_mp'`.
+    voltage: numeric
+        The amount by which to multiply the voltages.
+    current: numeric
+        The amount by which to multiply the currents.
+
+    Returns
+    -------
+    scaled_data: DataFrame
+        A scaled copy of the input data.
+        `'p_mp'` is scaled by `voltage * current`.
+    """
+
+    # as written, only works with a DataFrame
+    # could make it work with a dict, but it would be more verbose
+    data = data.copy()
+    voltages = ['v_mp', 'v_oc']
+    currents = ['i_mp' ,'i_x', 'i_xx', 'i_sc']
+    data[voltages] *= voltage
+    data[currents] *= current
+    data['p_mp'] *= voltage * current
+
+    return data

pvlib/test/test_pvsystem.py

@@ -300,6 +300,31 @@ def test_PVSystem_singlediode_floats():
         assert_almost_equals(expected[k], v, 5)
 
 
+def test_scale_voltage_current_power():
+    data = pd.DataFrame(
+        np.array([[2, 1.5, 10, 8, 12, 0.5, 1.5]]),
+        columns=['i_sc', 'i_mp', 'v_oc', 'v_mp', 'p_mp', 'i_x', 'i_xx'],
+        index=[0])
+    expected = pd.DataFrame(
+        np.array([[6, 4.5, 20, 16, 72, 1.5, 4.5]]),
+        columns=['i_sc', 'i_mp', 'v_oc', 'v_mp', 'p_mp', 'i_x', 'i_xx'],
+        index=[0])
+    out = pvsystem.scale_voltage_current_power(data, voltage=2, current=3)
+
+
+def test_PVSystem_scale_voltage_current_power():
+    data = pd.DataFrame(
+        np.array([[2, 1.5, 10, 8, 12, 0.5, 1.5]]),
+        columns=['i_sc', 'i_mp', 'v_oc', 'v_mp', 'p_mp', 'i_x', 'i_xx'],
+        index=[0])
+    expected = pd.DataFrame(
+        np.array([[6, 4.5, 20, 16, 72, 1.5, 4.5]]),
+        columns=['i_sc', 'i_mp', 'v_oc', 'v_mp', 'p_mp', 'i_x', 'i_xx'],
+        index=[0])
+    system = pvsystem.PVSystem(series_modules=2, parallel_modules=3)
+    out = system.scale_voltage_current_power(data)
+
+
 def test_sapm_celltemp():
     default = pvsystem.sapm_celltemp(900, 5, 20)
     assert_almost_equals(43.509, default.ix[0, 'temp_cell'], 3)