Surely it just becomes an efficiency/heating rate calculation tho. I mean, they install heat-pumps here for Central Heating, and we get down to sub-zero, they still work although there is obviously a crossover point where NG or Propane makes more sense.
Looking at the specs online, and applying my patented napkin-math, it would appear at 80/50/63 (so heating 80 degree water on a 50 degree day) you'd need 5.1kw for 75000 BTU.
For me, thats roughly a dollar for 75000btu.
My 250kBTU NG heater has an 83% efficiency rating. and my cost per therm (delivered) is roughly $0.82 - so 210000/10000 = 2.1 x 0.82 = $1.72 , so 210000 btu for 1.72, or 122k BTU/Dollar. (and going up like 25% soon!)
So bottom line, in the 'shoulder months' you'd be looking at something that was maybe 50-70% more than an NG heater.
It'd certainly be cheaper than Propane, and if you don't have NG supply to the house...what other option do you really have ? Last I looked, Oil-fuel pool heaters out there would run maybe gallon/hour, and that would be for 100,000 btu, at $3/gallon
If he has Solar, he's probably getting his electrons cheaper than I am, so the NG to Electric comparison would narrow some - Sure, it might be slow...but I'd take it over no heater at all.