Hi Niels,
The "fixed cost by capacity" in $/kW-yr category of operation and maintenance (O&M) cost on the System Costs page scales with the nameplate capacity of the system, which for PV Battery systems, SAM considers to be the DC capacity of the photovoltaic (PV) array, which has been the convention for PV systems.
The "variable cost by generation" in $/kWh O&M cost category is calculated from the system's total annual AC electricity delivered to the grid by both the PV system and battery. Note that for a given PV array size, a system with batteries delivers slightly less electricity to the grid over the year than one without batteries because of battery throughput losses.
For systems with battery storage, there is an additional O&M cost category in $/kWh of battery bank capacity (in kWh) to account for the cost of replacing batteries that depends on both the size of the battery bank and the frequency of replacements. The replacement frequency depends on the Battery Lifetime options you choose on the Battery Storage page and accounts for things like number of cycles, depth of discharge, etc.
We have been having internal discussions about the cost units for battery storage -- we do have costs in both $/kWh and $/kW for battery installation cost, but not for the O&M cost. We generally try to follow the conventions used by the NREL Annual Technology Baseline (ATB), for example:
atb.nrel.gov/electricity/2019/index.html?t=st
Best regards,
Paul.
