All water heaters (tankless or tank storage) are rated as to their “Universal Energy Factor” (UEF). This is a measure of how energy efficient that particular water heater is. It is also one of the factors to go into whether a given water heater qualifies for Energy Star rating.
For example, Energy Star requirements from the Energy Star official website for gas tankless water heaters are:
| Criteria | Energy Star Requirements |
| Uniform Energy Factor | UEF = 0.87 |
| Maximum Gallons Per Minute | Max GPM = 2.9 over a 67°F rise |
| Warranty | Warranty = 6 years on heat exchanger and = 5 years on parts |
| Safety | ANSI Z21.10.3/CSA 4.3 |
As we see, Energy Star requires not only energy efficiency but also specifies output performance, the minimum warranty, and adherence to certain safety standards.
When you are looking for a tankless water heater, we suggest you choose an Energy Star rated model.