Even in areas where the peak coincides with a peak in sunlight that helps very little since the reliability of sunshine is much lower than the required reliability for power, so you still cannot close power plants and need to keep the fixed costs. You can optimize your systems for a lower average output in those areas, especially in areas with lots of air conditioning where the peak load is expected to drop with lower sunshine, but we are talking percentage points, nothing drastic. Just look at the cost structure of an energy company, there is lots of staff driving around in vans, etc, the cost of fuel is only a small part.