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Originally Posted by Servicetech
There are 29.3KWH in a therm of gas. 29.3 x.11 cents per KWH (do you stay under 600KWH most months?, most people don’t) = $3.22 Gas is 44 cents, you need a COP of 3.22/.44 = 7.3 for the HPWH to make sense. Most tank water heaters get about 78% THERMAL efficiency, which means you need a net COP of 5.85 to make sense.
Odds are the bosch tankless is NOT modulating, which makes temp control difficult. Fine for a cabin or a single person, but it ain’t gonna fly if you have a family.
Sad to say, but the plain boring NG tank is probably your best option. Natural gas is cheap, it doesn't cost THAT much to heat water to begin with...
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Also consider natural gas service charges. It can be high enough to offset the savings. That would be unlikely up north, but pretty likely down in regions with low heating demands.
Flipping the question around, would it make sense to use CHP? The generator would only need to be better than 13.7% efficient to break even on electricity alone, pretty easy to do. Up north, the heat would be put to good use for a large part of the year, meaning it makes even more sense.
Should be a good place to DIY a generator out of a small car engine and an industrial induction motor.