If your kWh estimates are off, duty cycle is usually the missing variable. Many appliances do not run continuously, so using 24-hour runtime often exaggerates energy demand.
Build better estimates using the WattSizing Calculator.
Quick Answer
Duty cycle is the percentage of time an appliance is actually ON during a period.
Average watts = Running watts x Duty cycle
Daily kWh = (Average watts x 24) / 1000
Why Duty Cycle Matters
A refrigerator rated at 180W does not consume 180W x 24h every day because the compressor cycles on and off. The same pattern applies to many devices:
- Refrigerators and freezers
- Air conditioners and heat pumps
- Sump/well pumps
- Dehumidifiers
For baseline load math, review How Many Watts Does a Refrigerator Use and How Many Watts Does a Heat Pump Use.
Practical Estimation Methods
Method 1: Simple Observation
Watch one full cycle and estimate:
- ON time in minutes
- Total cycle time in minutes
Duty cycle = ON time / Total cycle time
Example: ON 12 min in a 30 min cycle -> 12/30 = 0.40 (40%)
Method 2: Smart Plug Data
Track power draw for 24 to 72 hours, then compute:
Duty cycle = Total ON hours / Total observed hours
This is usually the most practical and reliable home method.
Duty Cycle Reference Table (Typical Ranges)
| Appliance | Typical Duty Cycle | Notes |
|---|---|---|
| Refrigerator | 25% - 45% | Higher in hot kitchens or frequent door opening |
| Chest/Upright freezer | 20% - 40% | Better insulation can lower duty cycle |
| Window AC | 30% - 80% | Depends strongly on thermostat and outdoor temperature |
| Mini split | 20% - 70% | Inverter models modulate and may appear "always on" at low draw |
| Dehumidifier | 30% - 90% | Humidity setpoint drives runtime |
| Sump pump | 1% - 20% | Highly seasonal and weather dependent |
Worked Example
You measure a fridge at 170W while running and estimate duty cycle at 38%.
- Average watts:
170 x 0.38 = 64.6W - Daily kWh:
(64.6 x 24) / 1000 = 1.55 kWh/day - Monthly kWh (30 days):
46.5 kWh
At $0.15/kWh, monthly cost is about $6.98.
Common Mistakes
- Assuming all appliances run 24/7 at nameplate wattage.
- Using a single day without considering weather/season.
- Ignoring user behavior (door openings, thermostat changes, occupancy).
- Forgetting standby loads that run continuously.
If you are designing backup power, combine this with How to Calculate Total Home Backup Load before selecting hardware.
FAQs
Is duty cycle the same as efficiency?
No. Duty cycle is runtime proportion; efficiency is how effectively power is converted into useful output.
Can duty cycle exceed 100%?
No. By definition it ranges from 0% to 100%.
How many days should I measure?
At least 3 days for a better average, and longer for seasonal loads.
Do inverter AC units have duty cycle?
Yes, but they often run longer at lower power instead of hard on/off cycling.
CTA
Want more realistic kWh projections for your appliances? Use the WattSizing Calculator to model runtime, duty cycle, and daily energy with clearer assumptions.
