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.

Different people like to look at the same investment in different ways. Some people like to consider the return on investment over the entire life of the system (or a part of it), which we look at in this article. Other people prefer to think about how it affects their monthly cash flow. We'll look at the cash flow approach here.

The most obvious problem with flat plate collectors is the relatively large sheet of glass taking up one whole side of the construction – glass that can't be insulated without stopping the sunlight from heating the absorber plate. To solve this problem, evacuated tube collectors were invented.

Much like flat plate collectors, evacuated tube collectors have a solid, durable, insulated frame with one glazed side which contains absorbers. In this case, the absorbers are individual transparent tubes rather than a single, flat plate under a pane of glass.