I was actually wondering last night (while sitting in my hot tub) if it would be beneficial to fill it from my on demand water heater. I'm using the same number of BTU's regardless of if it comes from the tubs electric heater or from the on-demands gas burner. The on-demand is pretty efficient (not quite as efficient as the hot tub). I'm sure my cost per BTU in natural gas is cheaper than my cost per BTU in electricity.
Almost certainly yes. When I compared this a few years ago I seem to recall an 80% efficient natural gas heater was 1/5 or 1/6th the cost of electric resistive heat.
Let's see, on my latest bill I pay 11.4 cents/kWh for electric, and 72.6 cents/therm for gas. A therm is 29.3 kWh, or 2.48 cents/kWh.
Divide that by your heater efficiency in decimal form to get cost/kWh_usable, so even an 80% efficient heater would run 3.10 cents/kWh_usable, or a mere 27% the cost of electric resistive heat.
Now, 450 gallons heated from 55 to 104 °F requires 54 kWh, so is it worth the $6.16 - $1.34 = $4.82
savings to bother with hooking up a hose to a hot tap instead of your regular outdoor tap? That's for you to decide.
But why more pools in a climate like AZ don't have exchangers off the AC systems in the summer is beyond me. I had a ground source heat pump in IA and it was cheaper than natural gas in the winter and practically free in the summer there... That 15Kgal pond should make a great heat sink in the summer... or even source in the winter...
I've wondered this too. There was a member here who said the Hotspot or similar was no good, you'd never pay it back. Said he worked in finance and was interested in the total lifecycle cost. I said I was too, and asked him to share his numbers for why it wouldn't work, but he never got back to me...
My gut feeling is that yes, it would be tough to pay back. But I think it's also a complicated equation. Since the heat after the upfront cost is effectively free, your pool will always
be hotter than otherwise, so you may swim more than if you had to think ahead and decide if you needed to turn up the heat/debated if turning up the heat was worth the cost.
Around here you would do much better if you can find a Solar PV outfit willing to install a small array for the pool equipment with enough power to run a heat pump. Then you can generate heat as you need it. Solar heating arrays are ok, but I know too many people that put those up on flat roofs only to regret it when it springs a leak and no one notices the water loss for weeks on end. Or, you need to get the roof recoated (white) and you have to pay the contractor extra to lift out the coils (and pray they don’t break something in the process).
Ugh. The engineer in me shudders to think of using PV panels to power a heat pump. Rough numbers you get maybe 20% efficiency in PV, but the heat pump may have a COP of 3-5. I can't immediately locate efficiency numbers for solar pool heaters, but I'd be surprised if they were less than say 80%. So for the same area, roughly the same heat output. But in one case you have expensive PV panels running an expensive heat pump (unless you already had the heat pump), vs just pumping water through some glorified pipes.
I suppose the flat roof could change things. Do a lot of people out there have flat roofs? I don't think I've ever seen a residential house out here with a flat roof. May have a lot to do with snow. In any case on a sloped roof a water leak would be easily and quickly spotted.
I'm idly planning on installing solar heat next spring.