Every standby generator gets a score from 0 to 100% built from five 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. Generators are grouped by rated output on propane (under 15 kW, 15 to 22 kW and over 22 kW), and value has its own scale for each group.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per kW on propane | 35% |
| Transfer switch | whether an automatic transfer switch is included | 15% |
| Noise | rated sound level in normal operation | 15% |
| Power quality | total harmonic distortion | 10% |
| Warranty | residential warranty length | 25% |
Where the numbers come from
Rated output, noise, THD and warranty come from the maker’s spec sheet, product page or manual. Prices are checked at Amazon, the maker’s own store and home improvement retailers, and the lowest is used. Prices are for the equipment only, never installation.
The factors
Value is the current price divided by the rated output on propane, on a log scale, so going from $400 to $200 per kW counts as much as going from $800 to $400. Under 15 kW, $500 per kW scores zero and $250 full marks; from 15 to 22 kW, $400 scores zero and $210 full marks; over 22 kW, $350 scores zero and $190 full marks.
Transfer switch is full marks when the price includes an automatic transfer switch and zero when it’s bought separately, because a switch adds several hundred dollars or more to the real cost.
Noise is the maker’s rated sound level in normal operation, usually measured at 23 ft: 72 dBA scores zero and 60 dBA full marks.
Power quality is the total harmonic distortion the maker states: 15% scores zero and 3% full marks. Low THD matters for electronics, variable-speed furnaces and heat pumps.
Warranty is the length of the residential limited warranty: 3 years scores zero and 10 years full marks.
What isn’t scored
Natural gas output, engine size, load management, remote monitoring, the self-test noise and the enclosure are listed but not scored. Makers rate noise at different distances and loads, so the figure is shown with how it was measured.
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.