Boaters traditionally think of battery capacity in terms of amp-hours, but if the battery will be used to power an inverter, the reserve minutes rating can be more telling. Reserve minutes, also called reserve capacity, is the number of minutes a fully charged battery can sustain a designated constant load — usually 25 amps — before it is fully discharged. For a 12-volt battery, that means battery voltage will have fallen to 10.5 volts. Recalling that we divide watts by volts to get amps, supplying a 300-watt inverter load from a 12-volt battery requires 25 amps.

So a battery with a reserve capacity of 120 minutes can theoretically handle this load for two hours, but the practical limit is just one hour since it is never a good idea to discharge a battery more than 50%. Running a 1,000-watt coffeemaker through an inverter will completely drain this same battery's usable capacity in less than 15 minutes. If you plan to install an inverter, you need a battery with a high reserve minutes rating.