charge. For instance, a typical 12V battery bank gives a reading of 12V with flat batteries. But when the batteries are fully charged, it may record 16V. This voltage swing causes most batteries to be measured in amp-hours rather than in watt-hours .

The formula is just as easy; forget about the voltage.

• Battery Bank Energy (in amp-hours):

10A x 30 hours = 300 amp-hours of energy

You can convert energy measured in amp-hours to watt-hours by multiplying the amp-hours with the nominal voltage of the battery. Similarly, a watt-hour battery bank can be converted to an amp-hour by dividing the watt-hours by the nominal voltage of the battery. Those are all the formulae you need to work out electrical energy.