Townsend snow loss

What this step does

The row measures the energy lost while snow covers the modules. The Model tab's Townsend snow step sets the model: Townsend's monthly loss fraction, from the month's snowfall, the tilt, the snow piled on the ground under the lower edge, the humidity, the temperature and the insolation. Each timestep loses its month's fraction of the module power after the spectral row and before the flat derates, and the operating point sees the covered share as lost current. The row is absent when the step is off.

The math

1. Running total
2. Loss at a timestep

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}$$ $$L_i = s_{m(i)}\left(N P_{mod,i} - L_{elec,i}\right) \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/kWp
$E_{after}$	the running total after this row	kWh/kWp
$E_{row}$	the row taken as a loss	kWh/kWp
$\Delta t$	the length of one timestep	hours
$P_{DC}$	the DC nameplate of the system	kWp
$L_i$	the power lost at timestep $i$ over the whole system	W
$s_m$	the Townsend loss fraction of month $m$	—
$m(i)$	the month of timestep $i$	—
$N$	the number of modules in the system	—
$P_{mod,i}$	the module power after the temperature and spectral rows at timestep $i$	W
$L_{elec,i}$	the electrical shading loss of timestep $i$	W

Reading the number

Negative, and zero in months without snow. Low tilts, low lower edges, deep snow and cold, humid months with little sun raise it; it can reach tens of percent in a snowy winter month on a low-tilt array and is zero in summer.

References

1. Townsend & Powers 2011, Photovoltaics and snow: An update from two winters of measurements in the SIERRA, IEEE 37th Photovoltaic Specialists Conference
2. Townsend & Previtali 2023, A Fresh Dusting: Current Uses of the Townsend Snow Model, Photovoltaic Reliability Workshop 2023, NREL/CP-5900-87918

API

loss.snow

Symbols follow the symbol registry.