My $.02. I bought a Tiger River Tondi ( made by Hot Springs) 2-person spa more than 20 years ago. The wife has rheumatoid arthritis and I have osteo. Ours is a 110v unit and like Gil said we run it 24/7. TBH I didn't know folks could heat that volume of water fast enough to make on demand an option. We use ours at least 3 times a week and much more often during the winter months. I have never checked to see how much juice it pulls to determine an annual cost but I don't really care. It is worth every penny for the joint relief we get. I have replaced the heater a couple of times and replaced the circ pump once, cuz I'm handy like that. Otherwise all of the other plumbing is original. The cover is the thing that we have had to replace the most.
I haven't checked the Ph in years. We did it religiously when we first got it. We drain and refill a couple of times, sometimes only once, a year using well water and a charcoal hose-end filter. We clean the filter often and add chlorine whenever we can't smell it. Our water stays crystal clear. We don't have skin issues or algae blooms or whatever the chemicals are supposed to take care of. It works for us, YMMV.