Sizing the Battery Bank

To determine the minimum battery bank ampere-hour rating that you will need to operate appliances from the inverter, and any DC appliances powered by the battery bank, follow these steps:

1.List the maximum wattage that the inverter has to provide (as above).

2.Estimate the number of hours the appliances will be in use between battery recharges. This will differ depending on appliances. As an example, a typical home-use coffeemaker draws 500 watts during its brew time of 5 minutes; it maintains the temperature of the pot at about 100 watts. Typical use of a microwave oven is only for a few minutes. Some longer operating time appliances are lamps, TVs, computers and refrigerator / freezers.

3.Determine the total watt-hours of energy needed. Then multiply the average power consumption in watts by the number of hours of run time. For example: 900 watts for 10 hours = 9,000 watt hours. Using the 900 watts (or 90 Amps) for 10 hours example as above, then 90 amps is needed for 10 hours. This provides us with the basic amp-hours (AH) of battery that is required. Ten hours at 90 amps equals 900 Amp Hours (AH). This answer is just a beginning because there are other conditions that determine actual run time. These include:

•AC appliance load and time in use (basic Amp Hour)

•Cable gauge and length (cable losses)

•Charge level of the batteries (between use, chargers have to be able to fully charge the batteries)

•Temperature of the batteries (colder batteries provide fewer amps)

•Age and condition of the batteries (older batteries lose capacity/amp hours)

•Compliance with turning off unnecessary AC and DC loads.

If there is any doubt about sizing the battery bank, it is safe to over estimate the amp hour requirements of the battery bank.