Appliance Electricity Cost Math: kWh, Watts, Duty Cycles, and Real Monthly Cost
Most household energy advice starts with a vague claim: "This appliance uses a lot of power." That is not enough. A kettle may draw more watts than a refrigerator, but the kettle runs for minutes while the refrigerator cycles all day.
To estimate monthly cost, you need four variables:
- wattage;
- hours of use;
- duty cycle;
- electricity rate.
The Core Formula
Use this formula:
Monthly cost = (watts / 1,000) x hours used per month x electricity rate
Example:
| Input | Value |
|---|---|
| Appliance draw | 1,500 watts |
| Use | 30 minutes per day |
| Monthly hours | 15 hours |
| Electricity rate | $0.18 per kWh |
| Monthly cost | $4.05 |
The appliance feels powerful, but the cost is modest because runtime is low.
Why Duty Cycle Matters
Some appliances do not run at full power continuously. A refrigerator, dehumidifier, heat pump, freezer, or electric water heater cycles on and off.
If a device draws 600 watts while running but only runs 40% of the time, its average draw is:
600 watts x 0.40 = 240 average watts
Then calculate cost using 240 watts, not 600 watts.
| Device | Nameplate draw | Typical cost mistake |
|---|---|---|
| Refrigerator | Compressor wattage | Assuming it runs 24/7 at full draw |
| Space heater | 1,500 W | Underestimating because it runs for many hours |
| Dehumidifier | 300-700 W | Ignoring summer duty cycle |
| Gaming PC | Power supply rating | Treating max PSU size as real use |
| Heat pump | Variable | Using one static wattage for all weather |
Reading an Energy Label
If an appliance has an annual kWh estimate, start there. It already includes assumptions about normal use.
Annual cost formula:
Annual cost = annual kWh x electricity rate
Example:
| Annual kWh | Rate | Annual cost | Monthly average |
|---|---|---|---|
| 450 kWh | $0.18/kWh | $81.00 | $6.75 |
Your actual result will vary with setpoints, household size, weather, and maintenance.
The Hidden Rate Problem
Many people use the energy-only rate from their bill. That can understate real cost because bills include delivery charges, riders, taxes, and fixed fees.
For appliance decisions, use an approximate marginal rate:
Marginal rate = changeable bill charges / kWh used
Do not include fixed monthly fees that stay the same whether you use the appliance or not.
Worked Example: Dehumidifier
A basement dehumidifier draws 520 watts while running. In summer it runs 10 hours per day. The local marginal rate is $0.20/kWh.
| Step | Math |
|---|---|
| Convert watts | 520 / 1,000 = 0.52 kW |
| Monthly hours | 10 x 30 = 300 hours |
| Monthly kWh | 0.52 x 300 = 156 kWh |
| Monthly cost | 156 x $0.20 = $31.20 |
That is meaningful. If a better unit cuts runtime or wattage by 30%, the savings can justify an upgrade.
Worked Example: Phone Charger
A phone charger might average 5 watts for 3 hours per day.
| Step | Math |
|---|---|
| kW | 5 / 1,000 = 0.005 |
| Monthly hours | 90 |
| Monthly kWh | 0.45 |
| Monthly cost at $0.20/kWh | $0.09 |
Unplugging tiny chargers may be fine as a habit, but it is not where most households find serious savings.
Priority Ranking
Start with appliances that combine high wattage and long runtime:
- resistance space heating;
- electric water heating;
- dehumidifiers;
- old refrigerators and freezers;
- pool pumps;
- electric dryers;
- always-on home office and media equipment.
Short runtime devices are usually less important unless used constantly.
FAQ
Is watts the same as watt-hours?
No. Watts measure power at a moment in time. Watt-hours measure energy over time.
How many watts are in a kWh?
A kilowatt-hour is 1,000 watts used for one hour.
Why is my bill still high after replacing one appliance?
Because the bill reflects the whole home: heating, cooling, water heating, cooking, lighting, plug loads, fixed charges, and rate design.
What to Calculate Next
For household planning, pair this method with the utility offset math guide and the heat pump ROI calculator guide.