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.
The thing with solar collectors is that the simple, logical answer isn't always the right one, because everything is so complicated. Take evacuated tube collectors , for example: you'd think that the inclusion of vacuum heat retention would make them the most efficient option... but a simple web-and-tube pool collector can sometimes outdo them for efficiency. Why? Because the web-and-tube option is the right one for a particular situation in a particular place.
That's an important lesson to learn before you go looking for the ideal collector for your solar thermal installation: the best collector is the one that fits your circumstances.
With that in mind, you're going to need some research resources. Thankfully, one of the practical outcomes of the rise in interest in solar power is the Solar Rating and Certification Corporation (SRCC). This independent company takes measurements from tests run by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) and uses them to rate almost all the collectors available on the market.
In practical terms, the SRCC standardises the information so that you can compare different brands fairly. They also certify that the results are accurate, so it's always worth buying equipment that has an SRCC certification. There are two parts to rating solar collectors: their performance and efficiency is one, their durability and reliability is the other. The SRCC takes both into account when certifying.
ASHRAE is online at www.ashrae.org and the SRCC's results are available from www.solar-rating.org (for free).
Out of interest, the efficiency test is often done indoors with bright lights but some times it's performed in real sunlight. In both cases, multiple measurements are taken so that efficiency is measured for clear, cloudy and mildly cloudy days (at 2,000, 1,000 and 1,500 Btu/ft² per day respectively).
The operating temperature is also varied, to ensure that efficiency is measured with different solar fluid temperatures to mimic real installations.
As you read through our guidelines for choosing a collector for your solar thermal installation, remember that it's always about choosing the right one for your situation. The SRCC's resources are there to help you do that, but it's always worth talking to a professional with local experience.