what is the average efficiency of flat plate solar collector?

Answer 1

They usually measure efficiency in the ability to convert the incoming light energy into electrical energy. Very expensive panels can be up to around 40% efficient. But newer ones are less than 20% efficient (and that's still good).

It all depends on the model. You can find some values if you go shopping for solar panels. A 10% efficient panel is still a good buy. If you don't have a lot of space, then shoot for greater than 5%.

Answer 2

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