Earlier quoted context omitted.
It's funny how removed we are from reality. Even if $15/month might be cheaper than a propane bill, it would be far far more expensive against the planet to heat it with gas/coal burned at a factory 100 miles away, converted to electricity, then converted back to heat inside the spa water heater.
I think that an air/water heat pump on the tub, with say a COP of 8, would be less expensive to operate than propane or straight electric. But, I couldn't justify the capital expense. (Small hot tubs are electric.) I like my wood/propane mix for heating, because it's immune to power outages. At some point I'll get too old to cut/split/haul wood, though, and will have to figure something else out. I'm not sure why the…
It can make a lot of sense in urban areas. In addition to removing the methane leaks (with their high GWP), it reduces the need to install/maintain expensive natural gas distribution infrastructure, and also improves indoor air quality by eliminating NO2 and CO production within the house. If the power goes out, you just use a propane grill in the backyard, or a small butane stove.