Tecnobosque · Clean air and backup power
The purifier with the lowest wattage is not automatically the best choice for a smoky room. Compare the air-cleaning requirement first, then the electricity needed to meet it.
Published 19 October 2026 · Calculation guide · All equipment examples below are hypothetical.
What CADR per watt actually compares
CADR describes clean air delivery; watts describe electrical power. With CADR expressed in cubic feet per minute, the ratio has units of cfm/W. If CADR is supplied in cubic metres per hour, the ratio is (m³/h)/W. Keep units consistent before comparing numbers.
ENERGY STAR describes its current efficiency metric as Integrated Energy Factor (IEF), which uses a CADR-to-watt ratio. The simple calculation in this article is not a certification calculation. Do not rename an informal smoke-CADR/power calculation “IEF,” infer certification from it, or mix database fields without checking their definitions.
For wildfire smoke, retain the pollutant-specific rating: a smoke CADR should not silently be replaced by a pollen CADR. Keep the source and operating conditions beside each number.
Worked comparison: efficient does not always mean sufficient
Assume, solely for this exercise, a room plan requires 200 cfm of smoke CADR. This is an invented planning target, not a universal recommendation. The three fictional configurations below have documented CADR and power at the same fixed setting:
| Configuration | Smoke CADR | Power | CADR / power | Meets the example target? |
|---|---|---|---|---|
| A | 240 cfm | 60 W | 4 cfm/W | Yes |
| B | 180 cfm | 30 W | 6 cfm/W | No |
| C | 240 cfm | 40 W | 6 cfm/W | Yes |
B has a higher ratio than A but falls below the stated target. C meets the target with less electrical demand than A in this example. This shows why the sensible order is adequacy first, efficiency second. It does not establish a real-world winner: noise, filter availability, placement, price and the quality of the supporting evidence still matter.
For room sizing, use the wildfire smoke CADR calculator and review its assumptions. The EPA filtration guidance recommends choosing sufficient smoke CADR and running a portable air cleaner continuously at its highest fan setting, with unobstructed airflow. A lower setting should not be selected solely to improve an electricity estimate.
The most misleading calculation: mixing fan settings
Suppose a specification lists 240 cfm at high speed, while a plug-in meter shows 10 W in sleep mode. Dividing 240 by 10 produces 24 cfm/W, but that ratio combines two operating states. It does not describe either state.
If you cannot find a CADR for the speed you intend to use, record it as unknown. Measuring watts does not measure CADR, and a quiet fan is not evidence of a particular clean-air delivery rate. Our electricity measurement guide explains how to obtain a useful load figure; it cannot replace an air-cleaning performance test.
Translate the difference into energy and battery demand
For 24 hours at constant power, configuration A uses 1,440 Wh (60 × 24); C uses 960 Wh (40 × 24). The difference is 480 Wh, or 0.48 kWh, delivered to the purifier. This calculation excludes any other equipment and assumes the stated load persists.
For a battery illustration, assume 1,000 Wh nominal capacity, an 85% conversion factor and a separate 10% reserve. The simplified available energy is 765 Wh. Estimated runtime is therefore 12.75 hours at 60 W and 19.125 hours at 40 W. These are calculated scenarios, not measured runtimes or guaranteed performance. They omit a separate inverter-idle term and assume the stated factors are appropriate.
Use the portable power runtime calculator to examine your own assumptions. Avoid double-counting losses already included in a usable-energy figure. Check output compatibility and restart behavior after an interruption separately; energy capacity alone does not establish that the arrangement will work.
A comparison sheet worth keeping
- Exact model and market: record the model code and electrical version, not just a brand name.
- Air-cleaning evidence: record smoke CADR, units, source, operating setting and any stated test conditions.
- Power evidence: distinguish a nameplate rating, a test result and your own measured average. Mark missing conditions.
- Efficiency field: preserve the original metric name. Keep a separately calculated ratio visibly separate from a certified database value.
- Practical fit: note room requirement, noise at the intended setting, filter replacement availability and power compatibility.
Common questions
Does lower wattage mean better efficiency?
No. It means lower power demand at the stated condition. Efficiency needs an output as well as an input: in this comparison, clean-air delivery and electrical power.
Can I calculate electricity cost from CADR?
Not from CADR alone. Use measured or supported power, operating hours and your electricity rate: cost = watts ÷ 1,000 × hours × price per kWh. Fixed charges are outside that calculation.
Should I buy solely on CADR per watt?
No. First establish adequate clean-air delivery for the room and a usable operating setting. Then compare efficiency alongside evidence quality and practical requirements. See our wildfire smoke air purifier buying guide for the broader selection criteria.