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.

No matter which manufacturer you ask, they'll all tell you they have the best solar collector. They may even tell you that theirs is the most efficient. There's a good chance that they're right, too – and wrong at the same time.

As you should already know, choosing the most efficient collector for your solar thermal installation is more about your situation, location and climate than it is about just picking the top-rated one off a list.