Want to add some boat solar panels, or wonder how to make the most of those you already have? David Berry has some advice...


Keeping batteries topped up without resorting to running the engine is an ongoing problem for yachtsmen. Boat solar panels are an obvious option, not just in the Med but also around the coasts of the UK as the price of panels has fallen over the years.

But choosing them can seem a bit of a black art: after all, how can you possibly predict how much sun you’re going to get during the season, or how much power your panel will produce if it’s not exactly aligned with the sun? But provided you accept a statistical approach using established databases, then prediction is easier than you think.

Ultimately the only thing we need to know is the conversion efficiency, or, how much sun turns into electrical power. NASA do a trick: they use multi-layer panels where each layer responds to a different wavelength so the usual 20% or so is doubled.

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And there is a new material called Perovskite which is also used to provide an overlay on the standard silicone panel and the tandem panel is claimed to convert up to 28% of the sun’s energy into electricity.

Don’t rush though, when I looked on Amazon for one I discovered even the books describing it cost around £100! The panels themselves seem to be still in development.

How much energy does my boat need?

The first step is to work out your boat’s energy requirements. All you have to do is add up all the energy in watt-hours used by each device on your boat, such as the fridge, lights, computers and so on.

Energy is power accrued over time, so if power is measured in watts, energy is watt-hours. This can be tricky, for example how long is your fridge running for? And how about overnight? How much energy do the instruments take? Or your computer?

You can measure the power by measuring the Amps and Volts and multiplying them together to get Watts, but somehow you need to come up with a table similar to the one below. Once you have your energy requirements worked out, I suggest you add a goodly margin for expansion and errors – at least 10%.

Device Current Duty Wh/day
Fridge 4 0.5 576
Computer 3 0.1 86.4
Lights 2 0.1 57.6
Fan 0.5 0.5 72
Losses 0.1 0.1 2.88

Energy requirement for Aderyn Glas over the course of a day. ‘Current’ is Amps. ‘Duty’ is the portion of the day the (12V) appliance is powered up. eg Fridge 4A x 12V x 12hr = 576Wh

Next, you need to decide how much of this requirement should be met by the boat solar panels. We sail our Moody 33 Aderyn Glas throughout the summer season from our base in Preveza, Greece, so our requirements are essentially those of liveaboard sailors.

We want the panels to be able to supply most of our power needs and allow us not to have to ruin the peace of a quiet anchorage by running the engine simply for charging.

Weekend sailors might be content with a small panel to recharge – over the course of a week – the energy used during a weekend’s sailing. Getting the balance right is important, especially if you want to install enough boat solar panels to more or less cover your power requirements.

David Berry’s Moody Eclipse Aderyn Glas has boat solar panels mounted on a stern gantry where shadows are greatly reduced

But will you get the quoted wattage from your panel? If the panel is a 100W panel, will you get 100W? Panels are rated as the electrical power produced under certain strict test conditions, and these are solar irradiation (called insolation) of 1,000W/m2 at 25°C and an atmosphere of a particular clarity. Do we ever get these conditions in practice? Well, yes, we do, but life is never that simple – in this article I explain about how to assess the real insolation over the course of a day in your chosen location.

Types of boat solar panel

Leaving aside the exotic new Perovskite panels, there are three types that you might consider – a choice that hasn’t changed in years.

There are amorphous panels (good in shady conditions but large for any given wattage), printed panels (manufactured with an inkjet printer but really low conversion efficiency) and crystalline panels either poly- or mono-crystalline.


The smaller amorphous panel is 15W and the larger crystalline panel is 100W. So the larger panel is approximately twice the area, but seven times the power

What type of boat solar panels should I fit?

This question is a bit of a red herring – in reality, the question should be ‘How much space do I have?’

The academic drive is to make panels that are more environmentally friendly, both to make and dispose of, and to reduce the cost per watt.

But ironically this has resulted in a much poorer conversion efficiency generation by generation, which is not helpful to sailors with a small area available to mount the panels.


These panels are printed on a continuous inkjet printer with the idea that they can effectively be an endless strip. The problem with that, though, is the current can build up and eventually burn out the connecting cables. Furthermore, they are no good for yachts because the conversion efficiency is so low so you’d need a battleship in order to have enough space!

So, back to the first generation: crystalline panels come in mono- or polycrystalline species, but they only differ slightly in cost per watt and efficiency.

Added to the mix is the flexibility aspect: flexible panels require a bit of clever manufacture, and this is passed on in higher prices.

Many sailors think flexible panels are a solution because they can be mounted on the deck or coachroof or even the bimini but I’ve not considered them because all those places seem to suffer from shading, more of which later.

A hypothetical energy balance. The red line is the clear-sky power that the panel could provide during daylight. The blue line is the power your boat requires. Light blue is surplus energy, where the panel is outputting more than the load, and dark blue is where the load is greater than the panel can deliver. When night falls, the boat’s energy needs have to be met by the battery. In daylight, the battery becomes a load for the panels as it is charged. The green line is a notional illustration of the state of the battery; it winds downwards when there’s a shortfall and upwards when there’s a surplus available for charging. In winter, the load profile will be different and the captured power will be lower.

So does this mean the default starting point for any installation is a rigid, crystalline panel? In short: yes!

Where should I install my boat solar panels?

If you’re just fitting a small panel to top-up batteries on a mooring, buy a cheap rigid panel and find places on your boat where you can tilt it to catch the sun’s rays for the majority of the day.

We do this during the winter in Greece, where we strap on 20W or so of car battery top-up panels, and it works well for us. Choose a spot that isn’t shaded for the majority of the day and this technique should serve you well.

Do the sums. I have heard of people whose batteries have been damaged by a constant high power being applied to an already charged battery, even through a regulator. My rule of thumb for trickle-charging is a panel wattage about 10% of the battery’s stated amp-hours, but that’s a guess.


Aderyn Glas is fitted with two monocrystalline panels which are rated at 100W apiece – they take the same space previously occupied by amorphous panels which could only muster 75W between them

If you spend more time on board and need to get the best from your panels, mono- or polycrystalline panels will give more power from the same space. We replaced our 75W bank of amorphous panels with 200W of monocrystalline panels in the same area.

But with the higher efficiency comes higher sensitivity to poor mounting conditions, so if you want the best from your panels you need to do your utmost to ensure they’re not shaded, and also that they are tilted as close to a right-angle to the sun as possible. This is why liveaboards often mount theirs on adjustable gantries at the stern or on the pushpit.

Intrinsic losses

Let me just revisit this: the relationship between the nominal power of a boat solar panel and what you really get. When the manufacturer quotes, for example, 100W for a panel, that is the expected output under test conditions.


Choose your site carefully – this coachroof panel will be shaded by the boom, reducing its output by a considerable margin

The test conditions are an insolation of 1,000W/m2 at 25°C. So a typical panel of half a square metre will receive 500W of insolation, then we multiply by the efficiency of 22% or 0.22 and you get roughly 100W. So if you get a sunny day that insolates your 100W panel at 1kW/m2 then you have the potential to get 100W output for a short time around solar noon.

But the energy over 24 hours will be below this owing to the declination of the sun over the course of the day. This can be partly remedied with a tracking panel, but as the sun declines the light has to pass at a more oblique angle through the atmosphere, hence losing power.

The insolation will also be affected by your latitude, and by any form of shading or scattering from atmospheric dust, haze or cloud. Another important intrinsic loss is that heat reduces a panel’s output by about 5% for every 10°C rise in temperature greater than 25°C. For this reason, it’s quite possible to get a higher output from a boat solar panel in cooler northern latitudes than on the Equator!

This is the comparison between an MPPT controller and a PWM type over a 30 minute periodSo you see what I mean when I say the question is all about how much space you have: the default option should be a crystalline panel because it has the best watt per unit area coupled with price per watt, and we only need to deviate from this simple first-generation solution if other factors are important.

Cost and efficiency

Since we’re only talking about crystalline panels the efficiency is always going to be around 20% and the cost less than £1/Watt.

There are a large number of suppliers out there now but I should stick to the well known names such as Kodak, Polaroid, LG, Panasonic and Victron.

For suppliers I would look at Amazon (of course) or, in the UK, Midsummer Energy which stocks not just panels but all the cables and bits you will need to fit them.

Victron MPPT controller

Other boat solar panel system losses


Losses in cables are proportional to the square of the current. The equation is P=(i x 2) x R where ‘i’ is the current in amps, R is the resistance in ohms and P is the power lost in watts. The voltage gradient from the high voltage at the panels to the lower one at the regulator is fixed by the cable resistance and current (Ohm’s law), which is in turn set by how sunny it is and the power required.

But the resistance is a matter of design. To minimise cable losses and prevent potential cable overheating, large-core cables are needed. Resistance is also proportional to the length of the cable, so long cables need to be even fatter than short ones. I tend to use car speaker cable which is fat and can insulate the 12V we need.


Of all the possible ways to lose power from a panel, this is the most significant. On a crystalline panel, even the stripe of a rope’s shadow can wipe out a huge amount of the potential output power. Why is this? The individual cells in a crystalline panel are wired in such a way that a cell which is in shadow and not producing will act as a sink for the power produced by the other cells it’s wired to, with the result that virtually no power escapes from the panels as a whole.


You must have a regulator. The job of the regulator is to throw power away. It does this to ensure the power passing on to the batteries or services is not too great for them to handle. Normally it does this by controlling the amount of power passing through it and hold the output voltage at some predetermined value such as the float charge voltage of 13.4V.

The value of the power it passes depends on the current required by the load: the sum of batteries, lights, fridge and so on that are sucking the current from the panels. If the fridge is on, for example, more power will flow through the regulator and it will throw less power away as heat.

Most common, older regulators use a pulse width modulation (PWM) system which is more efficient than simply controlling the output voltage. Maximum power point tracking (MPPT) devices provide more usable power by seeking the panel’s optimum power voltage although they are expensive.

Chief among the suppliers is Victron (avoid the so-called MPPT types from ebay, they are almost certainly not MPPT controllers). And if you have the room it might be better to spend the money on a larger boat solar panel than on an MPPT controller. This is what we’ve done on our canal boat, settling for a PWM type.

To the boat solar panel, the regulator is part of the load – a consumer of power – which is why the entry to the regulator is an appropriate place to measure the voltage and current if you want to see exactly how much is being generated by the panels.

What power do I really get from boat solar panels?

If you do the sums, the unavoidable losses on a new panel operating at 65°C (measured in full summer sunlight in Greece) are going to be in excess of 20% from the temperature increase alone. Our example 100W panel is therefore only putting out 80W, and that’s only for a few hours.

If you really need every scrap of power then you need to find a way to keep the panel cool, and you need to invest in an MPPT regulator. As the panel gets older its performance will drop off even more. All a bit depressing, isn’t it?

I have to say, though, that in our particular installation on Aderyn Glas we regularly get more than 10A from our 200W panels, and our highest recorded value was 170W, which suggests that these loss figures are conservative.

In reality, with a well set-up installation you can expect to get a maximum of 75% of the power you would expect from a continuously insolated panel operating at its rated power.

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This feature appeared in the May 2021 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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