Observe the indicator
Time compressor, burner, element, or pump operation across several normal cycles. This is simple but requires representative conditions.
Measurement
Appliance Duty Cycle and Running Time
A device can be connected for 24 hours without drawing its rated watts for 24 hours. Duty cycle describes the share of time its main load is active.
What appliance duty cycle means
A refrigerator compressor, air conditioner, heater, or pump often switches on and off in response to a thermostat, pressure switch, humidity control, or water level. A 40% duty cycle means the controlled load runs for about 24 minutes during a representative hour. It does not mean the appliance uses exactly 40% of rated power at every moment.
Active hours = Connected hours x Duty cycleCost = Watts / 1000 x Active hours x Electricity rateTo calculate the observed percentage, divide active minutes by total observed minutes and multiply by 100. A compressor active for 24 minutes during one representative hour has a 40% observed duty cycle.
Duty cycle (%) = Active time / Observed time x 100Duty cycle cost examples
| 1,500W load over 8 hours | Active time | Energy | Cost at $0.18/kWh |
|---|---|---|---|
| 25% duty cycle | 2 hours | 3 kWh | $0.54 |
| 50% duty cycle | 4 hours | 6 kWh | $1.08 |
| 75% duty cycle | 6 hours | 9 kWh | $1.62 |
| 100% duty cycle | 8 hours | 12 kWh | $2.16 |
These examples treat 1,500W as the active draw. Controls, fans, and standby loads may continue using a smaller amount between active cycles.
Example: a cycling refrigerator
| Input | Value | Meaning |
|---|---|---|
| Running power | 180W | Power while the compressor is active |
| Connected time | 24 hours | The refrigerator remains plugged in all day |
| Example duty cycle | 33% | Illustrative assumption, not a universal refrigerator value |
| Equivalent active time | 8 hours | 24 hours x 0.33, rounded |
| Daily cost at $0.18/kWh | $0.26 | 180 / 1000 x 8 x $0.18 |
The same refrigerator entered as 180W for 24 full hours would produce $0.78 per day, illustrating why connected time and active time should not be confused.
Loads that commonly cycle
Refrigerators and freezers cycle compressors and may add defrost heaters. Air conditioners, heat pumps, dehumidifiers, and portable AC units respond to temperature or humidity. Tank water heaters, space heaters, electric blankets, irons, ovens, and aquarium heaters cycle resistance elements. Sump and pool pumps cycle or follow schedules for entirely different reasons.
Each case needs the same basic distinction: identify which component creates the major load, then estimate how long that component actually operates. Small fans, controls, or displays may continue drawing power while the compressor or element is off.
Three ways to estimate active runtime
Use the most direct evidence available for your appliance.
Measure accumulated kWh
A compatible energy meter captures cycling automatically. Divide measured kWh by running kilowatts to estimate equivalent full-power hours.
Use equipment records
Smart thermostats, EV chargers, pumps, and modern HVAC controls may report runtime or energy. Confirm what each metric includes.
Use annual kWh
For labeled appliances, annual kWh can bypass the duty-cycle guess. Multiply the label value by your rate for an annual planning estimate.
Why duty cycle changes
Temperature difference is a major driver. A refrigerator in a hot garage, an aquarium in a cold room, or an air conditioner during a heat wave may run longer. Insulation, seals, airflow, filters, frost, equipment size, thermostat settings, and how often doors are opened also matter.
Avoid using one extreme day as a yearly average. Keep separate mild-weather and peak-season scenarios, then weight them by the number of months or days when each condition is plausible.
When duty cycle is not enough
Variable-speed compressors, inverter air conditioners, computers, and electronically controlled motors may modulate power instead of switching between one fixed wattage and zero. For these loads, accumulated kWh is more reliable than a simple on/off percentage. Multi-stage equipment may also need separate wattage and runtime estimates for each stage.
Air conditioner and heater duty cycle examples
An air conditioner may have a low duty cycle on a mild afternoon and approach continuous compressor operation during peak heat. A thermostatic space heater behaves similarly as room heat loss changes. Calculate mild and peak conditions separately instead of applying one observed percentage to an entire season.
Duty cycle FAQ
How do I calculate appliance duty cycle?
Divide active running time by total observed time and multiply by 100. If a compressor runs for 24 minutes during a 60-minute observation, its observed duty cycle is 40%.
Is duty cycle the same every day?
No. It can change with weather, room conditions, settings, load, maintenance, and user behavior.
Should I enter 24 hours for an always-plugged-in appliance?
Only if the watts input represents its average draw across the day. If you enter compressor or heater running watts, use equivalent active hours instead.
How do I convert measured kWh to active hours?
Divide measured kWh by running kilowatts. A device using 1.2 kWh with a 0.15 kW running load has about eight equivalent full-power hours.
Sources
Government and ENERGY STAR context for estimating cycling appliance energy.
- Estimating Appliance and Home Electronic Energy UseU.S. Department of Energy
- RefrigeratorsENERGY STAR
- Air ConditioningU.S. Department of Energy
Try a cycling appliance calculator
Compare active runtime assumptions for common thermostatic and intermittent loads.
Heating & CoolingSpace Heater Cost CalculatorTest active runtime with a 1500W starting estimate.Heating & CoolingAir Conditioner Cost CalculatorTest active runtime with a 1200W starting estimate.Kitchen AppliancesRefrigerator Cost CalculatorTest active runtime with a 180W starting estimate.Hobby & SpecialtyAquarium Heater Cost CalculatorTest active runtime with a 100W starting estimate.
Related measurement guides
Measure the input, understand the formula, and compare wattage ranges.
MeasurementHow to Measure Appliance Electricity Use AccuratelyMeasure appliance electricity use with label watts, a plug-in meter, annual kWh, or circuit data.BasicsHow to Calculate Electricity CostUse watts, hours, and price per kWh to estimate running cost, including cost per hour by wattage.Appliance PlanningAverage Appliance WattageCompare typical wattage ranges for common household appliances.