To get started, we used the battery monitor to map the power usage of each electrical device and determined how much electricity each would consume in a day. For instance, the refrigeration compressor draws up to 4.5 amps, and in the Bahamas and southern waters, where the boat was headed, we figured it would run 12 hours daily. We went through each piece of gear the owner had aboard, and we were able to estimate that 60 to 80 amp-hours would be used in a 24-hour period when the boat was in use, and 15 to 20 amp-hours would be required when the boat was left unattended. If your boat doesn’t have a battery monitor, you can still get an idea of the current drawn by the respective units by referring to the manufacturer’s specifications. Some multimeters can also measure current draw; you’ll need to disconnect and bridge each unit’s positive power-supply lead.