How much does a dehumidifier cost to run?

Almost every figure you will read for what a dehumidifier costs assumes it runs continuously at its rated power, and almost no dehumidifier does. On a humidistat, the compressor stops once the room reaches the target humidity and only the fan continues, so over a night or a day the machine may be actively extracting for a third to a half of the elapsed time. Getting the duty cycle into the sum is what turns a scary number into a real one. Everything below uses your machine and your tariff, because both vary far more than any article can allow for.

What you'll need

• The wattage from your dehumidifier's rating plate
• Your electricity unit rate in pence per kWh, from your bill or meter
• A plug-in energy monitor, if you want to measure rather than estimate (optional)

Step by step

1. Read the wattage off the machine itself.On the rating plate, usually on the back or base. Compressor models are commonly 200 to 400 watts, desiccants 500 to 700. Use the figure on your machine, not a typical one — the range is wide.
2. Convert watts to kilowatts by dividing by 1,000.320 watts becomes 0.32 kilowatts. This is the only unit conversion in the whole calculation.
3. Get your own unit rate from your own bill.Pence per kilowatt hour for electricity, on the bill or on a smart meter display. Rates vary by region, tariff and date, so a national average is a poor substitute. If you are on a time-of-use tariff, note the rate for the hours the machine actually runs.
4. Multiply for the cost per hour at full draw.0.32 kW times 25p is 8p per hour. That is the worst case: the machine drawing full power every minute.
5. Now apply the duty cycle, which is the step everyone skips.On a humidistat in a room that is not badly damp, a compressor typically runs 30 to 50 percent of the time once the room has come down to the target. Multiply the hourly cost by that fraction. 8p an hour at 40 percent duty over 8 hours is about 26p.
6. Treat the first days as the exception.A genuinely damp room will run the machine near-continuously for the first day or two while it pulls the moisture out of the fabric of the room as well as the air. Cost that period at full draw, then expect it to settle to cycling.
7. Cost laundry mode separately.Laundry or continuous mode deliberately disables the humidistat and runs the machine hard, so the duty cycle is close to 100 percent for as long as it is on. That is the mode worth comparing against a tumble dryer, and it usually compares well.
8. Measure it if the number matters to you.A plug-in energy monitor left on it for a week gives cumulative kilowatt hours with the real duty cycle already included. Multiply that by your unit rate for a figure with no assumptions in it at all. Some libraries lend monitors.
9. Set the humidistat rather than running it flat out.Around 50 percent relative humidity is a sensible target. Every step you set it lower increases the duty cycle sharply for diminishing benefit, and this setting affects the bill far more than the choice of machine did.
10. Set the cost against what it replaces.If it is drying washing that would otherwise go in a tumble dryer, the comparison is favourable and the machine is doing two jobs. If it is running because condensation keeps appearing, weigh it against fixing the ventilation, which costs nothing to run.

Tips

• A desiccant model draws roughly twice the wattage of a compressor one, but in a cold room it extracts far more water per hour, so cost per litre removed is the fairer comparison than cost per hour.
• Running a dehumidifier in one closed room is much cheaper than running it in an open-plan space, because it reaches the target humidity and starts cycling instead of working on the whole house.
• The fan-only portion of a cycle draws a small fraction of the full figure. That is why timing the compressor rather than the machine gives the duty cycle you need.
• If a smart plug with energy monitoring is already in the house, it will give you the weekly kilowatt hours without buying anything.

Common mistakes

• Costing it at full power for every hour it is switched on — It ignores the humidistat, which is the entire point of the humidistat. It typically produces a figure two to three times the real one and puts people off using the machine at all.
• Using a wattage found online rather than the one on the plate — Domestic dehumidifiers range from about 150 to 700 watts. Using a generic figure can be wrong by a factor of three before you have started.
• Setting the humidistat as low as it goes — Below about 45 percent the machine runs far more for no useful benefit, and very dry air is uncomfortable. The running cost rises steeply for the last few percentage points.

Questions people ask