Through 2016 the IRS will allow 30% of the cost with no cap on the dollar amount. This applies to Solar Water heaters and Solar Panels. However, I believe if you have already taken a tax credit for prior installation of energy efficient products, you cannot qualify a second time. I may be wrong on that one, so don't quote me. In general though, solar conversion are extremely expensive, even with the tax credit. And the return on your investment is simply not worth it in most regions of the country. Whereas the payback on things like a standard energy efficient gas boiler or new windows will pay back much faster.