"Donna Ohl, Grady Volunteer Coordinator" wrote:

Good point ... dead capacity vs active usage!

Darn. I wish I understood this EF thing better, especially given two
identical situations where the *only* difference is the CAPACITY.

Based on what you implied, if I inferred correctly, if the USAGE was
exactly the same for two water heaters with the same EF, then the costs to
operate a 50-gallon water heater would be EXACTLY the same as the costs to
operate a 100-gallon water heater (if the Efficiency Factor were the same
for both).

Did I understand the math (and your point) correctly?

Not exactly. There are two types of heat use/loss in a water heater: One is the
heat used to heat the water you are actively using. The other is to reheat the
water that's sitting in the tank all day when you aren't using it.

Both tanks will use the same amount of energy to heat the water you are using
directly. If both tanks have the same efficiency and the same insulation, the
smaller tank will lose less energy to the outside air and thus be slightly less
expensive to operate over the course of a year.

The actual difference in cost is probably not that much. Look at the estimated
annual cost of the two heaters on the yellow energy tag. They normalize for all
that. If one say $180 and the other says $200, that's a rough idea of the
difference in operating costs.