Battery duration determines the inverter sizing that gets modelled alongside the battery storage capacity. So, for example, if you have a 100kWh battery with a 1 hour duration then that will model a 100kW inverter coupled to your 100kWh of storage cells. If you select a 2 hour duration then a 50kW inverter, 4 hour duration a 25kW inverter. You get the idea. You can model the same sized battery with multiple different inverter rates (battery durations) to see which gives best commercial return.