Retired engineer and boat electronics expert David Berry explains how insolation can affect your boat’s solar panels.


The sun is 93 million miles away, give or take. Packets of energy called photons leave it and travel at 186,000 miles per second just to fall on us and keep us supplied with energy.

If you held up a boat solar panel outside our atmosphere, it would be irradiated by broad-spectrum sunlight at 1,370W/m2: this is called insolation.

Because the Earth is tilted on its axis, we see more of the sun in the summer than the winter: in the winter it just about creeps above the horizon, and the stream of photons has to travel through a far longer atmospheric path than when it is overhead in the summer.

Article continues below…

At sunrise and sunset, the atmospheric path length is about 11 times the noon path length. The longer the path length, the more of the sun’s energy is lost by atmospheric absorption and through scattering.

Latitude is the other great influencing factor. If you were sitting at the North or South Pole, the atmospheric path length would be almost constantly 11 times as long as the path length above someone sitting on the equator.

Can an angled boat solar panel catch substantially more power?

At solar noon, the horizontally-mounted panel performance will get as close to the panel on a tracking mount as it will ever get. However, as the sun declines the power drops off rapidly.

The graph (below) shows how the irradiance of a horizontal panel varies as a percentage of the irradiance of a tracking panel. I mounted the panels on Aderyn Glas on a tilting mount so that I could track the sun in elevation but not in azimuth.


This is a compromise: if you cruise a particular latitude and can tilt your panels to the optimum angle for your latitude and track the sun in azimuth from east to west.

You will get much more from your panels. Suitable two-axis mounts are available, but it may be better to spend your money buying bigger panels and settling for a more modest mounting arrangement. We just try to swing Aderyn Glas to track in azimuth – but of course we can’t always do so.

An alternative is to mount your panels at a fixed angle so that they are permanently tilted and do not track the sun at all. Again, this is better than a horizontal panel and is the technique used by solar panel ‘farms’.


The tilting mount on Aderyn Glas allows us to track the sun’s elevation

It’s most useful if you tend to sail in a constant latitude (but only if you can point the panel at the sun in azimuth), since the optimum tilt angle is latitude-dependent.

If you choose to do this, consider whether to fix the tilt at a summer or winter position: you may decide there’s little need for winter energy input so you’ll set the angle for summer cruising.

Why not subscribe today?

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.

Subscribe, or make a gift for someone else, and you’ll always save at least 30% compared to newsstand prices.

See the latest PBO subscription deals on