Every UV water purifier 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. Purifiers are grouped by rated flow, up to 12 GPM and over 12 GPM, on the same scales.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per gallon a minute at the Class A dose | 25% |
| Certification | NSF/ANSI 55 class, or at least a published UV dose | 20% |
| Monitoring | UV sensor, lamp alarm, shutoff valve and lamp countdown | 20% |
| Lamp life | rated hours before the lamp needs changing | 10% |
| Warranty | warranty on the controller | 25% |
Where the numbers come from
Flow, dose, lamp power and life, monitoring features, ports, pressure and warranty come from the maker’s spec sheet, manual or product page, and from the Amazon listing where the maker publishes nothing else. NSF/ANSI 55 certification counts only when the certifier lists the model or the maker names the certification for that exact model. 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 flow at the 40 mJ/cm² dose that NSF/ANSI 55 Class A requires, on a log scale, so halving the price per GPM always counts the same. $300 per GPM scores zero and $20 full marks. A unit treats less water at a higher dose, so a flow rated at a lower dose is scaled down by the dose over 40: 12 GPM at 30 mJ/cm² counts as 9 GPM, and is marked estimated. When the maker gives no dose at all, the flow is taken at 16 mJ/cm², the lowest dose any standard recognizes, so 12 GPM counts as 4.8 GPM: a flow rating with no dose behind it says little about how well the water is disinfected.
Certification is the NSF/ANSI 55 class. Class A, for disinfecting untreated water, scores full marks. Class B, for supplemental treatment of water that already tests safe, scores 50%. A unit that isn’t certified but states the dose its flow is rated at scores 30%; one that states no dose scores zero. A maker saying a unit meets or is designed to the standard doesn’t count as certification.
Monitoring adds up four features: 40% for a UV sensor that measures the light reaching the water, 30% for an alarm when the lamp fails, 20% for a shutoff valve (or an output to drive one) that stops the water on a fault, and 10% for a countdown to the next lamp change.
Lamp life is the rated hours before the lamp needs changing, on a log scale: 8,000 hours (about a year of running) scores zero and 50,000 hours full marks.
Warranty is the maker’s warranty on the controller or ballast: 1 year scores zero and 10 years full marks. Chamber and lamp coverage are listed but not scored.
What isn’t scored
Lamp type (mercury or LED), lamp power, port size, maximum pressure, chamber material and voltage 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. Monitoring is neutral only when none of the four features is published either way. 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.