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Originally Posted by Nords
I'm surprised at those efficiency ratings of 0.58 and 0.63. When we bought an electric heater a few years ago we found a 52-gallon high-efficiency model with a rating of 0.92, $200 followed by a $50 rebate from HECO. Maybe it's different for gas...
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Electric resistance units can hit close to 100%. The best you can find in gas units is around .70 and I didnt come across any of those for residential installs. What surprised me is that PG&E's rebate program kicks in at .62 and almost everything you could get at the mass market stores (HD, Lowes, etc) is lower than that.
From what I saw, the key to hopping the last few points are a little item you can add on to an existing water heater that came standard on mine...little plastic or metal ball valves in the cold and hot pipes that inhibit thermal migration from the tank to the pipes when the heater isnt on. I peered into mine and saw them. Without them, the heat from the water in the tank is inducted away to the pipes in the house. Insulating the pipes helps, but doesnt completely solve the loss.
Sears had some add-on ones for existing water heaters.
Article http://www.eere.energy.gov/consumer/.../mytopic=13100
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