This paper from the American Economic Review found that consumers use a discount rate in excess of 15% in valuing residential solar power credits, compared to a social-wide discount rate of 3%. The implication is that a government can incent the same amount of solar investment through an upfront credit for as little as half the cost of a per kilowatt-hour ongoing subsidy.
The California Solar Initiative had two different incentive methods, the Performance Based Incentive (PBI) which was paid out over 5 years and the Expected Performance-Based Buydowns (EPBB) paid out upfront. The former was preferred by policy makers but the latter was more popular with homeowners. Now we know the degree of difference in the preference.