Monday, 12 March 2012

Optimising your solar water heating

The days are getting longer, the sun is getting higher and the sun gave us 9 kWh of hot water yesterday. Solar hot water panels haven't been getting much attention recently which is a shame because hot water must be one of the few luxuries you can have for free - well free marginal cost anyway, once you've got the panels all plumbed in, for a good half of the year. In the last year we have had about 1700 kWh of free hot water.

To put this into context, one hot shower uses typically 1-2 kWh. A bath can be 4 kWh or more. A 200 litre hot water tank heated to 60C from say 15C would store 10.5 kWh of heat.

You should be able to get at least half your hot water from a solar panel, averaged over the year. However, it is possible to configure your heating system so badly that you get almost none.The key point is to ensure that when the sun shines your hot water tank is not already as hot as it can be.


Like many people we use most hot water in the evening for washing up and in the morning for washing ourselves. We have the gas boiler programmed to top up the heat in the tank in the evening if necessary, around 7pm, up to 60C, so there is plenty for the washing up. If this cools a little overnight it is not a problem because you don't need 60C for showers - 40C is fine. When the sun gets around to the panels (which are slightly shaded from the east) the water tank is as cool as it is going to get and we absorb as much heat as we can. The solar panel is allowed to heat the tank right up to 80C. This is safe because we have a mixer valve (called a tempering valve) that ensures that the hot water drawn off the tank is mixed with enough cold so that it won't scald you when you wash your hands under a tap. We have not changed our behaviour at all. The key points are:
  • The boiler does not reheat the water after morning washing - we wait for the sun
  • The boiler only tops the tank up to 60C which is all we need
  • The solar panel is allowed to heat the tank to 80C to store more heat
  • The tempering valve ensures the hot water taps aren't dangerously hot.

The renewable heat incentive (RHI) applies for solar hot water panels and it pays 8.5p/kWh. Unfortunately the RHI is only available at the moment for commercial installations and it requires metering of the heat supplied, which most people don't have. The RHI will eventually be rolled out to domestic customers too but in the meantime, there is a grant you can get towards the installation of solar hot water panels - the Renewable Heat Premium Payment Scheme.

For more information about solar hot water panels in general see the Transition Cambridge Solar Hot Water FAQ.

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