Your best bet, just from a fiscal point of view, is probably in solar water heating.
As for the photovoltaics (electricity-producing solar cell panels), the state-of-the-art, laboratory test-result efficiency rating is now up to 36.9%. But for commercially available solar panels, think more along the lines of 16-17%, or even less.
One thing you’ll have to look into with your prospective panels is how big an efficiency drop-off there is with 1) temperature and 2) obliqueness of the angle of the solar rays. Older-technology (and probably the newest, too) panels lost some of their efficiency when they got very hot.
As for the angle issue, some solar arrays are set up to move throughout the day, to track the course of the sun. That would involve greater start-up costs, but if the efficiency gained was substantial enough, presumably it could be worth it.
Another thing to consider: not all solar panels are made to be installed on top of and apart from the roof; at least one [Australian] manufacturer makes them to be integrally incorporated with your roof, embedded right in, so that the panels don’t even stick out. Presumably, if you go with that option, you would be buying many arrays and getting your roof done at the same time.
There are also a few durability issues to consider: some panels are more cracking/breaking-resistant; some may promise longer usable lifespan; some may offer the option of replacement of a broken part…
Check into your local zoning and tax/rebate situation. FWIW, 35 states offer some photovoltaic incentive – be it by rebates, tax write-offs, or property tax reductions. By way of example, Florida’s photovoltaic rebate program works like this: you’d have to have your panels installed by the properly state-certified workers, with a meter that reads how much juice the panels generate. Then, for I don’t know how long a period of time, you’d have to report your readings to the state every month, to remain in compliance with the program. The rebate was substantial, amounting to roughly about a fifth of the cost of the panels.
U.S. Govt. Dept. of Energy website offering basic info
Tilt angle info; much other useful technical info on this site.
An economic perspective.
Personally, I’m really enthusiastic about the long-term potential for solar tech to help reduce our consumption of coal and petroleum, reducing the production of greenhouse gases, etc. etc. But unless you’re rich, your best bet is probably in: conservation and high-efficiency upgrades (compact fluorescent lightbulbs, energy-efficient appliances & electronics, double- or triple-paned glazed windows, more/improved home insulation, and roofing/home exterior/landscaping choices. As for solar power, solar water heating is probably the best logical choice, with the photovoltaics running a distant second.