It’s at the heart of the matter: Lights, plotter, autopilot… everything works by electricity on a sailboat and catamarans are no exception to the rule. Before knowing how to achieve electrical autonomy, you need to make a realistic assessment of consumption…
Do you remember anything of the physics you learnt in school? Whether it’s a cabin light or a radar, the quantity of electricity consumed by a device is measured in Ampere-hours (Ah): it is the product of its consumption (expressed in Amperes) and the time in use (h).
To realistically calculate the amount of electricity you’ll need to produce over a 24-hour period, it is therefore necessary to identify all the devices on board and to know their consumption. Then a table can be completed. We recommend that you do two tables for different types of sailing: either stopping to anchor or passage-making.
Typical example of a table:
So you just need to know the actual consumption of each item.
You’ll find this on the base of a bulb, on the manufacturer’s label or in the instruction manual. But as it’s not always easy to access it and the consumption figures are sometimes optimistic, the best thing to do is to take the time to measure each device’s consumption.
Generally, electrical panels are equipped with an ammeter. Turn off all instruments, make sure the ammeter is set to zero. Now turn on the VHF on standby. You will reliably know the appetite of each consumer. It’s a bit tedious work but it’s well worth doing.
Then ask yourself the question of the length of time in use, trying to be accurate. Are you sure that on this short 150-mile passage, the pilot will only be in use half the time and that everyone’s going to volunteer to helm? Have you thought about measuring the laptop’s consumption when the kids are watching a DVD? The refrigerator will certainly consume more now that you’re going down in latitude, it’s warmer and it’s emptying out. How many showers will be taken per day? And by the way, who forgot to turn off the reading light in their cabin before going on watch?!
Take a close look at the big power consumers, such as the autopilot.
Record its consumption in light upwind conditions, and then have fun re-measuring on a day when you’re sailing downwind in big seas. The figure could easily triple…
From experience, it’d be pretty unrealistic to imagine consuming less than 200 Ah per 24 hours on a modern, well-equipped boat, sailing with a family.
300 Ah wouldn’t be surprising. Beyond that, you should be starting to ask yourself questions about your lifestyle and whether you’d be able to consume less, a concern that is beginning to interest land-dwellers too, and is part of the interesting learning curve of cruising, especially for children…