The UPS system battery capacity (Amp Hours or KWH) should be 2 to 4 times the size needed to provide power during the peak rate hours. For example, if the load is an average of 4,000 watts from 4 p.m. to 9 p.m., that is 20,000 WH (watt hours) or 20 KWH (kilowatt hours). The battery capacity should be at least 40 KWH.