The Compact Solar Engine is the only solar engine that does not take up the top slot in Steve's Carts, using the sides instead. It generates energy at the same speed as the Solar Engine, but it has a much smaller power capacity and is more expensive to craft. A minecart equipped with a solar engine must have a clear view of the sky, so a solar engine on an underground mining cart would not be useful. Leaves will also block the view, so a Wood Cutter cart should have a tiny or regular coal engine as an alternate power source when it is chopping down trees.
Please note this engine has the same generation rate as the standard Solar Engine, and a very small max power capacity.