This really depends on what type of water heater you've installed.
If you've installed solar water heating equipment, you'd qualify for the Residential Clean Energy Credit in Part I of Form 5695, which is non-refundable, but can be carried over if not used during the tax year.
However, if you've installed an electric heat pump water heater -- which it sounds like you may have -- this would be entered as an Energy Efficient Home Improvement Credit, which is claimed on Part II of the form. The credit would be limited to $300, and can't be carried over.
Please see the IRS' Instructions for Form 5695 for more information.
