The more energy you consume, the bigger the storage facility you need. Buckets are measured in liters or gallons; battery capacity is measured in watt- or amp-hours. If the battery uses a higher voltage source for charging (like a full-strength PV array), you will enjoy a higher voltage recharge – a battery bank of 24V can record a 30V voltage when it goes through equalization charging.

But the battery voltage fluctuation creates difficulty in energy rating calculations. The normal formula is the product of the circuit's voltage (pressure) and current (electron flow) and the time taken by the flow of current. The issue here is determining the right voltage for a battery.

Energy (watt-hours) = Voltage x Current x Time

Thankfully, battery manufacturers are smart – they ignore the voltage in the calculation to avoid rating confusions. Thus, their rating calculation is simply current multiplied by time.

Battery Capacity (amp-hours) = Current x Time

How does this relate to the battery's storage level for home use when the grid goes south?

As an example, let's assume 4,000 watt-hours (4 kWh) energy consumption per day for a day-long blackout, which is a reasonable amount of energy for people on the grid who wish to be prepared for power failures.

We would still have to consider the usable energy in the battery bank and not just the rated or gross amount. The typical maximum discharge depth is 50%, although it can be less. We'll need twice as much energy:

4 kWh capacity required x battery derating factor of 2 = 8 kWh capacity

Battery capacity can be rated in watt-hours or kilowatt-hours by manufacturers, but the common unit is amp-hours. Watt-hours (kWh) can be converted to amp-hours (Ah) by dividing the battery rating by the nominal voltage of the battery as per system requirements – which is usually 24V or 48V (here we assume 24V).

8 kWh capacity (8,000 Wh) ÷ 24V = 333.33 Ah capacity

As you would expect, there is no battery with an exact capacity of 333.33 amp-hours. If the batteries are connected in series, the voltage will be increased; if in parallel, the battery capacity will increase. Hence, two parallel sets of 150 Ah batteries will give a capacity of 300 Ah. It's a bit lower than the calculated amount but the difference is negligible.

An undersized battery causes over-discharging as well as a decrease in the life span of the battery. For battery size estimations, it is better to round up than down. Battery capacity can offer great savings.