Ensure your inverter and battery are properly matched by checking voltage, current draw, and required battery capacity. Formula: Battery Capacity (Ah) = (Inverter Power × Runtime) ÷ (Voltage × Efficiency). The number of batteries you need depends on three factors: your inverter size, how much power you actually use, and how long you need to run. Properly matching your inverter. How many batteries do you need for a 3000 watt inverter? The size of the battery needed will depend greatly on the total amount of watts your appliances uses, as well as climate conditions and exposure to sunlight. Let's break it down: Battery Capacity Calculati. So I have made it easy for you, use the calculator below to calculate the battery size for 200 watt, 300 watt, 500 watt, 1000 watt, 2000 watt, 3000 watt, 5000-watt inverter Failed to calculate field. In this article, we'll break.