Virginia and North Carolina are full of older services where a separate meter socket was provided for the water heater.
Cheaper off-peak electricity tariffs have been used in the U.K. for decades for water heating and for space storage heating. For anyone who has missed previous discussions about storage heaters, they are usually individual heating units in each main room which have one or more elements set inside large thermal bricks, so that the bricks can be "charged" with heat at night and release that heat gradually during the day, often controlled by simple thermostatic dampers. (Some large, centralized storage heat units with forced ducted air around the house were also installed in the 1960s/early 1970s.)
One of the original schemes had a completely separate meter to record the cheap rate usage, with a timeclock-controlled contactor which turned on power to a separate distribution panel at night. It was commonly referred to as the "white meter tariff," because the second off-peak meter was in a white casing to identify it instead of the usual brown-black. All 24-hour circuits were still charged at the normal rate on the regular meter.
These days though, the general Economy 7 tariff is the norm. There is still a timeclock and contactor to feed power to a separate panel for storage heating, but just a single meter with dual registers. As well as energizing the contactor at the appropriate time the clock also switches the meter to so that
all usage during the off-peak hours is charged at the lower rate. It's usually midnight to 7 a.m. in winter, 1 a.m. to 8 a.m. in summer, or thereabouts (hence "Economy 7" -- 7 hours cheap rate each night).
We have some more options now which aren't quite so common, such as Economy 10. This gives the usual 7 hours per night cheap rate but also provides 3 hours afternoon boost for heating at an intermediate rate.
Also, in place of the conventional timeclock some systems now have remote radioteleswitch systems which change rates (and close the off-peak contactor) in response to signals from the PoCo.