Electrical Energy Calculator
Energy = power × time, and this calculator handles the unit gymnastics for you. Enter a device's power draw and how long it runs — get energy in joules and kilowatt-hours, plus the electricity cost when you add your tariff rate.
Optional — leave empty to skip cost
Electrical energy
1 kWh
3.6 megajoules · costs 0.2
- Energy (J)
- 3.6 M
- Energy (kWh)
- 1
- Cost
- 0.2
- Equivalent
- 86 phone charges
How it works
- Enter the device's power consumption in watts (check the label or the power calculator).
- Enter how long it runs, in hours.
- Optionally enter your electricity rate (per kWh) to see the cost.
- Energy appears in joules and kWh, with cost when applicable.
- The math is shown so you can reproduce it in your lab report.
The formula
Energy
E = P × t
Joules = watts × seconds. This calculator converts your hours to seconds for you.
Kilowatt-hours
kWh = joules ÷ 3.6 × 10⁶
1 kWh is exactly 3.6 megajoules — the unit your electricity bill is priced in.
Cost
cost = kWh × rate
With a rate like $0.20/kWh, a 100 W lamp on for 10 h costs 1 kWh × $0.20 = $0.20.
Worked example
Worked example — the gaming PC question
- Gaming PC: 500 W average, used 4 h/day
- Daily energy = 0.5 kW × 4 h = 2 kWh
- At $0.20/kWh → $0.40/day → ~$12/month
2 kWh per day — about $12 a month at $0.20/kWh. That's roughly a quarter of a typical student electricity bill.
Common mistakes
- Mixing watts with kilowatts — 100 W is 0.1 kW; forgetting the 1000× costs you on the bill math.
- Confusing W (power) with Wh (energy) — a 100 W bulb left on for 10 h uses 1000 Wh = 1 kWh.
- Using the surge power instead of average — PCs and fridges draw peak power only briefly; average matters for energy.
Frequently asked questions
How do I read energy from my electricity bill?
Bills charge per kWh. Your monthly kWh = (total watts of devices × hours used) ÷ 1000, summed across everything.
What's the difference between W, Wh and kWh?
W is power (rate), Wh is energy over an hour, and kWh is 1000 Wh — the billing unit. 1 kWh = 3.6 million joules.
Can I estimate the cost of my laptop?
Yes — a 65 W laptop charger used 6 h/day draws 0.39 kWh/day. At $0.20/kWh that's under $2.50/month.
Is the calculator exact for real devices?
It's exact for constant power. Real devices vary (standby, load), so treat the result as a solid estimate.
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