The basic problems with solar energy are dependability (must you be able have hot water on a cloudy day?), energy storage (which is expensive, but deals with the first problem), and wide-area distribution (whech can deal with the first two problems, but requires a new,massive and expensive infrastructure.
Direct solar heating (hot water, etc.) works well in terms of efficient enegy transfer, but presents problems in reaching sufficiently high temperatures (it takes a lot of energy to raise water temperatures from 40F to 75F, but that isn't much use when you're trying to take a hot shower.
Back-up energy supplies can solve all these problems, but at a cost. If you're in, say Vermont, a solar water heater needs an electric bck-up water heater (gas back-up heaters have excessively high storage losses)and, if you were originally using natural gas to heat your water, the combination of solar heating and electric back-up may well cost you more to runthan your old system. If you live in Texas, on the other hand...