The three significant factors to consider when setting up a UPS are the intended load (i.e., the combined voltage and amperage of all connected electronics), the capacity (i.e., maximum power output), and the runtime (i.e., how long it can supply battery power for). A UPS is most. The capacity of your UPS is its maximum power output (AKA VA rating). When shopping for a UPS, the VA rating should be listed along with. The available runtime for a 300 watt UPS largely depends on the make and model itself and how close to capacity the connected load is. The commonly-used formula for calculating the runtime of a UPS is pretty straightforward, but you have to know a few additional values first. 1. Find the capacity of the UPS.