Full disclosure: I am a fan of solar and am saving up to install a PV system on my house here in Anchorage.

That said, I do think a logical discussion regarding the tax credit should take place.  As stated above, the system installed should theoretically pay for itself in 10yrs.  The credit is 30%, meaning without the credit, it would pay for itself in 13yrs-ish.  If, as stated below, the lifespan can reach 40yrs, it seems like this investment doesn't need the tax credit to make sense.  Additionally, if you own the building, it adds value to your property.  A study of residential PV systems in SoCal from '05-'10 found that the home value increased by approx. 95% of the investment (after credit).  So, even without the credit, it's a higher rate of return then redoing your bathroom or kitchen.