Solar hot water system: Work during a blackout?

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Solar thermal systems—also called solar hot water systems—utilise the sun's energy to warm water for domestic use in a tank. Solar hot water systems can either have a tank on the roof and collectors on the ground or a tank on the side of the house and collectors on the roof.

Both models utilise a pump to transfer water from the collectors to the tank. To deliver hot water on cloudy days or when more hot water is required than the solar system can produce, most solar hot water systems additionally contain an electric booster.

The electric booster won't function during a blackout, which means hot water cannot be accessible until the electricity is back on. However, the sun's energy could heat the hot water tank once more on a sunny day.

If I have a solar hot water system, will I lose my hot water during a blackout?

If you have an older style solar hot water system (also called solar thermal system), the hot water tank sits on your roof, usually with two collectors and gets heated through the sun. Therefore, during a blackout, there are situations where you might have hot water and situations where you may not. Please let me explain.

A solar thermal system is very similar to when you have a hose lying unrolled across the lawn in your garden, and it is a hot day. As a kid, you might remember when you opened the hose, suddenly the water that came out was very hot.

The reason was that the heat of the sun, shining onto the hose, had transferred into the water.

Solar hot water system on roof
Solar hot water systems can hold a moderate amount of hot water for you during a blackout

A solar hot water system is very similar in that principle. For example, aluminium or copper tubes go zigzag up and down in the solar hot water roof collectors. The water in these tubes heats up as the sun shines on the collector. 

As hot water is lighter than cold water, hot water will travel to the top of the collector and then get collected by the tank positioned on top of the collector. These tanks usually carry 180 litres or 250 litres.

Another solar hot water system model involves water collectors on the roof, with the tank on the side of the house, and a pump is used to get the water from the collectors into the tank. Such systems commonly feature tanks with 250 litres, 315 litres and 400-litre capacities. 

Most solar hot water systems use electricity via an electric booster

So, if it’s a cloudy day, your solar hot water system will not perform at its optimum, and you will not get the entire tank at a maximum hot temperature. For that reason, many of these systems have an electric booster, which kicks in on cloudy days, or when more hot water is needed than what the solar hot water system can produce.

That means the booster will start coming on if the temperature in the hot water tank goes below a certain degree. Then, heat your water using normal electricity, usually around 3200 Watts which can be converted to kilowatts here

During a blackout, let’s say you’ve had a long shower and used all the hot water available in the tank, then after that, obviously being a blackout, you don’t have the booster working.

Then, you would only get cold water for your shower. During a blackout, even if you have a solar thermal system, it can still affect the hot water output.   

The following morning, when the sun rises, you will have hot water again if you’ve got a sunny day from 10 to 11 a.m. But till your electricity is working again, you are only limited to generating the hot water that the sun will heat for you. For more information and alternate systems, refer to Energy.Gov’s article about solar hot water systems.

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