I'm in the process of a deep energy assessment for a Gainesville, FL couple. They have 18 cent kwh and $1.35 therm NG. They have a conventional gas water heater feeding kitchen, laundry and mostly unused 2nd bath. They have an electric water heater (tank type) in the attic serving the master suite. I suspected, but now am quite certain (owing to a recent post by Dana1) that the EF of the gas heater is well below the standard 0.60 owing to very low demand.

My thought was to suggest that the electric heater CW inlet be tee-connected to the gas heater's HWoutlet. That would give the gas heater more to do, raising its EF, and reduce the load on the electric water heater to little more than standby. NG btus work out to 1/4 the cost of electric btus given their utility situation. The incremental btu cost of gas fired hot water would seem to be able to be calculated at combustion efficiency rather than EF since the gas heater is already in place and busily losing btu up its stack.

There would be a loss associated with the connecting water line - basically the heat content of its water volume would be lost before and after each draw...probably four times per day...not much.

Thoughts, anyone?

The other option would be to get rid of the electric and put in a circulation system, preferable convection. The only reason to keep two tanks might be capacity, big bathtub or small refillable hot tub? But even going with what you suggest, feeding the nat gas in to the electric will save a bunch and you could likely turn the electric down to 110 or so to further reduce it's runtime and get the benefit of almost instant hot water and lots of it