Appliance Wattage Calculator

The label on the back of an appliance tells you its wattage. What it doesn’t tell you is what that number costs. A 1,500 W space heater and a 1,500 W kettle use the same power but for wildly different amounts of time. This calculator bridges the gap. It converts raw wattage into kilowatt-hours, applies a duty cycle to account for appliances that cycle on and off, and multiplies by your electricity rate. The result is a yearly cost figure you can compare against the next appliance you’re considering. Useful for budgeting, for comparing energy ratings, and for spotting which device is driving up the bill. Everything runs locally.

How to Use the Appliance Wattage Calculator

1. Choose your currency from the dropdown at the top.

2. Name the appliance and set how many identical units you’re calculating.

3. Pick a power input method. Watts is the default for most appliances.

4. Enter the wattage, or fill in volts and amps, or type the BTU/h figure for an air conditioner.

5. Set hours per day, days per week, and weeks per year to match your usage pattern.

6. Add a duty cycle if the appliance cycles. A fridge runs about 35% of the time it’s plugged in.

7. Enter your electricity price per kWh.

8. Read the annual cost in the results panel.

9. Check the breakdown table for per-day, per-week, per-month, and per-year energy use.

How the Appliance Wattage Calculator Formula Works

The calculation chains power, time, and price together.

Formula: Watts = Volts × Amps × Power Factor
Formula: Watts = BTU/h ÷ 3.412142
Formula: Effective watts = Watts × Duty cycle %
Formula: kWh = (Effective watts ÷ 1000) × hours of use
Formula: Yearly kWh = kW × hours/day × days/week × weeks/year
Formula: Cost = kWh × price per kWh
Formula: CO₂ = kWh × grid intensity (kg/kWh)

Each power input method produces the same unit wattage before the rest of the math runs. Direct watts is the simplest. Volts × amps matters when the label lists voltage and current separately. Power factor adjusts for inductive loads like motors, where current and voltage don’t peak together. BTU/h converts to watts at 3.412142 BTU per watt, so a 12,000 BTU air conditioner draws about 3,517 W of cooling capacity, though actual electrical draw is lower because ACs move heat rather than generate it.

Duty cycle handles appliances that cycle. A fridge compressor runs about 35% of the time it’s plugged in. An air conditioner might run 65% of the time during peak cooling. Continuous loads like a TV or a PC sit at 100%. The calculator multiplies raw watts by the duty percentage to get the average draw.

Validation requires positive wattage. Other fields fall back to defaults if left blank. The badge classifies total wattage: under 100 W is low, 100 to 500 W is moderate, 500 to 1,500 W is high, and above 1,500 W is very high.

Worked Example

A chest freezer in a UK home draws 120 W. It runs 24 hours a day, 7 days a week, all year. The compressor duty cycle is 35%. UK electricity costs £0.24 per kWh.

Effective watts: 120 × 0.35 = 42 W
Yearly kWh: (42 ÷ 1000) × 24 × 7 × 52 = 366.9 kWh
Yearly cost: 366.9 × 0.24 = £88.06
Monthly cost: 88.06 ÷ 12 = £7.34
Daily cost: 88.06 ÷ 365 = £0.24

Carbon at the UK grid intensity of 0.233 kg/kWh: 366.9 × 0.233 = 85.5 kg CO₂ per year.

The badge reads “Moderate Power Draw” because 120 W sits in the 100-to-500 band. The same chest freezer run on a French grid (0.052 kg/kWh) would emit just 19 kg CO₂ per year, a fifth of the UK figure.

Now swap the freezer for a 3,000 W tumble dryer used twice a week for one hour at 90% duty. Effective watts: 2,700 W. Yearly kWh: 2.7 × 1 × 2 × 52 = 280.8 kWh. Cost at £0.24: £67.39. Fewer running hours but more power per hour, which is why appliance choice matters as much as usage pattern.