Maintenance of the power inverter, batteries, wiring, and the panels themselves means that marginal cost is not zero, and as long as we don't know the solution to entropy, will never be "essentially" zero.
All of those things happen the same, and cost the same amount, whether 10 kWh are produced or 11 kWh. The marginal cost of the last kWh is zero for these items.
There is thermal wear on the power management and voltage step-up electronics, but it is small unless design parameters are exceeded in going to 11. So maybe those components would have to be replaced ten milliseconds earlier than otherwise.
There is thermal wear on the power management and voltage step-up electronics, but it is small unless design parameters are exceeded in going to 11. So maybe those components would have to be replaced ten milliseconds earlier than otherwise.