Marginal cost = cost of producing one more unit of something (energy in that case) once your capital has been spent. It just says that if you have a solar pane

Marginal cost = cost of producing one more unit of something (energy in that case) once your capital has been spent. It just says that if you have a solar panel, you don't need to spend additional money to make it produce electricity. Unlike, say, a diesel generator. It is purely capital spending, zero marginal cost. Another way of counting would be to say that _not_ producing costs you money as you are losing capital over the panels' lifetime.