flat-plate solar collector
What is the average efficiency of solar flat plate collector?
In general, efficiency is measured in the ability to turn next light energy into electrical energy. Very expensive panels can be up to 40% efficiency. But the newer ones are less than 20% efficiency (and still is good). It depends on the model. You can find some value if you go to buy solar panels. A panel of 10% efficiency is still a good purchase. If you do not have much space, then shoot for more than 5%.