Introduction to Solar Hot Water

Image: Filomena Scalise /

Solar Hot Water (SHW)

Solar radiation totals 5.4 million exajoules (EJ) per year, of which 30% is reflected back into space leaving 3.8 million EJ for use on earth, over 10,000 times greater than the 230 EJ consumption of fossil and nuclear fuels in 2002 (Boyle).

Solar thermal systems use the sun’s energy to heat water for use in a property for washing and other domestic uses; they do not usually heat the property itself.

In the UK solar water heating can deliver around 50-60% of year round water heating. In the summer months, 90% of the hot water needs of a typical home may be met by these systems. However, this can fall to less than 20% in winter.

The two standard collector panel types are "flat plate" and "evacuated tube". Flat plate systems use a dark plate in an insulated box to transfer energy into the water system. Evacuated tube systems are more expensive and sophisticated, using metal strip collectors in vacuum tubes but have the advantage that smaller panels are needed.

Solar Hot Water Rules of Thumb

1m2 of evacuated tube is required per person

1.5m2 of flat plate is required per person

1m2 = 500-800 kWh saving per year

1m2 = 120-160 kgCO2 saving per year (based upon replacing gas)

1m2 = £20-30 saving per year (on gas at 3.9p per kWh)

1m2 = 50 litres of hot water per day (relates to cylinder size)

Note savings depend upon which fuel the comparison is made against.

Solar Hot Water Example

Typical requirement for luxury dwelling would comprise a 300 litre cylinder served by 6.0m2 (three 2.0m2) solar collector panels. Installed cost estimate for panels, cylinder and pipework is £8,000. This would yield roughly 4,200kWh per annum, save 840kgCO2 per year and provide a RHI payment of around £756 per annum. Payback is 10.5 years.

SHW Design Notes

Ideally solar panels for generating hot water or electricity should be installed at an inclination of 45° on a south facing roof clear of shading. There is little reduction on performance if the inclination is varied between 20° and 50° or the panels face up to 20° either side of south. Installing in a horizontal or vertical plane will add 20-30% to the required absorber area.


Homemicro article on solar electric Solar Electric
Energy Saving Trust - Heat water for your home using energy from the su
Look up solar annual insolation in your area Encraft
Get some fast facts on solar energy

Solar water - last updated 30th January, 2014 by Corny