A cool-mist humidifier running overnight costs about 5p.
A warm-mist one doing the same job costs about 50p, and the difference is not the mist. It is the kettle element hidden inside.
Quick facts
- Unit rate assumed: 25p per kWh โ check your own bill
- Cool mist (ultrasonic): 20โ35 W, roughly 5p for an eight-hour night
- Warm mist or steam: 200โ400 W, roughly 50p for the same night
- A 150-night season: about ยฃ8 cool, about ยฃ75 warm
The heating element is the whole cost, and the fan is a rounding error
An ultrasonic humidifier vibrates a metal disc at a frequency you cannot hear and shakes water into a fine cloud.
That takes almost no power, which is why a 5-litre machine and a 60-watt light bulb are not in the same conversation.
A warm-mist machine boils the water instead.
Boiling anything is expensive, and the tariff does not care that the kettle is shaped like a bedside lamp.
Work it out from the plate on the back, not the box
Multiply the watts by the hours, divide by 1,000, and multiply by your unit rate.
A 25 W cool-mist unit running eight hours uses 0.2 kWh, which is 5p at 25p a unit.
A 250 W warm-mist unit over the same eight hours uses 2 kWh, or 50p.
The figure printed on the packaging is usually the maximum draw, reached only on the highest output setting with the heater on.
The rating plate moulded into the base is the honest one, and it is the number to work from.
Hybrid machines that offer both mist types sit between the two.
Expect somewhere around 100 to 130 W with the heater engaged, and a fraction of that without it.
Three tiers, and the gap across a season is about ยฃ67
Electricity for eight hours a night across a 150-night heating season, at 25p per kWh.
A British heating season runs roughly from the middle of October to the middle of March.
Call it 150 nights of eight hours, which is what most people actually manage before the machine goes back in the cupboard.
Cool mist costs about ยฃ8 across the whole of that.
Warm mist costs about ยฃ75, which is no longer a rounding error on a winter bill.
Across a winter, the choice between cool mist and warm mist is worth about sixty-seven pounds.
Warm mist still has its arguments โ it is quieter, it kills what is in the tank on the way out, and it does not chill the room.
Our Levoit LV600S review covers a hybrid that lets you decide which of the two you are paying for, night by night.
A humidistat saves more than a lower wattage does
The largest saving available is not buying a lower-rated machine. It is not running the one you have when the room does not need it.
A model with a built-in humidistat measures the room and switches the output off at a set percentage.
Set it to 45% and, in a typical British bedroom, the machine spends much of the night idle.
Without one, it runs flat out until the tank empties, whether the room is at 35% or 65%.
The same logic applies to the heater switch on a hybrid: leave it off unless the room is genuinely cold, and the running cost falls back to cool-mist money.
Assumptions matter here, and 25p per kWh is only a working figure.
Unit rates vary by region and by tariff, so the sums above are worth redoing with the number on your own bill.
Check the current capped unit rateOfgem, the energy regulator for Great Britain โ
