Two core decisions we must make when designing a cruising boat’s electrical system for living aboard full-time and making offshore voyages are:
- the size of the battery bank, and
- which charging sources we will need: generator, solar, main engine, wind, etc.
But the first thing we need to do, before getting into all that fun stuff, is think about electrical consumption and how to keep it reasonable.
That’s what I’m going to cover in this chapter, and then in the next we will move on to battery bank specification.
It’s Worth It
Whatever your situation, do make the effort to go through this process, particularly if you have not yet got out there cruising and/or done a multi-day passage, since unrealistic assumptions about power usage, leading to undersize battery banks and charging sources, is one of the most common cruise and/or voyage ruiners.
And even for me, after all my years of cruising successfully on the same boat, this exercise has proved both interesting and useful.
The Old Way
Typically, when designing an electrical system, we are supposed to fill out a form listing every single load on the boat in order to calculate our expected consumption.
But there are several problems with that approach:
- It pushes us into thinking about the details ahead of the big picture—pretty much always a bad idea.
- It’s difficult to estimate electrical usage for a given piece of kit. Sure we can look up the specification, but that doesn’t help much unless we can also make an accurate assessment of how much of the time the machine in question will be running, and how its consumption may vary over time; not always, or even often, easy.
- On a modern cruising boat, with scores of machines that use electricity, this kind of bottom-up analysis will take a huge amount of time.
A Better Way
Given that, I’m going to come at this from the top down, by looking at the loads that really matter, along with rules of thumb for estimating them based on my some-25 years of living on boats.
Will this approach be dead-nuts accurate? No, but it will let us get as close as we need to. And, anyway, the inaccuracies in the bottom-up approach are probably as bad or worse—just because we write down a lot of numbers doesn’t make ’em right.
Do We Even Need To Fix It?
The other advantage of this approach is that we can reverse the process by using the same rules of thumb to understand what level of electrical consumption the system we already have will support (more on how in Part 2). Increasing battery bank size and charging sources is a huge project, at least when done right, that can seriously screw with our cruising budget both money- and time-wise.
When thinking about at-sea loads, I will be primarily focusing on sailboats, since a motorboat, at least one with proper alternators, does not need to worry about loads while underway. But I will also cover loads at anchor, which will be of interest to both sail and power owners.
A couple of conventions:
- I will be expressing usage, load, and capacity in amp hours (Ah) for a 12-volt system. If you have a 24-volt system just divide my numbers by two.
- When I write “day” I’m referring to 24 hours, unless otherwise stated.
Let’s start by looking at the four horsemen of the flat battery: