Solar Hot Water Panels vs The Rain: Who Will Win?


On one side, I have my Solar Hot Water Panel. On the other, rain. Will there be enough hot water for me to take a shower? If you’re considering using solar water heaters then you’ve probably had this thought. I did too, so I put it to the test.

Solar Hot Water Heater temperature output drops considerably when it starts raining.  The combination of the cooler weather and cold rain cools the panels down to the point where the water temperature can drop by as much as 30 degrees, possibly more in colder climates. 

But this does not mean solar hot water heaters can’t save you tons of money, even if you live in a wet climate.  Let’s dive into that a bit more and explore how we can boost the effectiveness of a Solar Hot Water Heater during the rainy season.

How My Solar Hot Water System Performed In The Rain

I have a 2 panel, 40 gallon capacity Solar Hot Water System on my Off Grid Home. My home is located in Hawai’i, where we get over 120″ of rain a year. That is a lot of rain. I wanted to know how well they actually worked in the rain, so I ran a few tests.

TimeAir TempWater TempWeather Conditions
10:00am79 degrees101 degreesFull Sun
11:00am76 degrees97 degreesJust Started Raining
11:45am75 degrees89 degreesConstant Rain for ½ hr
1:00pm71 degrees73 degreesConstant rain for 2 hrs

As you can see, the water temperature the solar hot water panels was able to produce dropped dramatically. That is why we employ back up measures to ensure that we have hot water even when it is raining.

Solar hot water heaters use the sun’s thermal energy to heat water. When it is raining, the solar hot water panels are only able to heat water up to the point of the thermal temperature around it.

I did take some readings on what happens when the weather turns from rainy to sunny. These numbers might give us some hope.

TimeAir TempWater TempWeather Conditions
11:00am69 degrees68 degreesRaining all Morning
12:00am70 degrees69 degreesPartly Cloudy 
1:00pm75 degrees78 degreesFull Sun
2:00pm82 degrees96 degreesFull Sun

Here the numbers show is that even when it has been raining all morning, a few short hours of sunshine can heat your water up quick! It can be possible to have hot water on rainy days…technically.

Will A Solar Hot Water Heater Work On A Cloudy Day?

Generally, solar water heaters work on cloudy days, because the sun’s energy is able to penetrate through the clouds.

Solar water heaters are typically advertised to carry approximately 80% of the domestic load of hot water.

Think of all the requirements of hot water in a domestic environment: laundry, washing dishes, cooking and bathing. Some households are able to cover all daily hot water needs with solar water heaters.

On a cloudy day, the percentage of effectiveness of a solar water heater may decrease, from, say, 80% to 60%, depending on how much of the sun’s energy is reaching the solar panels. The amount of hot water available may still be sufficient for domestic use, depending on your individual demand for hot water.

How Long Does Water Stay Hot in a Solar Water Heater?

This depends on the type of solar water heater used. An active-circulation system uses a small amount of electricity to operate a pump and store water in a tank, and can keep water hot for a longer period of time, than say a passive system. 

A passive system uses no pump or electricity and depends on the basic process of convection for heating water. In the passive heating system, insulated storage tanks are able to keep water hot for a period of 24 hours.

Again, the time may vary depending on your type of tank. Please refer to the manual or distributor for a more exact estimation.

3 Ways To Backup Your Solar Hot Water Heater

  1. Hot Water On Demand – Hot Water On Demand instantly heats up water as it travels through a pipe. This option may be found on special shower-heads to provide hot water for bathing, or under a kitchen sink for cooking or washing purposes.

    It is a low-cost option, but usually requires electricity to operate and does not store hot water for later use.  Off-the-grid users may consider a gas-powered Hot Water On Demand system, instead of electric.
  2. Conventional Water Heater Tank – If you already have a conventional water heater tank, consider keeping it as a back-up option. An electric water heater tank stores hot water for days (sometimes even weeks!), and can save you in the event that your solar water heater is not working optimally due to subsequent rainy days.

    If the rainy season in your location is predictable, consider disconnecting or switching off your hot water tank during the dry, sunny seasons and only utilize your conventional water heater a few days before the start of the rainy season.
  1. Heat Water With A Cookstove – Rainy season doesn’t last forever. If you want a temporary solution for a temporary challenge, consider heating water the old-fashioned way – in a pot on the stove!

    Simply fill a large pot with water and heat to your desired temperature, or heat to boiling and cool down with room-temperature water in a separate bucket or basin.

    This is generally appropriate for washing dishes or taking a bucket-bath, not for filling an entire bathtub or taking a lengthy shower.

Solar Water Heaters are an excellent, environmentally-friendly choice. They work in all climates and can provide hot water even on cloudy days. If you live in a climate with a rainy season, don’t rule-out solar water heaters – just consider a back-up option for any temporary limitations on hot water.

Sean Jennings

Sean has been living simply Off-Grid in Hawai'i for over 18 years. He lives debt free on Hawai'i Island with his family and over 40 chickens. When he's not tinkering around the homestead, he's off exploring the shorelines for fish & surf.

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