Energy Consumption Calculator - Watts, Hours and kWh

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Energy use is power multiplied by time. A 100 W device does not use 100 kWh. It uses 0.1 kWh every hour it runs. That little unit mistake is how people scare themselves with nonsense bills.

Formula at a glance

  • kWh = watts x hours / 1000
  • monthly kWh = watts x hours per day x days / 1000
  • cost = kWh x rate

Field note: The nameplate shows maximum draw. A fridge marked 150 W does not run the compressor 24 hours a day.

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Energy Consumption Calculator

Calculate electricity usage in kWh

W
hrs/day
days
Result
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Formula

kWhE = P × t / 1000

Where P = watts, t = hours

Common Appliances

Appliance kWh/day
Refrigerator 1-2
AC (window) 3-5
TV (4 hrs) 0.4
Washing machine 0.5

Related

Electricity BillkW to kWhWatts to kWh

How to use the Energy Consumption Calculator

Use this as a unit check for energy, power and time. Keep the time period honest, because a one-hour run and a one-month run can make the same load look completely different.

Worked example

Example: a 75 W ceiling fan running 10 hours a day uses 0.75 kWh per day. Over 30 days, that is 22.5 kWh.

Practical checks before you trust the number

  • Use measured watts for variable-speed devices when you can.
  • Standby loads are small one at a time, but routers, chargers and DVRs run all month.
  • Motors and compressors cycle, so use average watts over time.

Common mistake

The nameplate shows maximum draw. A fridge marked 150 W does not run the compressor 24 hours a day.

Sources and references

Related calculators

Frequently Asked Questions

kWh = (W × hours) ÷ 1000. Or kWh = kW × hours. Sum across all appliances and runtimes to get total energy use. Example: a 1500 W AC for 6 hours = 9 kWh. Same as the watts-to-kWh formula, applied across multiple loads.

Energy = power × time. For appliances rated in watts: kWh = (W × runtime hours) ÷ 1000. For monthly: multiply by 30 days. For appliances with duty cycles (fridges, ACs), multiply by the on-time fraction (typically 30 to 50%) to get realistic figures.

(100 × 24) ÷ 1000 = 2.4 kWh per day. For a month: 72 kWh. At I6/kWh, that's I432/month. Even small always-on loads add up — five 100 W loads running 24/7 cost over I2000/month.

Monthly kWh = (W × hours per day × 30) ÷ 1000. Example: 200 W refrigerator running effectively 12 hours per day (50% duty cycle) → (200 × 12 × 30) ÷ 1000 = 72 kWh/month. Sum each appliance for total household energy use.

Duty cycle is the fraction of time an appliance is actually on. A fridge with 40% duty cycle running 24 hours is only on for 9.6 hours of energy use. Multiply runtime by duty cycle for realistic energy estimates. Always check actual cycling on a clamp meter or smart plug for accuracy.

Yes, and you should include it. Standby loads (TVs in sleep mode, chargers plugged in, set-top boxes) often draw 5 to 30 W around the clock. Five such devices at 10 W each = 50 W × 24 h × 30 = 36 kWh/month. That's 10 to 15% of a typical residential bill.

Yes. Enter watts and runtime for two appliances and compare monthly kWh and cost. Example: 5-star AC at 1.2 kW × 8 h vs 3-star at 1.5 kW × 8 h → 288 kWh vs 360 kWh per month. The 5-star saves 72 kWh × I7 = I504/month, paying back the price difference in two summers.