Methodology

Core formula

Energy in kilowatt-hours (kWh) equals power in watts (W) multiplied by hours of use, divided by 1,000. Cost equals energy in kWh multiplied by your price per kWh.

Example: 1,000 W for 1 hour is 1 kWh. At an illustrative price of $0.20 per kWh, 1 kWh costs $0.20.

Units

• W (watts) and kW (kilowatts) measure power. 1 kW = 1,000 W.
• kWh (kilowatt-hours) measure energy, which is what electricity is billed by.
• An air conditioner's cooling capacity in BTU/h is not its electrical power. Use its rated electrical input in watts instead.

Time periods

• Per day of use: power × hours used that day ÷ 1,000.
• Week: energy per day of use × days used per week.
• Average calendar day: weekly energy ÷ 7.
• Year: weekly energy × 365 ÷ 7, using a 365-day year.
• Average month: annual energy ÷ 12. A 30-day month is never used.

Calculation methods

Each method produces a weekly figure; the year, average month and average calendar day follow from it as above.

• Power and hours: weekly kWh = watts × effective power ÷ 100 × hours per day × days per week ÷ 1,000. Effective power is 100% unless you change it.
• Energy per cycle: weekly kWh = kWh per cycle × cycles per week.
• Energy per year: the yearly kWh from the label is used as the year; weekly kWh = yearly kWh × 7 ÷ 365.
• In use plus standby: weekly kWh = in-use watts × hours in use per week ÷ 1,000 + standby watts × (hours plugged in − hours in use) ÷ 1,000. Plugged-in hours include the hours in use, so no hour is counted twice.

Effective power assumption

Appliances with a thermostat or compressor, such as air conditioners, do not always draw their full rated input. The optional effective power percentage (0 to 100) is your own assumption and is applied as percentage ÷ 100. At 100% the estimate assumes full rated input for every hour entered. No typical percentage is provided.

Season estimates

The air conditioner calculator can multiply the weekly figure by your weeks of use per year. This season estimate is shown on that page only: it is not saved with the appliance and does not change comparison or dashboard totals. Everywhere else, including the comparison and dashboard, a year means use every week of a 365-day year.

kWh calculator

Energy (kWh) = watts × hours ÷ 1,000; average power (W) = kWh × 1,000 ÷ hours; time (hours) = kWh × 1,000 ÷ watts. Dividing by zero hours or zero watts has no answer, so those inputs must be above 0.

Totals for several appliances

Totals add up each appliance's weekly, yearly, average-month and average-calendar-day figures. A combined “per day of use” total is not shown, because appliances are used on different days.

Input limits

• Power and standby power: 0 to 100,000 W (100 kW). Hours of use per day: 0 to 24. Days of use per week: 0 to 7.
• Hours plugged in per week: 0 to 168, and at least the hours in use. Effective power: 0 to 100%.
• Energy per cycle: 0 to 50 kWh. Cycles per week: 0 to 100. Energy per year: 0 to 100,000 kWh.
• kWh calculator: time up to 8,760 hours (one year) and energy up to 100,000 kWh.
• Electricity price: 0 to 100 per kWh. A price of 2 or more triggers a check, because it is often a price in cents or pence entered as whole units.
• Numbers can include decimals (for example 0.5 hours). Negative numbers, exponents and text are rejected.

Currency and rounding

Choosing a currency changes only how amounts are displayed. No exchange rates are applied and no tariffs are looked up. Calculations keep full precision and are rounded only for display: energy to 2 decimal places and money to 2 decimal places. Non-zero values smaller than 0.01 are shown with 2 significant digits instead of 0.00. CSV downloads round energy to 3 decimal places and money to 2.

Sources

The calculator relies on the figures you enter. If reference values are added later, each will be listed here with its source and the date it was checked.