Solar cells are the first heat producing unit in the game. One costs $200 and produces 3 heat per tick. It expires after 100 ticks. A solar cell must be placed adjacent to a generator. The generator must be able to convert all the heat it absorbs to power otherwise it explodes, together with the heat-producing component. A solar cell lifetime generates 300 power which is converted to $300 yielding $100 as profit.
After a solar cell expires, it stops producing heat. An expired solar cell is distinct for its weak opacity. Replacing it costs another $200. You need to buy Solar cell manager, under the Research tab, to make this automatic.
Solar cell lifetime and efficiency may be upgraded in the Upgrade tab.