Of course their is a formula. What does it cost to heat water? How much can WDRP heat say 200K gallons water for vs the competition.
Same temp change, same time frame. Take into account product cost, upkeep, installation and of course cost to run it. Isn't that the bottom line? The consumers total out of pocket expenses? Isn't that what it's all about? Using less power than we do now to save the planet and our pocketbook?