stretch wrote:

Electric water heater efficiency is 3.413 BTU/Watt.

Well no, it's just 100%.

That conversion factor does not change with age of the heater.

And that conversion factor is 3.413 Btu _per hour_ per watt.

If the power is going in the heat ends up in the water.
It has nowhere else to go!

Then again, as crud builds up, recovery time lengthens.

Nick