I would like to start a discussion around developing features that implement PVsyst style losses that act as direct physical operators rather than as percentage factors on just the dc section of the ModelChain. For instance, implementing soiling losses as monthly percentage inputs that operate on the irradiance input leading to effective irradiance, DC ohmic losses as a percentage at STC loss from which a resistance value is calculated and operates on the current of the DC output, and/or AC ohmic Losses as a STC loss percentage that operates on the output of the ac_model.

It is my understanding that loss models in the ModelChain are currently implemented as a single operation after the dc_model has been run and before the ac_model and only affect the dc outputs. Therefore "pvsyst_losses" functionality would not be an additional loss model at the losses_model part of the chain, but would require extra steps added to the chain.

I wanted to bring this up and open it for discussion about preferred ways to implement this. For instance , a soiling_loss function could be added that executes before the aoi_model. This potentially depreciates the losses_model as a single self-contained function.

I suppose what I am suggesting is to remove the losses_model in favor of new functions like soiling_losses, dc_losses, ac_losses. What to run for each of these could then be defined per model strategy whether it is pvwatts, sapm, or pvsyst, etc,. If this strategy seems valid I would like to attempt a pull request to begin implementing it.