Power, Baby!

For hundreds of years, ships sailed just fine, thank you, without any electrical power at all. In those days, copper was only something you used to line the bottom of the hull (as antifouling protection.) These days, there are wires everywhere. You’d swear that sailing was mostly about managing power: generating it, storing it, using is sparsely, and measuring it constantly. Sailors are obsessed with power! And not just the people hoping to run a freezer in the tropics, either. Watch an America’s Cup race and you’ll see two people sailing and all the others are grinding away, making power to control the boat.

I’ve spent way, way, way too many hours learning about the intricacies of power management. I’ve learned about how a half-full battery accepts power more readily than a nearly-full one. And how the output of solar panels varies depending how they are connected. I’ve learned about the particulars of each type of battery chemistry and how well they charge and how long they hold a charge. I’ve cataloged the power needs of every device on our boat and plotted their usage throughout various stages of the day. It’s all a bit much, really. But for the tech buffs and nautically minded, here’s a blow by blow description of Spartan’s electrical systems.


Batteries, like ice cream, come in various flavours. Flooded acid batteries are the vanilla of batteries. Flooded is the oldest battery technology still sold and it is very commonly used for a boat’s batteries. These batteries are very heavy and bulky. They tend to fail quickly if you discharge them too much or too frequently. They need maintenance. But you can buy them at Costco or anywhere else, cheaply.

At the opposite end of the spectrum – perhaps the cardamom gelato of the battery world – is the Lithium Iron Phosphate battery. Lifo batteries are more like the battery in your phone than the one in your car. They are light, they withstand more discharging and they charge faster. Ones large enough for a boat are still a bit hard to get, and they are about four times the cost of flooded batteries.

We split the difference, opting for a technology called Carbon Foam. Our FireFly carbon foam batteries are heavy and bulky like flooded batteries, but they are supposed to charge faster and tolerate deeper discharge, like Lifo batteries do. As you might expect, they are also the mid-range price point.

We have 2 x 115ah batteries. Which means we should have about 100 amp hours of power to get us through the night without pulling too deeply from our power reserves.

Power Production and Consumption

Coincidentally. we estimate that, at worst, our boat will consume 192 amp hours of power per day. But, during the day, the sun is up and generating electricity for us. So the batteries will reach full charge during the day and only draw down throughout the night. According to my calculations, 2 days of sailing in relatively sunny days should look something like this, assuming that we start day one at 70% charge:

As you can see, our power trough, from 7-9am, is lower on day 2 than it is on day one. If we consume this much power all the time and we never run the engine, we’ll begin to run critically low by day 3 or 4. And even before then, running low is not great for the life of the batteries. Fortunately, I’ve assumed the worst about our consumption and haven’t factored in any engine use at all. So in reality, we should be able to keep our batteries well above 75% for multiple days in a row.

(If this sort of geekiness turns your crank, click here to see how deep this rabbit hole goes!)


In our first season of boat ownership, we purchased two 100w Coleman panels from Canadian Tire. Not the best quality panels, but we wanted to dip our toe in the solar panel world. The panels have worked surprisingly well.

Our Canadian Tire specials, on duty all winter long, trickle charging the batteries while every other component of Spartan goes into deep hibernation.

This year, with the trip in sight, we purchased two more panels. These ones are from a leading producer named SunPower and have a great reputation. Combined, the four panels generate 420 watts of power. We expect that in the course of the average day, they have the potential to give us about 225 amp hours of electricity. But some power will go to waste, I’m afraid.

Considering and ultimately rejecting the idea of having new panels sewn into our bimini. Instead they are just lashed to the bimini frame now.

Excess Power

Whenever the batteries are too full, there’s nowhere to put the power, of course. On some boats, power that can’t fit into the batteries gets diverted to an alternate load. Watermakers are a very good choice.They can convert your excess power into fresh water, saving you from lugging water jugs up from a dinghy. However, watermakers are also about $8000CAD and quite temperamental. They might be nice to have for the 3 months we plan to spend in the Caribbean, but kinda silly to have one aboard for the years ahead when we are back on the Great Lakes – literally floating on the world’s freshwater supply! Unfortunately, our excess sun will go to waste. We’ll either need to time our usage to ensure we leave charge capacity when the sun is coming out, or – fingers crossed – we’ll have such an abundance of power that we just won’t need to think about it much.

Front-End Loaded

You don’t get free electricity from solar power, you just pay for it up front. Similarly, if you want to live on a boat without thinking much about electricity, there’s a whole lot of thinking to be done up front.


  1. Impressive work Thom, thanks for sharing your spreadsheet, will save those that really care and appreciate this a lot of headaches.


    • Thanks Attila. I will start living on the boat today, so I’ll be watching the power consumption more closely and adjusting some of the assumptions I’ve made – particularly on charge efficiency – as I learn more.


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