Energy Production

How much hot water will I get?

Solar thermal panels should provide 90% of your hot water from April/May to September/October, and make a 30% contribution in the remainder of the year. Annually that works out to be 60% – 80% of your hot water.

How much you actually benefit depends on

  • How well you use your solar gain i.e. having baths / showers in the evening rather than the morning.

  • Insulation of the cylinder and the pipes carrying hot water. It is all about stopping heat waste and storing heat from rainy cloudy days.

  • Storage; it’s all about the size of your cylinder – the bigger the better. Most boiler led cylinders are sized only hold sufficient water for a day, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater, whereas a large cylinder will store water for two or three days.

  • How you use existing boiler. Your solar control panel should allow you to programme the hot water and central heating separately, thus you will get the maximum benefit from the solar panels when the heating is turned on by only boosting the hot water once the sun has gone down.

  • Reduce the temperature you need. Most cylinders have a “blender valve” to blend in cold water with your hot water so that you do not burn yourself. If you reduce the temperature the system will not call for back up from your main boiler so often. However, it is important to make sure your cylinder reaches more than 60 degrees centigrade at least once a week to avoid risk of Legionella.

Choose your system supplier carefully

The Energy Savings Trust identified a huge range of performance, with the best system producing 98 per cent of the household’s hot water, and the worst just 9 per cent. The median across all systems was 39 per cent.

The following graph illustrates the annual energy gain.

Download Graph

Too many Panels

A system should be designed to provide 100% of hot water during the summer and up to 30% in the winter. Adding extra panels to increase heat in winter is not beneficial and is not cost effective as during the summer months the excess heat needs to “dumped” and as the system will be oversized it will run below optimal efficiency for the majority of the year.

An oversized system will have a high upfront cost and will not be cost effective to run.