Most households break even on their monthly bills, or low-use households get a slight credit selling electricity back to the power grid. Trust me, the rates you sell power at are nowhere near the rates you pay to buy if you go over your capacity!
When in Sac, a friend put one in. SMUD has some major incentives for homeowners to do so; it only ended up costing her about $5k. However, since the cost was financed, it will end up costing her a lot more than that. She lived alone and was a very low-usage household, and most months made about $5 selling electricity back to SMUD.
It's a nice system -- you generate all or most of your own power while being on the grid for special events (guests, etc.) when you need more power. But you have to be in the house a LONG time to pay for the system.
If you are buying a house on an FHA program, you can usually install the system (and other energy saving features), roll it into your house payment, and not have it affect your LTV ratio.