A 72V 20Ah battery theoretically stores 1440 watt-hours (Wh) of energy and will last about 14. 4 hours powering a 100-watt device at ideal efficiency. This is calculated by multiplying the voltage (72V) by the amp-hour rating (20Ah). . If you're working with a 72V battery system and need to know how much electricity it can provide through an inverter, you're in the right place. Let's break down the math in simple terms – no electrical engineering degree required! Pro Tip: Always check your battery's spec sheet for accurate. . The numerical digit that you get by solving the above equation will give you the total energy value in kilowatt hours. The energy E in kilowatt-hours (kWh) per day is equal to the power P in watts (W) times number of usage hours per day t divided by 1000 watts per kilowatt: E(kWh/day) = P(W) × t(h/day) / 1000 (W/kW) Energy consumption calculator. . Enter electric appliance in the dropdown menu or enter manual wattage rating in watts or kilowatts (kW) and the daily usage of the device in hours. Electric energy or power consumption. .