[Greenbuilding] is it ever sensible to use PV to heat water?

[Reuben Deumling]

My instinct says 'rarely.' but in the following case I wonder:

two person household; very frugal with hot water usage; already flip
breaker on electric tank water heater off most of the time

planning on adding a PV system that will produce more than they use in the
summer (minimum system size requirements for grid connected subsidies....
you know).

Neither solar PV or solar thermal contribute much of anything in the winter
here (cloudy PNW)

Are there rules of thumb?

Thanks.
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