Solar water heating (known as solar thermal) captures the free heat from the sun and uses it to heat up water for use in the home. It’s a simple process:

• panels on your roof absorb heat from the sun – they are known as the collector
• the water in the panels heats up
• this hot water is pumped through a coil in your cylinder
• which transfers the heat to the water in the cylinder

Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. Outside of that estimates vary depending on who you ask.

The Energy Savings Trust says solar panels will ‘provide about a third of your hot water needs’, the Centre for Alternative Technology puts it at about half and a number of solar panel suppliers promise 60 to 70 per cent of annual hot water usage heated by the sun.

The reality will depend on a variety of factors:

• How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.
• The size of your cylinder makes a difference. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.
• If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on.
• If you have an electric shower it won’t use your solar hot water.
• If your dishwasher and / or washing machine are cold-fill they will still have to heat the water using electricity, and won’t use your solar hot water.

Research commissioned by Viridian Solar confirmed the significance of the household’s behaviour on performance. Its panels installed in six housing association properties were monitored by the Building Research Establishment (BRE) over 12 months. Energy savings in heating hot water varied from a lowest level of 26 per cent to a highest of 70 per cent. The average was 50 per cent, which is an average saving of 1,200 kWh of energy per year.

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The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west. As a rule of thumb you need between 1 and 2 m2 of panels per  person.

Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).

Solar panels are compatible with most existing hot water systems, although if you may well need a new cylinder that is tall and thin, with two coils, and ideally big enough to hold two days worth of hot water.  It is more difficult with a combi boiler. If you have a combi boiler it is important to check with the manufacturer that it will accept pre-heated water.  The Centre for Alternative Technology has an information sheet: solar water heating with combi boilers.

If your present system is gravity fed, it will need a control (such as a valve and pump) for the hot water circuit, if the panels are to work effectively in winter when the boiler is running for central heating.

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There are two types of solar thermal panel: flat plate panels and evacuated tubes.

Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.

Instead of a plate, Evacuated Tube collectors have glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum (the air is ‘evacuated’ hence the name), which minimises heat loss.  

Unlike the rest of Europe, the UK tends to favour evacuated tubes over flat plate collectors. This is in line with a  DTI test of eight panels which found that evacuated tubes performed better (clicking this link will download a pdf document).

This is contradicted by research done in Switzerland by the Solartechnik Prufung Forschung. Its research found that the gross efficiency of flat plate collectors is considerably higher than evacuated tube collectors. The average gross efficiency of 120 flat plate collectors it tested was about 70 per cent, compared to an average of 50 per cent for the 42 evacuated tube collectors examined. For more information, Sustainability magazine has done a detailed analysis entitled solar energy myths.

The Swiss research also gave clear indications of where the best and worst performing panels are made. The most effective  flat plate collectors are made in Austria and Germany and the best evacuated tubes in Switzerland and Northern Ireland. Six of the 10 worst performing evacuated tube collectors are made in China. Its tests show that the best performing collectors are more than twice as efficient as the worst ones.

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Domestic solar thermal panels installations are subject to a huge range of prices. Researchers at Oxford University's Environmental Change Institute analysed the prices of systems getting low carbon buildings programme grants between 2006 and 2008. They found prices ranging from £1,000 to £8,000. The cost will vary a good deal from one contractor to another, and will depend on the type and quality of the panels, whether you need scaffolding, and how easy it is to integrate into your existing plumbing system.

As a guide, the researchers published the following benchmarks for solar thermal prices:
Fixed costs: average of nearly £3,000, but varied considerably
Equipment: £500 per MWh/year
Example: a 2MWh/year system should cost an average of £4,000.

Even allowing for different installation costs at different sites, the authors found profit margins highly variable, and no correlation with the generation capacity of the panels.

Evacuated tube panels are generally more expensive than flat plate panels, but you need a smaller surface area.

Panels are available for DIY installation, but buying them this way means that you are not eligible for a grant and have to pay the full rate of VAT (instead of the 5 per cent rate), so check that it’s worth the effort. Some people make their own from old radiators and the like. CAT publishes a book Solar Water Heating: a DIY Guide to help the enterprising.

Heating water accounts for 17 per cent of energy use in the UK and for 20 per cent of domestic carbon emissions. How cost effective your solar thermal system is depends on a wide range of variables and is different for each system. Payback times can vary from five years to 50! To maximise benefit, a system should be capable of producing around 50 to 70 per cent of all water heating that is not associated with space heating.

Solar thermal is eligible for the renewable heat incentive, which is due to come into force on 1 April 2011. Final details won't be confirmed until the government responds to the consultation in autumn 2010, although installations in the interim will be eligible to receive it. To qualify you must use a MCS accredited installer and product.

There are grants programmes for Scotland, Northern Ireland and London. To find out if there are any local grants available in your area you can ring your local energy efficiency advice centre on 0800 512 012 or search the Energy Savings Trust database.

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Solar thermal panels are generally considered ‘permitted development’ in England and Wales as long as they are not installed above the ridge line of your home. However, if you live in a listed building or in a conservation area, restrictions will apply. Up-to-date advice is available on the government’s planning portal.

For those of you living in Wales, from the beginning of September 2009, the Welsh Assembly Government announced new planning rules to encourage householders to install renewable energy equipment. A leaflet has been published to explain the changes - Domestic microgeneration permitted development: A guide for householders.

From the blog:

Solar hot water delivered quicker and cheaper

Six tips to save you from solar panel cowboys

How to avoid bugs in your solar hot water

Avoiding the solar cowboys

Seven steps for servicing your solar thermal system

Calculators shed light on renewable choices

Can I have solar hot water with a combi boiler?

Evacuated tubes: make sure you don't get burnt

Getting the best out of solar panels

Good Energy rewards renewable heat generators

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

How to store heat in a thermal store

Planning for solar panels

Solar thermal for space heating

Storing your solar heat

Support for solar hot water in Warwick

Sources:

Centre for Alternative Technology

Daily Telegraph

Devon Association for Renewable Energy

Energy Saving Trust

National Energy Foundation

Sustainability Journal, Ireland