How big are we talking here, really? Or should insulation be considered too?
I have two tanks in my house that are heated by different elements of the (fossil fuel based, unfortunately) heating system - the stove and the furnace, which used to burn coal, but now we're doing wood and briquettes.
Either one of those can be heated to near boiling temperatures in about an hour, carry about 100-150 liters of hot water, but sadly will cool down over less than 24 hours so if it's freezing, but not "fuck my balls are going to fall off" freezing, I have to use the electric boiler at great expense. But both of those are bare metal tanks, no insulation at all AFAIK.
PS: House was built by grandparents, I'm too broke to put 10-30 grand on either an air to water or ground to water heat pump right now, but if anyone's interested, I can tell horror stories about the current heating system and house altogether. Soviet era design choices and modern prices for literally everything make for an uh... interesting combination. When it hit -28C (around -18F), I was up all night fueling the furnace, literally feeling myself burn through money.