Best for
- People comparing long-term ownership cost
- 24/7 purifier users
- Buyers comparing a small unit at high speed with a larger unit at lower speed
Watch for
- Using standby power as running power
- Assuming rated watts equals actual draw at every speed
- Ignoring filter cost, which may exceed electricity cost
Use the basic energy formula
A 40 W purifier running 24 hours uses 0.96 kWh per day at that power draw. Multiply by your electricity rate to estimate daily cost, then by 365 for a rough annual figure.
Actual use may be lower if the purifier spends time at lower fan speeds.
Auto mode changes the calculation
Smart purifiers can reduce fan speed when sensors report cleaner air. That can save energy, but the savings depend on the room and control logic.
For a conservative comparison, calculate both a maximum-power case and a moderate-use case.
Filter spending belongs in total cost
Electricity is only one recurring cost. Add replacement filters, optional pre-filters and accessories when comparing ownership over three to five years.
A cheaper purifier with expensive proprietary filters can become the more expensive choice.
Use the same sizing basis across brands. Room-size claims can use different air-change assumptions. When possible, compare published CADR and calculate the airflow your room actually needs.