I can only speak to the electrical side of things. Most above ground hot tubs are electrically heated. I don’t think I’ve ever seen a gas one (I didn’t even know that was a thing until this thread. Still not quite sure I believe it and not going to do any research).
Most hot tubs require a 220 volt 50 (some require 60) amp circuit. It requires GFCI protection and a disconnect means within sight AND within 25 feet of the hot tub. The most common way to do that is to bring the circuit out of whatever power source you have the easiest access to and then put the GFCI protection at the disconnect spot by the hot tub. A 2 pole 50 amp GFCI breaker costs > $100 for just the breaker, if you can find them. Ironically, Home Depot and Lowes sell spa disconnects with that breaker already installed in the enclosure for $99. Again, if you can find them. The last two I did, I couldn’t find those disconnects in stock anywhere in the three metro counties and had to buy one from my normal supplier at over twice the cost. That is all based on the 50 amp size. Good luck if it’s 60 amps. Price easily doubles. But I’ve only ever done one of those.
For comparison, electric dryers use a 30 amp circuit. An electric oven is on a 40 amp circuit. That doesn’t mean the hot tub is using all that power, all the time. But just like your gas water heater has to monitor the water temp and ignite to keep the water temp accurate, the hot tub heater would have to run to keep the temp up. Then add the pumps and the jets and the lights and that’s when the power usage really goes up.
If the gas thing is true, you’d have to consider that the time it would be used most is when your gas is being most to heat your house, in the winter. So you’d be adding more cost to the expensive part of your gas bill. It would probably take a bit of research to see if your house has a big enough gas line to be able to add that extra BTU load.
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