When it comes to your local climate, there are some factors that can seriously affect your choice of collector type. The worst non-catastrophic climatic event for solar thermal installations is, without any doubt, snow. It's the worst because, unlike rain or total cloud cover, it can physically pile up on top of the collector and block out the sun's light.
If you live somewhere that gets snow regularly, a flat plate collector is often the best bet: the glazing on the front will lose gathered heat to melt any snow that falls on it, which reduces efficiency but keeps the aperture clear.
Conversely, an evacuated tube setup can retain its heat and efficiency at the cost of allowing snow to pile up on the outsides of the tubes and block the sun. Even worse, the snow can pile up between the tubes and reduce the temperature even further, allowing more snow to accumulate. There are some locations where an evacuated tube collector will become snow-bound early in the cold season and stay that way until the thaw, effectively making it useless for extended periods. This is why it's so important to mount evacuated tube collectors at a significant angle, to encourage snow to slew off.
It's also important to consider the absorber plate coating: if you live in an area which stays quite mild or gets very hot all year round, the coating can be much lower efficiency than if you're living somewhere cloudy and cool. Choose a coating that suits your climate.
No matter what coating you decide to use, you'll also need to take the total surface area of the absorber plate into account. Manufacturers usually provide three measurements: gross collector area, absorber area and net aperture area. The easiest is the first: the gross collector area is the overall size of the collector construction, including the box and all its contents – its footprint.
The absorber area is the surface area of the absorber plate. In a flat plate collector, that's only slightly smaller than the gross area as the plate reaches almost to the edges of the frame. In an evacuated tube setup, the absorber area is much smaller as each plate is inside a layer or two of glass and separated from the other tubes by a small amount of space. Flat plates are, on average, 25% smaller for the same amount of absorber area (in size, not taking efficiency into account).
The aperture area is the amount of surface through which the sun's radiation will pass and, obviously, needs to be as high a percentage of the overall frame and plate sizes as possible.
It's important to note that the amount of energy falling on each square foot of the Earth as solar radiation where you live is absolutely constant: you cannot change it or influence it by using different collector types. Consequently, you'll need to calculate your best option by taking the three collector areas and efficiencies into account, then relate them to the amount of space you have, the type of sunlight and your