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How much of my hot water can I get from the sun?

Posted by Gabriel Wondrausch on 13 December 2008 at 9:01 am

Have you ever wondered how much energy falls on your roof each year? Well probably not, but it is really quite interesting.

The average UK house with a south facing roof of 30m² will be exposed to around 30,000KwH of the sun’s radiation every year. When you compare that to the amount of energy that is used to heat the hot water for a 3-4 bed house (approximately 3,000-5,000KwH per annum) we have far more energy than we need.

While it is technically possible to use this energy to heat all the hot water you need, it’s not that straightforward. This energy is not constant. Most of it falls in the summer months. So, while only a small collector (panel) is needed to harvest energy in the summer, a much bigger collector is needed to gain the same amount of energy in winter.

Simply installing a bigger collector isn’t the answer. If the system is big enough to meet all your hot water needs in winter, it will be too big for summer and will heat all the water needed early in the day. Once the system has produced all the hot water required, any additional radiation falling on the panels is not used, making the system inefficient.

You don’t want to pay for collector area that is not being used! In the summer, an oversized system also leads to stagnation within the collector. This is the term used when the temperature in the collector exceeds 140C causing the fluid to turn into steam. This puts added stresses and strains on the collector and components which can lead to damage or a shorter life for the system.

When designing a system we normally aim for a solar coverage of around 50-70 per cent. This enables the system to provide most of the annual hot water requirements while still being cost effective and efficient.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one


AlternativeComment left on: 15 June 2010 at 2:02 pm

I am going to install 30 EV tubes on my house, but in summer I will rotate maybe ten of them, so that I will not have excess heat produced. In the winter I will turn back all the tubes to face the sun, in order to get the maximum energy harvested.

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Martin Crundall

Martin CrundallComment left on: 22 February 2010 at 8:54 pm

Is it not possible or useful to over-specify solar thermal panels and to shade out part of them in summer so as to allow for greater production in the winter?

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