A system that heats water in tank for bath by heat pump at idle time is popular in Japan since mid 2000s (named "Eco Cute"). It expects unused power is found in midnight because of nuclear plants keeps generating power all time. It looks works well until 2011 earthquake. After that, not much nuclear plants working so the assumption isn't very good. Modern systems has connected to HEMS to heat water when power is generated by solar panel. AFAIK currently not connected with smart power grid.