Sizing your battery bank is an easy exercise once you have estimated the number of Amp Hours you plan to use each day. The rule of thumb is that your battery bank should be 4 times as large as your estimated energy needs. Therefore, in the example above we estimated that we would use 164 Amp Hours each day. If we multiply that by 4, we reach 656 Amp Hours (164 x 4 = 656). Therefore, when planning for your battery bank, you should size it accordingly so that the total of all the batteries could supply 656 Amp Hours. If you remember from the battery articles, you should never let your batteries become more than 50% discharged and if you want to make them really last, you should try not to go below 20% before recharging. In this scenario if you sized your battery bank to supply 656 Amp Hours and used 164 Amp Hours each day, your batteries would never be discharged by more than 25%, which fits in nicely with what we know about keeping our batteries in tip top shape.

If you plan on using an appliance that draws large amounts of power for short periods of time such as a microwave, you may need to increase the multiplier to 6. So in this scenario if you knew that you wanted to regularly use a microwave while running it off of your inverter, you would want to size your battery bank at 984 Amp Hours (164 x 6). The reason for this is because deep cycle batteries are not meant to supply large amounts of power over short periods of time. Instead, they are meant to supply less power over longer periods. Supplying large amounts of power over a short period could damage them. Therefore, in order to accommodate this need without damaging the batteries, you need to spread the load out across more batteries. My suggestion: if you have a need such as this, don’t run it off the inverter. Just crank up your generator for a few minutes.

One Last Note:

I have included watts in my worksheet for the AC items. If you know the amperage already, then you don’t really need to know wattage to calculate Amp Hours. However, knowing the wattage does come in handy when you are trying to size your inverter. Most inverters are rated by the maximum amount of wattage they can supply. Therefore, I suggest you calculate the wattage using the formulas above and include it in your worksheet so that when it comes time to determine the size inverter you need, you have the numbers readily available. Since the DC items are not running through the inverter, you do not need to know the wattage of these items. I will leave inverter sizing for the articles related to inverters.

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