Watts to kWh Calculator - Energy Use from Power and Time

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

Watts tell you how hard the load is working right now. kWh tells you how much energy it used over time. Multiply watts by hours, then divide by 1,000.

Formula at a glance

  • kWh = W x hours / 1000
  • cost = kWh x rate
  • W = kWh x 1000 / hours

Field note: A 1,000 W load is only 1 kWh after one hour. People mix that up constantly.

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Watts to kWh Calculator

Convert power to energy

W
hours
Result
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Formula

kWhE = W × h / 1000

Quick Reference

Power × Time Energy
100W × 10h 1 kWh
1000W × 1h 1 kWh
1500W × 4h 6 kWh

Related

kWh to WattsElectricity Bill

How to use the Watts to kWh 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: 500 W for 8 hours is 4 kWh. At $0.20 per kWh, that costs $0.80.

Practical checks before you trust the number

  • Use average watts for cycling appliances.
  • For battery systems, include inverter efficiency.
  • For monthly use, multiply daily kWh by days.

Common mistake

A 1,000 W load is only 1 kWh after one hour. People mix that up constantly.

Sources and references

Related calculators

Frequently Asked Questions

kWh = (W × hours) ÷ 1000. Divide watts by 1000 to convert to kW first, then multiply by hours. Example: a 500 W TV running 6 hours per day → (500 × 6) ÷ 1000 = 3 kWh/day, or 90 kWh/month. The /1000 step is what trips up juniors most often.

kWh = (W × h) ÷ 1000. Example: 100 W bulb running 10 hours → (100 × 10) ÷ 1000 = 1 kWh. That's 30 kWh per month if used every day, or about I180 to I240 per month at residential tariffs. Same formula scales for any appliance.

kWh = (100 × 24) ÷ 1000 = 2.4 kWh/day. Over a month, that's 72 kWh. At I6/kWh, the cost is I432/month for a single 100 W device running 24 hours. This is why even small always-on loads add up — they're called phantom loads, and they can total 5 to 10% of a residential bill.

Monthly kWh = (W × hours per day × 30) ÷ 1000. Example: 200 W refrigerator running 24 hours → (200 × 24 × 30) ÷ 1000 = 144 kWh/month. Sum each appliance's contribution for total monthly use. Always include duty cycles for cycling loads like fridges (typically 30–40% on-time).

Because watts is power and kWh is energy in kilowatt-hours. To convert from W × h to kWh, divide by 1000 to go from W to kW. Without that division you'd get watt-hours, which is a 1000-times larger numerical answer. The /1000 keeps the unit consistent with how electricity bills are measured.

No. Energy requires both power and time. kWh = (W × hours) ÷ 1000. Without runtime, you only know the rate of energy use, not the total. Always include daily or monthly usage hours when estimating energy cost from a wattage rating.

Yes. Multiply watts by hours used, divide by 1000 for kWh, multiply by tariff for cost. Example: a 1500 W AC running 6 hours/day → (1500 × 6) ÷ 1000 = 9 kWh/day → 270 kWh/month → I1890/month at I7/kWh. This is a fast way to show customers which appliance is the real bill driver.