Water. Cost per joule is really good so long as you have room for the large insulated tank. You can use a heat pump to bring the water up to 180 degF, a fail s

Water. Cost per joule is really good so long as you have room for the large insulated tank. You can use a heat pump to bring the water up to 180 degF, a fail safe mixing valve to use it to produce 120deg water for the baths and taps, and a heat exchanger for radiant heating loops. It's much more cost effective than batteries to the extent that your energy use is driven by heating demands. If the tank is inside the building envelope the lost heat from imperfect insulation still heats the space too.