This ones is 2000L. Lots of thermal mass but also lots of surface area exposed when the lid is open.It's probably 3-4x the water volume than ours however, so more thermal mass.
And I think I fucking jinxed things talking so fondly about the tub. Decided after coming back from a ride this afternoon and having a little chill on that we'd go hop in the hot tub.
Found it with about 12" of water on the bottom. Pump still running dutifully keeping that 12" of water nice and hot, however.
Mine has had two issues with water disappearing. An lighted logo cracked and of course they put it in a 4" hole with the top of it 6" below the water line. As an added bonus, that dumped the water onto of the control board and pumps. Great design. Second was a slow leak right after refilling. Fitting to hose was leaking right at the heater so all the water had to come out again to repair.