Successful Sailboat Project: Solar Energy and Self Suffiency

Successful Sailboat Project: Solar Energy and Self Suffiency

In the midst of busy boat projects you never have time to write down everything that goes on. Later on it seldom feels like a tempting subject, especially if there are interesting travel stories waiting to be told. But sometimes a perfect opportunity presents itself – like when you’re waiting on a Galician ría to get some engine service done, and it’s freezing cold and cloudy – so I decided on a little article about the solar energy system we installed a year ago. Besides, we now have a whole year’s worth of experience and insight. If you’re into technical stuff and boat projects, you might find this interesting – if not, travel stories will appear soon enough!

Self Sufficient Energy Production

Originally our boat had 280 Ah, that’s amp hours, worth of AGM batteries. You could charge them with an engine alternator, which is powerful enough, giving 150 amps. There was also a wind turbine, that kept the batteries charged while the boat was on the hard during the winter. That’s still about as much as we know about the wind turbine’s ability to produce electricity – we mostly keep it lashed down, as it makes a very unpleasant noise.

All the boat’s systems that consume power were designed around that engine alternator. The cooling compressors only ran while the engine was running, and the fridge compartments had large cold plates inside, that could keep them cool between charging cycles. I’ll describe our new cold box installation a bit later, but just want to mention that we wanted a system that works with 12 volt electricity – running the engine daily just to keep normal house appliances working is definitely something from a bygone era.

Solar panels on boats are now very common and popular. They have also developed a lot lately – the same footprint will now produce twice as much electricity as it did a decade ago. The panels have also gotten a lot cheaper. A well designed and built system is very care free and easy to maintain. As we planned to sail south, into sunnier latitudes, choosing solar energy was a no-brainer.

The biggest challenge with solar energy is that the required surface area, unshaded by the sails and rig, is not often easy to find on a boat. Many cruising boats have a stern arch for installing the panels, and that’s what we decided to add to our boat, too. We got a great deal to have one built in Poland, so that’s where we headed at the start of our journey. The project took a little longer than expected, but we were very pleased with the quality and finish of our arch.

Solar System – Batteries, Panels and Controllers

To get the project started, we made an energy budget – a list of all the existing and planned electrical systems onboard our boat, and their energy consumption. Doesn’t sound easy, and it isn’t! It’s hard enough finding info on the amps and watts needed, let alone trying to estimate how many minutes you might use the VHF radio or the radar on an average day. We read user’s manuals and techy boat forums, and eventually came up with a daily budget of 150 amp hours – more while sailing, less while swinging at anchor. As you should never discharge a lead battery below 50% of its capacity, your batteries have to be at least twice the daily budget – and really quite a lot more, to take into account cloudy weather, emergencies and other safety factors. We doubled our budget twice, and as the battery compartment had its limits too, we ended up with 4 x 160 Ah, giving us a total capacity of 640 Ah. In addition to that, we have two starter batteries and one dedicated to the anchor windlass.

We chose the same kind of Mastervolt AGM batteries as before. They are maintenance free and sealed, and particularly suited to be used on boats. They are also long lasting – our old batteries seemed fully functional at the age of 10 years or more. We were not ready to jump on the lithium train, because it would have required a more substantial electrical refit than we had time or budget for – or the know-how, even if we might have had enough of the DIY spirit required.

For sizing the solar array you often see a recommendation of 1 W for each battery amp hour, when it’s your primary means of producing energy. We chose two larger panels to go over the stern arch, at 245 W each. Additional two panels of 100 W were attached to the side rails, where they can be tilted to a desired angle and folded down while underway or in a marina. So the rated capacity of the whole system is almost 700 watts.

Rated capacity is not the same as actual yield, which is affected by clouds, temperature, and the angle of the sun. To make the most of it, you need an intelligent charge controller between the solar panels and the batteries. We chose Victron MPPT control units, that can talk to each other and to a battery monitor of the same brand – and all the information can be accessed with a mobile phone app. It tells us, that on a random, cloudy day in Galicia, our solar panels have produced about 2 kW of power, that the batteries have been fully charged before noon, and that the system has been in absorption mode or floating the rest of the day, which means only replacing the little power used during the day.

Energy usage


So, we had decided upon the battery and solar panel capacity more or less scientifically. They’re always limited, so it’s also wise to figure out ways to conserve energy. An obvious and easy one is to replace incandescent lamps with LED lights. On some lamps you can just swap the light bulb, but we also added new lights here and there, where lighting is needed.

Navigation electronics

We kept most of the existing navigation systems, so we just have to live with how much power they need. The autopilot needs quite a lot, but while we sail, we can use the windvane that doesn’t need electricity. The radar is another power hungry beast, but in power saving mode it only spins once every 10 or 15 minutes. We use an iPad for a chart plotter, and it doesn’t need much power.


The fridge and freezer are usually the biggest power consumers on a boat. The warmer it gets, the more power they use – but fortunately, solar panels tend to work better in sunnier climes, too! As we planned to sail towards warmer latitudes (which we obviously haven’t reached yet) we wanted our cool boxes to be as efficient as possible. Another reason was that the boxes are of substantial size, American style, the fridge about 150 litres and the freezer, after added insulation, 120 litres (that’s over 5 and 4 cubic feet respectively, for those of you who are that way inclined). You need sizable compressors to keep big boxes like that cold enough, so they’d better be efficient. We didn’t like the idea of making our boxes smaller, because cooking and eating well is an important thing for us. Also, having a big fridge is great when provisioning for a long journey, and when you have a big freezer, you can buy a lot of cheap stuff when it’s available.

Often, boat fridge compressors are air cooled – in the Northern countries that’s usually the case. Another alternative are water cooled devices. They are more energy efficient, but also more expensive. In Northern Europe an air cooled system is just fine, in those cool climates there’s not much difference between the two systems. But the difference increases in warmer climates. Air cooling also makes the cabin warmer. After a lot of research and pondering, we chose a water cooled system, with so called “keel coolers”. They are chunks of bronze shaped like gold bars, that are attached on the outside of the boat’s hull. The cooling pipes circulate through the coolers and into the compressors, and another circuit brings the coolant into the evaporators inside the cool boxes.

We chose Frigoboat for our system, which we ordered from England and installed ourselves. The compressors are tiny and could easily fit on the bottom of a small galley cabinet. We drilled two holes through the hull, close to the boat’s center line, for the keel coolers, and threaded the pipes through the bilges and under the cabinets into the compressors. That was probably the hardest part of the project! We attached the evaporator plates to the back walls of the cool boxes, after we had bent them to an L shape by warming them with a heat gun. Finally, the pipes were attached to the compressors. Then we connected the electrical wires, and lo! we had a working system for storing food onboard!

The Joy of Success!

Of the various systems onboard our boat the refrigeration has proved the least problem free. A little bit of ice accumulates in the freezer over time, and we have to defrost the box once a month or so – otherwise the whole food storage business is quite painless. It’s also very energy efficient. The two compressors use about 50-60 Ah per day, the water temperature being around 20°C (70 °F) which is very little, especially considering the size of the boxes. They’re also very quiet.

The solar panels are even more painless. As long as you remember to wash the salt, dirt and bird poop off of them, you can pretty much forget they exist, and still they keep pushing power into the batteries. Apart from December and January, they have sufficiently produced all our power. Since leaving our winter base we have not once hooked onto shore power – we are completely self sufficient! So far our battery charge has never dropped below 90%, so I guess we also have enough safety factors!

More about our boat’s systems and equipment under the Boat tab.

2 thoughts on “Successful Sailboat Project: Solar Energy and Self Suffiency

  1. Very interesting approach and strategy. I’ve learned a lot even if all cases are different on details. Many thanks. BR

    1. Thank you! I’m glad you found it interesting and maybe even helpful. It’s always good to read about other people’s projects, sometimes you even get to learn from their mistakes 😉

Leave a comment!

This site uses Akismet to reduce spam. Learn how your comment data is processed.