This was for the old Picaroon, whose electrical system was very simple. The principles still apply to more complex systems, but remember to account for draw from sensors and instruments that may be hardwired to the battery.
It took me a long time to get around to doing this, partly because I kept forgetting to look up the current draw of the Picaroon’s various electrical appliances, but also because I expected it to be harder than it was.
For various reasons, mostly having to do with trying not to run out of power at inconvenient moments, sailors need to know exactly how much electrical power their boat uses. You can do this by installing diagnostic instruments in your boat, but that sounds like a real pain in the arse, so most people just figure it out on paper (actually, most people pay someone to do it, but to hell with that).
1. Draw up a load chart.
This is easy: copy the one below. I did it in Microsoft Excel, which does the sums for me, but you can do it just as easily with a pen, paper and slide rule, if that’s how you roll.
2. Walk through your boat from stem to stern and note down every single appliance on board.
Unless your boat is very new to you or you have a lot of appliances, you can probably do this in your head. By appliance, I mean anything that uses electricity: light bulbs, fans, bilge pumps, chart-plotters, hot tubs . . .
3. Write down the current each appliance draws, in amperes.
You can do this using a multimetre, but the easiest way by far is to gather up all the manuals, instructions, and box labels of every appliance you wrote down. Somewhere in that lot there will be a power or current rating. If you can only find the power (in watts), just divide that by your voltage (12, probably), to get the current.
4. Figure out how many hours each appliance will be used on an average day.
You will probably want to draw up different tables for day trips and passage-making, since the demands are so different. Remember that some high-load appliances, like the starter motor on a diesel engine, will run for only a very short time, but at huge currents (the starter on Picaroon’s tiny YSM8 draws 83 amps!).
5. Multiply the draw in amps by the hours of use to get your average daily consumption in amp-hours.
Now you are probably saying to yourself, ‘Sweet! I have this fabulous load chart with my average daily consumption all worked out. Now what the hell do I do with that?’ Well, I don’t know. You’ll figure something out, I expect.