Electrical Power Calculator
Find power consumption, energy usage, and electricity cost.
Calculate
Formulas
P = V × IPower in Watts
Cost = P × t × rate / 1000Energy cost from power, time, and electricity rate
Power & Energy
A 2000W heater running 5 hours uses 10 kWh. At €0.35/kWh, that costs €3.50. Use this to estimate monthly appliance costs.
How Electrical Power Is Calculated
Power is the rate at which electrical energy is converted to another form — heat, light, or motion. In a DC or resistive AC circuit it is simply voltage multiplied by current:
P = V × IP is power in watts, V is voltage in volts, I is current in amps.
Using Ohm's law, power can also be written as P = I²R (useful when current and resistance are known) or P = V²/R (when voltage and resistance are known). All three give the same answer.
Worked Examples
A device drawing 2.5 A from a 230 V mains supply:
P = 230 × 2.5 = 575 W
A 10 Ω heating element carrying 6 A:
P = I²R = 6² × 10 = 360 W
Energy and Cost
Power (watts) measured over time gives energy (watt-hours). To estimate running cost, multiply power in kilowatts by hours of use by your electricity tariff.
| Device | Typical power | Cost / hour at €0.30/kWh |
|---|---|---|
| LED bulb | 10 W | €0.003 |
| Laptop | 60 W | €0.018 |
| Microwave | 1000 W | €0.30 |
| Electric kettle | 2000 W | €0.60 |
Frequently Asked Questions
What is the difference between watts and watt-hours?
Watts measure the rate of energy use at an instant; watt-hours measure total energy consumed over time. A 100 W bulb on for 10 hours uses 1000 Wh (1 kWh).
Why is AC power sometimes different?
For reactive AC loads, real power (W) is less than apparent power (VA) by the power factor. Use the power factor calculator for those cases.
Which formula should I use?
Use whichever matches your known values: V and I → P=VI; I and R → P=I²R; V and R → P=V²/R.