(Electric) heat pump.
You got it.
Well, it fluctuates just like your AC bill but in reverse. When the air/water is warm it doesn't need to do much and it's very efficient and cheap. When the air/water is cool, it bogs down like an AC in August and the bill spikes.
The gas heater has 3X the BTUs but always sucks the same amount. It's nice when you need the BTUs extending the season, but stinks when it's expensive mid season.
Are you looking to extend the season or just warm the pool during your existing season ?