Now, it has been a long time for most of you since you had to think consciously about how much power you were using in your house. Household power systems have been upgraded to the point where the supply almost always exceeds the possible demand.
Not so here. The shore power cord, the power panel and all the circuitry onboard are designed to handle 30 amps maximum. So how much is that, really?
- Heat pump: 12 amps
- Refrigeration: 5 amps
- Water heater: 10 amps
- Interior lighting: 4 amps
- Battery Charger: 15 amps
- Espresso maker: 10 amps
- Microwave: 15 amps
- Vacuum cleaner: 10 amps
- Hair dryer: 15 amps
Off the dock, the equation changes. The limit doesn't change, but the supply does. Most of the boat is set up to run off of 12 volt power, and we have 8 large batteries onboard to supply that power. To use battery power to run standard 120VAC appliances, we have a large inverter, which transforms 12VDC into 120VAC. A 100% efficient translation of power like that says that (volts x amps) on the input side of the inverter must equal (volts x amps) on the output side. Therefore, since we are stepping up the voltage by a factor of 10 (12V -> 120V), the input current must be 10 times the output current. This means that your hair dryer, which pulls 15 amps at 120VAC, is pulling 150 amps out of the batteries. This is a huge load - twice what the starter in your car pulls when starting it, if you have a V8. Imagine cranking two recalcitrant V8-equipped cars continuously the whole time you are running the hair dryer. So running big loads off the inverter is possible, but uses prodigious amounts of battery power. But running the espresso maker every morning is not optional on Eolian.
Therefore, we also have a generator onboard. It is diesel fired, produces 30 amps at 120VAC. But it disturbs the serenity of a quiet anchorage, so I try to minimize its use. It is the choice if we need the hair dryer (its noisy too...). It also supplies the battery charger so that when the generator is running, the battery banks are recharging.
So, typically the electrical budget when we are off the dock requires about an hour of generator in the morning and 30-60 minutes again at night. The output is essentially directed into the batteries to replenish what has been taken out by the daily operation of the boat. You learn to think in terms of amp-hours at 12 volts. Refrigeration uses 60-100 amp-hours per day. Making the obligatory two rounds of lattes in the morning uses about 20 amp hours. Running the anchor light all night used to use about 15 amp hours, but now we have an LED anchor light that uses about 1/10 of that - small enough to be ignored, and a blessing.
If we run the engine that day, the engine alternator will typically eliminate the need for running the generator
Its a lot easier when you just have to pay the power bill once a month, isn't it? Living on a boat is a lot like that in many ways: You are, and must be consciously aware of things that are invisible, behind the scenes, ignored, on shore.
But being conscious is a good thing, I think.