Every grow light 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. Lights are grouped by power, under 300 W and 300 W and up; value is scored on its own scale in each group, and the other factors on the same scales.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per µmol/s of plant light | 30% |
| Efficacy | plant light per watt of electricity | 30% |
| Features | dimming, controller and water rating | 15% |
| Warranty | warranty length | 25% |
Where the numbers come from
Power draw, PPF, efficacy, coverage, spectrum and warranty come from the maker’s product page, spec table or manual, or the maker’s own copy on its Amazon listing. Power draw is what the light actually uses, not the “equivalent” wattage many listings headline, and PAR readings under the center of a light are not used, since they aren’t PPF. Lights whose makers publish no PPF aren’t ranked. Prices are checked at Amazon and the maker’s own store, and the lowest is used.
The factors
Value is the current price divided by the PPF, on a log scale, so halving the price per µmol/s always counts the same. Under 300 W, $1.00 per µmol/s scores zero and $0.20 full marks. At 300 W and up, $0.80 per µmol/s scores zero and $0.15 full marks.
Efficacy is the light a fixture makes per watt of electricity, in µmol/J, as the maker publishes it, or PPF divided by power draw when the maker gives only those. 2.0 µmol/J scores zero and 3.0 µmol/J full marks.
Features is 40% for dimming, 30% for connecting to a controller or app (including a controller sold separately), and 30% for a water rating of IP65 or better.
Warranty is the maker’s warranty: 1 year scores zero and 5 years full marks.
What isn’t scored
Coverage, spectrum, form (panel or bar), weight and release date are listed but not scored.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated. The exception is warranty: a maker that doesn’t publish one gets no warranty counted, so that factor scores zero, the same as the shortest warranty on the scale, and is marked 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.