For anyone here with a hot tub, who doesn’t want to pay the extortionate electricity costs immediately turns to Home Assistant.
It probably starts out pretty simple; get the climate control so you can manage heating, etc remotely.
Next is to add a bit of scheduling - I used the schedule helper to only heat up at the weekends. The problem with this is twofold.
- To have it ready for a particular time (say 19:00 on a Friday), you’d have to estimate how long it would take to heat up. With the outside temp and the water temp ever changing, I found myself tweaking the schedule constantly.
- As much as I hate to admit it, I am a solitary geek in this very full house. So, if someone else wanted to change the schedule etc. teaching them the HA app was not an option.
So, I wanted a better way with two goals in mind:
- I just want to say when it should be at temperature. Everything else is calculable.
- I wanted the schedule on a shared Google calendar. My family can manage that!
For the eagle eyed, you may see an odd entry - Duration Till Next Heat. My next calendar entry is at 19:00 tonight, and it’s calculated it’ll take 6hrs 48mins to get it up to temp, given the volume of water, water area, power of the heater, outside temp and the target temp. It’ll need to start heating in 25 mins to get there in time.
I can’t take credit for the formula - that’s an AI special! But I had it implemented in a spreadsheet, and once it was working, ported it over to HA, making extensive use of Helpers & Templates.
I’m not sure how to “package” it up for others to use, but if there’s interest, I’d be happy to write some detailed instructions.
But for now, I’m proud as punch with no one local who understands nor cares! 
