I looked at ResStock's documentation and source code and it is difficult for me to understand how they have modeled the COP equations in their code. Does anyone have any explanation/understanding of it or any better source to get an equation to model the temperature-dependent coefficient of performance for cold climate air source heat pumps?
https://github.com/NREL/resstock/blob/main/resources/measures/ResidentialHVACAirSourceHeatPumpSingleSpeed/measure.rb