Department of Energy Distributed Energy Program Solar Hot Water

Energy Star guide to High Efficiency Water Heaters

Department of Energy Guide to Solar Hot Water 

National Renewable Energy Laboratory Consumer Guide Heat Your Water with the Sun
Solar Rating and Certification Corporation 

Department of Energy Consumer's Guide Estimating a Solar Water Heater System's Cost
DOE Consumer Guide Drain Water Heat Recovery

Gama Association of Appliance & Equipment Manufacturers website to find water heaters by EF and tank size.