Even if they only provided 10% of your electriciy, if everyone had them on their roofs it would all add up. That might mean 1-2 less nuclear power plants having to be built, which would be a good thing.
Having said that, the sums are quite clear. Solar power - useful. Wind, wave, geothermal...all good and helpfull, but if we are to maintain civilisation as it now stands, there is no real alternative other than nuclear. Everyone “in the know” knows this (even the “greens”), but no-one wants to take the decision to build because it will be so controversial. Moral cowardice really.
With the current technology, it would be an overall net loss of energy, though.
I agree that nuclear power is the future and we'll eventually drag the greenies to it kicking and screaming because there is no real, viable alternative unless somebody gets cold fusion to work or unless there is an order of magnitude decrease in the cost of production of PV's and an order of magnitude increase in efficiency of PV's, not to mention energy storage mediums.
To me, in regard to solar, what makes more sense in certain areas is solar heating of water and living space. Using a hydronic radiant floor heat system coupled to a mass tank and solar collectors, most houses could easily be heated, even during cloudy periods. I designed and installed an LP fired hydronic radiant floor heat system in my house and in my hangar. I'm currently investigating putting collectors on the roof of my hangar to ease the costs of heating a 6000 sq ft building with 14' ceilings. So far, the cost/benefit analysis I've done looks very promising just using the floor slab as the storage mass. Radiant floor heat is the nicest heat there is, too. It's like the Sun shinning out of the floor. Toasty toes!
Passive solar is another alternative. I designed my house with a lot of south facing glass that is appropriately shaded in the summer months by a roof overhang that designed to shade the glass in the hot months when the sun is high in the sky to ease the sun load on the house during hot weather. In the winter when the sun is lower, the overhang does not come into play. In winter, here at 2300' msl, where temps can easily dip below zero in the winter (currently 14.4F), my house will stay 70F on a bright, sunny winter day with no other heat source on. This is just with south facing glass. Include a thermal mass in the form of a "trombe" wall and it really starts to work well, even on cloudy days. Cheap to build, too. If everybody would look at passive solar designs we could save one hell of a lot of energy.