Solar panels should be placed on the top of machines or wires/cables with a view of the sky. During the day when it is not raining, they will generate their stated power level (1V, 8V, 32V, etc). At night or when it is raining, they will generate 1/8 of their stated power level, except for the 1V panel, which will produce zero.
Solar panel covers will be become 'dirty' over time, very slowly reducing power output. The covers can be cleaned by left-clicking on the cover with an empty hand, with a bucket of water in the inventory. The water in the bucket will be consumed, and you should get a message saying the panel has been cleaned.