To figure out what capacity you need for your boat's house or domestic battery bank, you need to get your calculator out...

In order to work out your boat’s battery capacity you need to know how much power you’re going to draw, and this requires you to get your head around three key equations:

  • Volts x Amps = Watts (example: 12V x 6A = 72W)
  • Watts / Volts = Amps (example: 300W / 12V = 25A)
  • Amps x Time = Ah (example: 2.5A x 5h = 12.5Ah)

Stu Davies, PBO’s engine expert, urged me to consider what might be a daily routine on our PBO Project Boat Maximus. “Would you turn your fridge on as soon as you get on board? Lights on at 6pm till 11pm? TV? Batteries are only good for half their stated capacity in reality.”

Stu then advised me to think about making passage. “VHF on standby, MFD for chartplotting, wind and depth? Nav lights for night passage?” he suggested. “The rest doesn’t really matter – a solar panel will keep on top of them.

“I imagine two x 110Ah batteries plus a starter battery is good enough for your needs. Add in a 100W solar panel and you’re good.”

That sounded good to me, but of course, I needed to do the sums. If your electrics are already up and running, as most boat owners’ are, then you can work this out with a multimeter or, if you have a battery monitor such as Victron’s, by turning on the items one at a time and reading the actual current draw.

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With Maximus we were starting from scratch so I sourced either the power rating (in watts) or current draw (in amps) for each item I planned to use, and then estimated how long, in a given day I might be running this for.

As long as you know either one of these, you can work out the current draw with the following equations:

  • Power (W) = Current draw (A) x System voltage (V)
  • Current draw (A) = Watts (W) / System voltage (V)

Then, once you know your current draw, all you have to do is work out the number of hours that piece of equipment will be switched on for. So, for example, our 3W Osculati tri-light in a 12V system will draw 0.25A (3 divided by 12).

If it’s switched on for an overnight passage of 10 hours, it will consume 2.5Ah (0.25 x 10). Assuming we replaced all our bulbs with LEDs, here was what I worked out (see table, below).

This was the most I could imagine using – it assumed being on night passage for 12 hours, and then at anchor or on a swinging mooring for the remaining 12.

I have no plans to sail across Biscay just yet… It was actually a really good exercise because, very early on in Maximus’s refit I was having to focus on what items I needed, and which of these should be installed while still in the boatyard.

For example, the through-hull transducers for the wind and speed instruments, and the tri-light and steaming light to be fitted while the mast was down.


Installing the batteries aft under Maximus’s cockpit

Which boat battery did we choose?

Let’s say I needed, at most, 130Ah out of my domestic battery, then I would be wanting house batteries with at least double that capacity (260Ah). We didn’t have the budget for lithium-ion batteries, and as weekend sailors they didn’t seem necessary.

We did consider lead carbon batteries, though our marine electrician knew very little about them and I didn’t know of a single boat owner who had them, other than PBO contributor Paul Sumpner, who fitted them on his narrowboat.

In his article, Paul pointed out that they can survive a regular partial state of charge (PSOC) and his Leoch lead carbon batteries had 3,000 life cycles, which – with the exception of lithium-ion – was over three times the lifespan of most other types of battery.

However, we couldn’t find space for the 53cm-long 160Ah lead carbon batteries I discussed with Nigel Vincent at Victron Energy, so decided to go with Adam’s advice and get three of the smaller Exide batteries which would be easier to manoeuvre into position.

Adam rated them highly and had fitted them successfully to many boats our size.

Thanks to our Project Boat Supporters

Dell Quay Marine, Osculati, Raymarine, Shakespeare Marine, TruDesign, Screwfix, Coleman Marine Insurance, MDL Marinas, Premier Marinas, seajet, Marine & Industrial, Clean to Gleam, Dometic, West System, Farécla, Navigators Marine, Lewmar, RYA, Aqua Marine, Ecobat, Victron Energy, Scanstrut, T Sails and XP Rigging.

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This feature appeared in the March 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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