There's an opportunity cost to having an expensive power plant just sitting around doing nothing during the day. Yes you use less fuel, but how much of the cost

There's an opportunity cost to having an expensive power plant just sitting around doing nothing during the day. Yes you use less fuel, but how much of the cost of power is the fuel itself vs infrastructure and other non-marginal costs? So only if the total cost of solar is less than the marginal cost of gas would it make economic sense to replace gas with solar during the day.