What is the Energy Factor Rating?

Here's one definition: http://www.businessdictionary.com/definition/energy-factor-EF.html

(One of the top hits from Googling "water heater energy factor")

The idea makes sense, but my hunch would be that testing protocols are likely to be somewhat arbitrary and imprecise.

Energy Factor (EF) is the ratio of useful energy output from a water heater to the total amount of energy delivered to the water heater. The higher the EF is, the more efficient the water heater.

This comment is plagarized from this site, the FAQ page of Energy Star.

Thus, an EF of 2.XX would not be possible. I guess it is that pesky rule about not getting more out than you put in.

Sort of makes tank water heaters look very inefficient.

I do not think I have ever seen an EF ratio applied to either a tank-less or instantaneous water heater. Have you?

One thing about instantaneous heaters: They need a lot of energy real time. So I wonder how often existing existing gas plumbing modified.

Add on the increased complexity, and there arises a question in my mind about the value of converting to an instantaneous whole-house water heater.

The standby losses of a water heater with a tank can be addressed with more insulations, aka., aftermarket water heater jackets.

Temperature loss in a new and well insulated tank are minimal - if you have an isolation valve at the tank outlet and a temperature measuring device in the tank you can check it for yourself.

There is far more loss between the tank and the consumer.