# Effects on PV System Output¶

## Loss models¶

 pvsystem.combine_loss_factors(index, *losses) Combines Series loss fractions while setting a common index. pvsystem.dc_ohms_from_percent(vmp_ref, …) Calculates the equivalent resistance of the wires from a percent ohmic loss at STC.

## Snow¶

 snow.coverage_nrel(snowfall, poa_irradiance, …) Calculates the fraction of the slant height of a row of modules covered by snow at every time step. snow.fully_covered_nrel(snowfall[, …]) Calculates the timesteps when the row’s slant height is fully covered by snow. snow.dc_loss_nrel(snow_coverage, num_strings) Calculates the fraction of DC capacity lost due to snow coverage.

## Soiling¶

 soiling.hsu(rainfall, cleaning_threshold, …) Calculates soiling ratio given particulate and rain data using the Fixed Velocity model from Humboldt State University (HSU). soiling.kimber(rainfall[, …]) Calculates fraction of energy lost due to soiling given rainfall data and daily loss rate using the Kimber model.

 shading.masking_angle(surface_tilt, gcr, …) The elevation angle below which diffuse irradiance is blocked. shading.masking_angle_passias(surface_tilt, gcr) The average masking angle over the slant height of a row. shading.sky_diffuse_passias(masking_angle) The diffuse irradiance loss caused by row-to-row sky diffuse shading.
 spectrum.spectrl2(apparent_zenith, aoi, …) Estimate spectral irradiance using the Bird Simple Spectral Model (SPECTRL2).