Once upon a time, figuring how much insulation a tank-type water heater had was easy. Right there on the label, there was an R rating, just like with wall or ceiling insulation. The higher the number after the "R," the thicker the insulation.

That, apparently has changed, in part. Rheem and its subsidiaries, Ruud/Richmond/GE, have dumped R factor and embraced Energy Factor, or EF. They say it's more precise. But the other makers still include it in their spec sheets, or tell how much insulation there is in inches, one inch being approximately R-8

Our feeling is that EF is ambiguous. EF comprises insulation, tank design, burner design and other factors. It's also true that an electric heater will have a much higher EF than a gas one, yet in some parts of the country, an electric tank will cost you a lot more to operate than a gas one because of utility rate structures.

You can still use EF to compare Rheem heaters, but you should remember to look for insulation figures in the spec sheets of other manufacturers, as those will tell you if the tanks have only one inch, or maybe two or three inches.