Found this info at the following site:

http://www.millsco.com/Heatsiphon.htm

THE LOWEST OPERATING COST OF ALL POOL & SPA HEATERS.
Using 220v electricity, based on US Dept. of Labor Average US City fuel prices, HEAT SIPHONŽ will cost you:
50% LESS to operate than NATURAL GAS POOL HEATERS!
80% LESS to operate than PROPANE (LP) GAS POOL HEATERS!
75% LESS to operate than ELECTRIC RESISTANCE POOL HEATERS!

Also some very good info on how a heatpump works at the bottom of that page.