I take it that was an indoor gas water heater? Which used your indoor cold air? So it's analogous to a decorative fireplace losing good air up the chimney.
The coefficient of power is like 5 or something, and, since it's a bit more expensive than a contractor-grade water heater, I think the manufacturer also spent an extra $10 on better internal insulation or something. It uses much less than 1/5th the energy I'd expected, even with the "turn yourself down at night" feature disabled.