Would it surprise you to know that the price you pay for fossil fuels could be as little as 20% of their real cost? That's because they're often subsidised by the government. Cynics would say that this is to keep prices low and ensure that business keeps booming for the people who control the resources and, in many ways, history shows they're right.

Many people who consider installing renewable energy systems in their homes are put off by the relatively high investment. Why would you choose a solar thermal system that could cost thousands of dollars when you can buy an electric water heater for a couple hundred?

If you look at the cost of installing an electric heater and compare it to a solar thermal system, you may wonder why you would bother with the latter. A solar water heater can cost more than five times the price of an electric setup, though rebates and government schemes can reduce that by almost half. However, the initial installation costs don't take your annual savings into account... and that's where solar heaters come into their own.

A solar thermal space heating installation with storage is one of the most complicated systems to size. There are so many variables: storage size, collector array size, expansion tank size, piping size, heat exchanger size and heat load requirements. There are two ways of working out how big everything needs to be: the complex mathematical way or by rules of thumb.