Will Solar Hot Water work when its 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. For this to occur, a solar water heater, consisting of collectors and a tank, is installed in a location that receives high amounts of sunlight. After the water is heated, it is stored in the tank until needed. The tank can be located on your roof or the side of the house.

The one where the tank is separated from the collectors is called a split system.  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: If your system has a well-insulated hot water storage tank, it may still get hot water from the energy collected on the day and from the previous sunny days. The storage tank acts like a battery for heat, holding onto it until it’s needed. 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 the solar collectors are not providing enough energy due to cloudy weather, this backup system can kick in to ensure that the water is heated to the desired temperature. This will ensure that hot water is still available but may lead to higher utility costs if the backup system is used frequently.
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.

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. This heater has temperature settings that identify if the water needs to be heated with gas or if it is at the ideal temperature. 

So should I be worried?

As you can see, a solar hot water system is considered a smart and cost-efficient way to make hot water for your home. 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. Because 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.