Peak power needs
Energy use is highest in the summer in exactly those places where you get the most sun. Assuming that a solar panel system has to completely replace line power for the average home is a mistake. Ideally, it would supplement line power, selling power back into the lines on hot sunny days, and drawing power from normal sources at night and on cloudy days.
A LOT of power plant capacity is built to deal with peak needs. If solar panels kick in across Arizona on a 100 degree day, it's going to make a big difference.
Every watt that comes from the sun is a watt we don't need from a power plant.
We've discovered that the hybrid car is a more practical solution right now than the pure electric. Same for home solar power.