Very much like cooker loads, there are some functions that you can't utilise at the same time, so not all of the pumps will be working at any one time. We have quite a few of these Spa/hot tubs over here, most owners have gone for the heat exchanger system, rather than those crazy KW ratings of the offered heaters (electricity is expensive here). So most here, are heated via the homeowners own central heating system.
The owners here don't normally need the water heated in the summer months. The sun is more than capable of heating the water to more than decent temperatures. Hell my 9m x 4m pool with a 2m depth at the deep end gets upto in excess of 28c in July!!! ...lol!!!