The final crucial piece of information you need to be able to scientifically specify your boat solar panels is how much sunlight you can expect to receive, explains David Berry.


This might sound impossible, but in fact it’s easier than you think, thanks to some handy scientific data published online by the European Joint Research Centre.

How can I find the maximum I’m likely to get out of my boat’s solar panels?

Most people want to know this either to check their installation or to plan the size and number of panels to install. The EU calculator is available online.

I was interested in finding out how much my nominal 200W installation of crystalline panels should deliver in my chosen cruising area of Corfu in the summer months and then seeing if I could work back to estimate the efficiency of the installation taking into account the panels, regulator, wiring and general losses – in other words, ‘from sunshine to battery’.

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The calculator is fairly straightforward to use. First use the map and zoom controls to navigate to the position you are interested in then click ‘off-grid’ and enter the data in the boxes.

Enter also your daily energy requirement in Watt-hours (Wh)then click ‘visualise results’ and have a look at the year’s predictions for your setup. The calculator allows you to download the results as a csv file which you can turn into a graph using Excel or something similar.

The following graphs are the output for Aderyn Glas in the Corfu channel with 200W boat solar panels and 220Wh of battery.

Fig 1: Insolation on a horizontal panel in Corfu in August (European Commission JRC)

Fig 2: Insolation on a two-axis tracking panel in Corfu in August (European Commission JRC)

Out of interest I changed the location to Cardiff and found there was only a shortfall in January and December.

The first obvious difference between horizontal and tracking installations is that the peak power output is less if you can’t tilt the panel. This is because the sun is never exactly overhead in Corfu so they can’t be equal.

Secondly, the shapes of the curves are different, with the tracking installation being much fatter. This is the result of being able to capture energy when the sun is low: try comparing the values at the edges of the curves at, say, 7am.


These semi-flexible solar panels have been cleverly incorporated into a sprayhood roof

Theory and reality

To compare theory with reality I wanted to extract something measurable from this result. One obvious thing would be electrical power. If I could measure voltage and current delivered to the battery at the same point and time then multiply the two I would get power. So this would be something I could use my multimeter to check.

I was aware from the start there’d be a number of assumptions and guesses in this calculation, some made by JRC (they explain these in their help sections) and some by me. Nevertheless, I should be able to get into the right ballpark.

If I calculated the theoretical power then took my highest-ever measured current and voltage readings for August, dividing measured by theory would give me the efficiency value I wanted. I have a tilting mount so I could use the tracking results.

Two-axis tracking mounts allow solar panels to make the most of the available sun

My panels are a 200W. The peak value of the tracking curve was 1,010 W/m2 (from the tabulated results) so the peak output was: 200W x 1,010W/m2 ÷ 1,000W/m2 = 202W. The result for a horizontal panel returns 181W.

Unfortunately for my master plan, catching the greatest current going into the batteries is like trying to photograph a dolphin; by the time you’ve seen it, it’s gone. The current depends on the load more than the source, so maximum currents are likely to occur when the battery charge is low, and that means in the morning – but the sun is not at its peak then.

The best I’ve seen to date is 13.1A at 13.4V, giving me 170W and an overall system performance of 170 ÷ 200 x 100% = 85%, which represents a system loss of 15%. In other words, 85% of the theoretical electrical output from the panel reaches my battery, which is pretty good.

Irradiation estimates by month for a horizontal panel and a two-axis tracking panel (European Commission JRC)


The total area under the curve represents the energy collected in a typical day, and I wanted to quantify the difference between a fixed horizontal panel and a tilting one.

This was fairly simple. I went to the PV estimation tab and filled in the blanks leaving the JRC estimate of system losses at 14% and entering my 0.2 kW solar panels.

I entered zero for the angle to generate a horizontal panel output and also selected two-axis tracking since this was the comparison I wanted to make. The result is pretty graphic: you gain a minimum of 33% more energy if you can follow the sun; much more in winter.

Power produced from my 200W tracking array compared to my 800Wh/day requirement (European Commission JRC)

I calculated my power requirement at 800Wh/day in the summer and plotted how this could be met. The blue bars show the energy produced, and these reach the target value of 800Wh/day throughout the cruising months of the summer. But the panels will need a bit of help from September onwards and before May.


If you’re choosing solar panels, start by working out what space you have available and what your energy requirements are. Then try to find an unshaded location where you can mount rigid crystalline panels, preferably on a tilting mount.

Many boat owners will want to mount them on deck, in which case semi-flexible crystalline panels may seem a sensible choice but can be up to 10 times as expensive as rigid panels. For tracking mounts or gantries, rigid panels will be necessary, so crystalline units will again be the best option.

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