The BIGGEST problem with clean energy

It seems to be always more efficient to store energy in its "native" form and only convert it to electricity on demand. For example, there is twice as much energy coming from the sun in the form of heat than as light. Using solar arrays to capture the heat, they transfer the energy to sand or molten salt and are easily stored underground, holding their energy for a long time. On a cloudy day when demand is higher, you just fire up your steam or sterling engine running off of that stored heat and you're good to go. The nice part... this system is very scalable (from 1 house to a whole city).