Every tankless water heater gets a score from 0 to 100% built from four factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Heaters are grouped by fuel, gas and electric, and each factor has its own scale for each group.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per 1,000 BTU/h (gas) or per kW (electric) | 35% |
| Heating power | gas input, BTU/h; electric, kW | 25% |
| Efficiency | Uniform Energy Factor | 15% |
| Warranty | heat exchanger warranty | 25% |
Where the numbers come from
Gas input, electric power, flow, UEF, venting, warranty and the rest come from the maker’s product page, spec sheet or installation manual, or the maker’s own copy on its Amazon listing. Prices are checked at Amazon and the maker’s own store, and the lowest is used. Where a model comes in natural gas and propane versions, they share one listing.
The factors
Value is the current price divided by the heating power, on a log scale for each group, so going from $8 to $4 counts as much as going from $16 to $8.
| Group | Unit | Scores 0 | Full marks |
|---|---|---|---|
| Gas | per 1,000 BTU/h of input | $12 | $4 |
| Electric | per kW | $45 | $12 |
Heating power is the maximum gas input or the rated electric power: for gas 120,000 BTU/h scores zero and 199,000 BTU/h full marks; for electric 11 kW scores zero and 36 kW full marks.
Efficiency is the Uniform Energy Factor from the Department of Energy test: for gas 0.80 scores zero and 0.96 full marks; for electric, where every model is close to 1, 0.90 scores zero and 0.98 full marks.
Warranty is the residential warranty on the heat exchanger (or the heating chamber on electric models): for gas 5 years scores zero and 15 years full marks; for electric 3 years scores zero and 10 years full marks. A limited lifetime warranty gets full marks. Parts warranties are listed separately.
What isn’t scored
Flow in gallons per minute is listed but not scored, because makers state it at different temperature rises and it follows heating power anyway. Minimum input, venting, recirculation, Wi-Fi and weight are listed too. Independent tests are listed where they exist, but there are too few to score.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.