Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <[email protected]>

Author: williamhobbs

pvlib/pvsystem.py

@@ -2922,11 +2922,13 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     This model has also been referred to as the power temperature coefficient
     model.
 
-    This function accepts an optional irradiance adjustment factor, `k`, based
-    on [2]_. This applies a piece-wise adjustment to power based on irradiance,
+    An optional adjustment can be applied to :math:`P_{dc}` as described in [2]_.
+    The adjustment accounts for the variation in module efficiency with
+    irradiance. The piece-wise adjustment to power is parameterized by `k`,
     where `k` is the reduction in actual power at 200 Wm⁻² relative to power
-    calculated at 200 Wm-2 as 0.2*`pdc0`. For example, a 500 W module that
-    produces 95 W at 200 Wm⁻² (a 5% relative reduction in efficiency) would
+    calculated at 200 Wm⁻² as 0.2*`pdc0`. For example, a module that
+    is rated at 500 W at STC but produces 95 W at 200 Wm⁻² 
+    (a 5% relative reduction in efficiency) would
     have a value of `k` = 0.01.
 
     .. math::