Does solar hot water work on cloudy days?

Fast read

Solar hot water systems are a popular way for Australians to produce hot water while utilising solar energy. Solar thermal panels convert sunlight into heat, which is then stored in a container until needed.

While the system will still work on cloudy days, multiple days of overcast weather can affect its efficiency and production rate.

Most solar hot water systems come with a backup booster, be it electric or gas, in case the temperature drops below a certain point. Then the booster kicks in to heat the water to the correct temperature.

Homeowners should not worry about having hot water on cloudy days as a backup is always available. It is important to install the correct size and model solar hot water system for your household's needs.

Does solar hot water work when there are clouds?

Solar hot water systems utilise solar energy to heat water, which can be utilised in homes for activities such as showers, dishwashing, or home heating. The impact of cloudy weather on solar hot water systems varies based on factors like system type, backup heating availability, and thermal storage quality, among others. Cloudy days don’t necessarily halt the system entirely but generally reduce efficiency, potentially requiring backup heating for desired hot water levels.

 What happens to my solar hot water system when it’s cloudy?

Australia has the highest solar radiation per square metre for any continent. This is why it’s no surprise that most Australians are turning to solar energy as a way to power their homes and also to produce hot water. Solar hot water is a product of solar thermal panels converting sunlight into heat.

To enable this process, installing a solar water heater with collectors and a tank in a sun-rich location heats the water, storing it in the tank until required. The tank can be located on your roof or the side of the house.

Whether for cleaning or housework, it makes sense to utilise the sun’s energy to create your hot water, as hot water can contribute up to 40% to a household’s energy bill.

However, despite the ever-growing increase in solar adoption, the one question always tossed around is whether your solar-powered systems will work when the sun isn’t shining. This question applies to solar PV but also to solar hot water systems.

The short answer is yes, your solar hot water system will still work, but let’s look at the details. 

How will clouds affect my solar hot water system?

If you have ever been sunburnt on an overcast day, you would know that it takes more than just a few clouds for the sun to have no effect. However, clouds can filter out how bright the sun is to the naked eye, as well as how hot it can get. This means that if you have a solar hot water system and it is a cloudy day, the ability of the system to generate hot water is reduced.

You can rest assured that the system is still generating some hot water, just not as much. In saying this, multiple days of cloudy or overcast weather can affect the efficiency and production rate of your solar hot water system in a serious way.

Verdict 1

On cloudy days, the solar hot water system will not work as efficiently due to decreased sunlight. Here’s a detailed technical explanation of what happens:

  1. Reduced Solar heat generating insolation: Solar insolation refers to the amount of solar radiation energy received on a given surface area in a given time. Cloudy weather will block some sunlight, so the solar collectors will receive less solar energy. This decrease in energy reduces the system’s efficiency, and less hot water will be generated.
  2. Collector efficiency: Different types of collectors, like flat-plate collectors and evacuated tube collectors. These collectors capture solar energy and convert it into heat. When sunlight is diffused through clouds, the intensity decreases, making it harder for these collectors to capture and convert energy. Depending on the collector’s efficiency, the effect will vary.
  3. Heat transfer fluid: If you live in an area prone to frost, your solar hot water system will not use water in the collectors but an antifreeze mixture and circulate through the system. The fluid goes through a heat exchanger to transfer the heat to the water, which then goes into the collection tank. On a cloudy day, the reduced energy available means the glycol fluid won’t get as hot, so it won’t transfer as much heat into the water.
  4. A quality storage tank: A well-insulated hot water storage tank allows your system to retain heat collected not only from the current day but also from previous sunny days. This storage tank effectively functions as a heat battery, preserving warmth until required. However, long periods of cloudy weather could eventually mean you run out of water, but there is a solution.

Is there a hot water booster

  1. Hot water booster in the tank: Many solar hot water systems include an electric booster element that acts like a backup heating system. There used to also be gas boosters available, but they are becoming less and less, as gas is a fossil fuel. That uses electricity or gas. If cloudy weather reduces the energy from solar collectors, the backup system can activate to heat water to the desired temperature. While ensuring hot water availability, frequent use of the backup system might result in increased utility costs.
  2. Split systems: Split solar system, which has a tank on the roof, includes a controller that monitors the temperature in the collectors and a pump that circulates the water. If the temperature in the collectors drops below the tank’s set temperature of, for example, 60 degrees Celsius due to clouds, the controller may stop the pump to prevent cold water from circulating into the tank, reducing the overall system efficiency.
  3. Impact on evacuated tube collectors: These types of glass collectors are more efficient in capturing diffuse sunlight compared to flat-plate collectors. So, if you have an evacuated tube system, you might experience a smaller decrease in efficiency on cloudy days.
  4. Cloudy days still give hot water: The impact of cloudy weather can vary depending on the time of year and your specific location. In some regions of Australia, even on cloudy days, there might be enough solar radiation to provide significant hot water heating.

solar hot water system

Your backup will save the day

Fortunately, nearly all solar hot water systems operate with a backup available. To ensure that you enjoy hot water when there are clouds.  If the temperature drops below a certain point, the booster will kick in and heat the water to the correct temperature. Furthermore, certain solar hot water systems offer a gas-booster model. This works similarly to the electric backup. Where the water flows from the solar hot water system to taps and passes through a continuous flow gas heater. 

Will my solar hot water work on cloudy days?

Even if it is a cloudy day, homeowners should still be able to enjoy hot water, although not necessarily as free as they would on a sunny day. However, you should not be worried about whether or not you will have hot water when purchasing one of these systems, as the modern models always come with a backup. Nevertheless, installing the correct size and model system is vital to any residential household. 

Therefore, the answer to the question is that they do operate on overcast days. However, on gloomy days, they might not always use solar energy to heat the water. Since these systems always have a backup, you shouldn’t worry about whether you’ll have hot water when you get one of them. Additionally, your hot water system can still use solar energy to produce some hot water on overcast days, even if the amount might be less.

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