One of my favorite Seinfeld clips is this one where Newman proclaims that he does not work in the rain. Well, neither do we, but for good reason!

Installing solar panels in the rain has it’s obvious dangers. Slippery roofs, visibility, potential roof leaks, and lightning are all problems that prevent work during rainy conditions. There are, however, somethings we can do inside when it rains. Solar water heaters, for example, need attic plumbing and water heater replacements, so this work is feasible during inclement weather.

Anyway, you probably want to know how solar panels perform during rainy weather. I will break this down into solar thermal and solar electric to answer more accurately.

Solar Thermal

Solar thermal technology relies on transferring solar radiation to a fluid (usually water). Solar pool heaters and solar domestic water heaters work on this principle. When it rains, not only is the sun’s irradiance lower due to clouds, but cold rain water sucks the heat out of the fluid inside the solar panel. Since water has a much larger thermal mass than air, any heat in the fluid is quickly absorbed. In short, solar thermal technology generally ceases any significant performance during rain.

The good news is that most solar thermal systems use differential temperature controllers to sense whether the solar panel can heat the pool or domestic water supply. There are many solar pool heaters that run without controllers, and these systems can actually cool the pool during rainy conditions, but on balance they still heat the pool over time. A controller is the best way to optimize solar pool heating performance, but adds cost and complexity to a system. Virtually all solar domestic water heating systems use a differential temperature controller, and some use a small solar electric panel which effectively senses when it’s not sunny and slows or stops the circulation pump.

Solar Electric

Solar electric panels definitely put our less power when the sun’s irradiance is reduced by cloud cover. Rain, per se, doesn’t have any significant effect on performance, but the amount of irradiance during rain is reduced by cloud cover and refraction from raindrops themselves. Therefore, the power output is reduced. The good news is that solar energy, especially in grid-interactive systems, is reliant on long-term power over time. All that really matters is how much energy you produce in the long run, and short durations (even multiple days) of rainy weather will not matter when considering the big picture.

In fact, solar electric panels perform better when cool, so rain followed by sunny conditions can often cool the ambient temperature and improve solar panel performance. In a perfect world for solar electric panels, it would always be sunny and cool. That’s why solar works so well in Colorado, where they get tons of sun and it is relatively cool (compared to Florida).

Below is a graph of my solar electric system’s output over the last 7 days. You can virtually tell the weather from the graph. February 1st and 3rd were bright, sunny days in Southwest Florida, with hardly a cloud in the sky. February 2 was a rough day for solar, with cloudy conditions and the week’s only rain.

You can tell the weather from a solar energy production graph.

Compare the graph above to the historical weather data below obtained online from Weather Underground. Check out February 2, where the middle of the day had overcast conditions and rain. You can clearly (pun intended) see on the graph above that solar power output was severely hampered during the cloudiest an rainiest conditions.

Keep in mind that residential utility customers are billed for electricity in units of electricity – that’s power over time. The instantaneous solar power output is not the key to solar panel performance. You have to look at long-run energy production. As a result, solar panels work just fine in the rainy Sunshine State.