Another thing about living on a boat is that you are always conscious of your systems - energy (diesel, propane, and electric) as well as garbage waste disposal and water. In a house in town, you have utility companies and municipal services for most of these On a boat you are the head of your own multi-faceted utility company. I have written before about water use, so will talk now about electricity, which I don't think we have blogged about in any detail.
When a boat owner is trying to work out their requirements for an electrical system, there are three areas to be concerned with - storage, use, and production of electricity. Storage is a matter of selecting the correct number of suitable batteries. These are usually lead acid deep discharge batteries and the common choices are vented wet cell batteries that require periodic topping up (e.g., golf cart batteries or marine deep-discharge units) or sealed units such as AGMs or Gel Cells. We chose to install three group 27 Trojan AGM batteries because we plan to leave the boat in warm sunny areas and will not be around to top up the batteries. AGMs also typically last longer than regular wet cell batteries, although they are much more expensive ($750 US for our three batteries).
It turns out that the number and size of batteries depends on both how much electricity you consume and how much you can produce in a day. You also need enough capacity to tide you over between recharge periods, which may be when the sun shines (if you use solar panels), when the wind blows (using wind turbines) or when you run your engine or a generator to generate electricity from gas or diesel. You don't want too many batteries because they cost plenty and weigh a lot and because if you have more battery capacity than you can recharge easily, this is just wasted.
To produce electricity, we rely mainly on solar panels (250 watts total) and an 80 amp externally regulated alternator on our diesel. As a rough way to calculate how much we will generate per day, I used a rule of thumb of taking the watts and dividing by 5 to get amp hours per day (although in the tropics on sunny days you may do much better, this allows for the odd cloudy day). Amp hours are a useful common unit for dealing with production and consumption. I also assume I will run the engine on average for an hour every other day, producing about 12 amp hours per day from this. I try to treat this as a bonus, because ideally I only run the engine when we need to as part of a passage or on leaving or entering anchorages. So that gives us a total of 62 amp hours generated per day.
That leaves consumption of power. To determine this, you do an energy audit, which is simply a list of all the things that consume electricity and an estimate of how much you use each one per day. This is different when on passage versus at anchor, so I did 2 lists. Here is one list for at anchor:
Fridge: (varies with air & water temperature - measure this to calc): 6 amps for 10 minute/hour: 24 amp hours
Interior lights: 2 @ .4 amps for 4 hours + 1 fluorescent @ 2 amps for .75 hours: 4.95 amp hours
Propane Solenoid: .5 amps for 1 hour: .5 amp hours
Anchor light: .3 amps for 12 hours: 3.6 amp hours
Net-book charging: 2 amps for 3 hours: 6 amp hours
SSB transmission/reception: 2 amps for 1/2 an hour + 25 amps for 10 minutes = 5 amp hours
This list gives us a total of about 45 amp hours per day. underway, we probably consume another 12 amp hours for instruments (24 hours @ .5 amp/hour) and 4 for periodic use of the radar for a total of roughly 60 amp hours. You can see that the fridge is the main consumer of power, especially in the tropics. We actually used about 15 amp hours per day in cooler Mexican waters. Our fridge is very small and we do not have a freezer. As an aside, we have friends whose refrigeration consumes far more than 100 amp hours per day - they run a generator every day to keep things cold.
It looks like we have a slight surplus here when in port and a rough balance when underway, assuming we do not get too many days of cloudy weather. If this happens, we must either cut down on our optional consumption (mainly Net-book time) or run the engine more often. Note that I have not included the water maker consumption here as we only run this when we have a surplus of solar energy on sunny days or when we are running the engine.
One final thing - how did we determine we needed 3 Group 27 batteries? I assumed we would need to last for two days without recharging. We do not want to run the batteries below 50% charge as this reduces their lifespan, so two days is 120 amp hours, which is a little less than the 150 amp hours (300 amp hours/2) the batteries claim as their rated capacity. You want this extra room for various reasons, but mostly because you rarely charge your batteries to 100% of their rated capacity.