Not to be picky, but surface area is actually an OK benchmark for figuring out how much solar, because it *generally* trends the same as pool volume, within reason, at least for IG pools.

For AG pools that don't have a deep end, I agree that volume makes more sense. But, if you use the area calculations for an AG, you are probably oversizing a bit, and as all of us in the solar world know, bigger is better (if you can afford it and have the space!!).