Ohm's Law Calculator (V = I × R)
Calculate voltage, current or resistance using Ohm's Law (V=I×R). The calculator also computes electric power. Fast, free and no registration required.
Science
Calculator · Science
Electric Power Calculator (P = U × I)
Electric power P describes how fast a device converts electrical energy into other forms — heat, light or motion. The core formula P = U × I links power (W), voltage (V) and current (A). This calculator lets you find any one of these three quantities when the other two are known. Results are shown in watts (W) and kilowatts (kW), and when both voltage and current are available the circuit resistance is computed as a bonus. The tool is useful for students, electricians, electronics hobbyists and anyone checking the parameters of a household appliance, sizing a fuse or estimating energy consumption.
How the electric power calculator works
The calculator supports three calculation modes: • Power mode: P = U × I — enter voltage (V) and current (A) to get power in watts and kilowatts. • Voltage mode: U = P / I — enter power (W) and current (A) to get voltage in volts. • Current mode: I = P / U — enter power (W) and voltage (V) to get current in amperes. Additionally, when both voltage and current are known, the calculator computes resistance R = U / I in ohms. All results are rounded to 4 significant figures. Division by zero is guarded — if voltage or current is 0, the result is returned as 0 rather than an error.
Example: light bulb 230 V / 0.26 A
A light bulb connected to a 230 V supply draws a current of 0.26 A. We calculate power: P = U × I = 230 × 0.26 = 59.8 W ≈ 60 W P in kW = 59.8 / 1000 = 0.0598 kW R = U / I = 230 / 0.26 ≈ 884.6 Ω Second example — electric kettle rated 2000 W on a 230 V supply: I = P / U = 2000 / 230 ≈ 8.696 A Knowing the current allows you to choose the right cable cross-section and fuse rating.
Frequently asked questions
What is electric power?
Electric power P (measured in watts, W) is the rate at which electrical energy is transferred or converted. The fundamental formula is P = U × I, where U is voltage in volts and I is current in amperes. One watt equals one joule per second.
How do I calculate electric power from voltage and current?
Use the formula P = U × I. Example: a device connected to a 230 V supply drawing 4 A consumes P = 230 × 4 = 920 W. Select 'Calculate power' mode, enter the voltage and current, and the calculator returns the power in both watts and kilowatts.
How do I calculate current from power and voltage?
Use the formula I = P / U. Example: a 2000 W electric kettle on a 230 V supply draws I = 2000 / 230 ≈ 8.7 A. This is useful for sizing fuses and cable cross-sections.
How do I calculate voltage from power and current?
Use the formula U = P / I. Example: a 100 W motor drawing 2 A operates at U = 100 / 2 = 50 V. This formula is often used when designing low-voltage circuits.
What is the difference between watts (W) and kilowatt-hours (kWh)?
A watt (W) is a unit of power — it describes how fast energy is consumed at a given moment. A kilowatt-hour (kWh) is a unit of energy — the total energy used over time. 1 kWh = 1000 W × 1 hour. Example: a 2000 W heater running for 3 hours uses 6 kWh of energy.
What is the typical power consumption of household appliances?
Typical power ratings: LED bulb 5-15 W, refrigerator 100-400 W, television 50-150 W, microwave 700-1200 W, washing machine 800-2500 W, electric iron 1000-2400 W, electric kettle 1500-3000 W, desktop computer 200-600 W.
What is resistance and how is it related to power?
Resistance R (measured in ohms, Ω) is the opposition a conductor offers to current flow. Power can also be expressed as P = I² × R or P = U² / R. When both voltage and current are known, R = U / I. The calculator automatically shows resistance alongside the power result.
What is a kilowatt (kW) and when is it used?
One kilowatt (kW) equals 1000 W. Kilowatts are used to describe higher-power devices: electric motors, heating systems, electric vehicles and household appliances. The calculator automatically converts the result from watts to kilowatts.
Why is the mains voltage in Europe 230 V?
European countries, including Poland, use the 230 V / 50 Hz standard defined by CENELEC EN 50160. A higher voltage allows the same power to be delivered at a lower current, reducing energy losses in cables. The 230 V figure is the RMS (root mean square) value of the AC waveform.
How do I use the electric power calculator?
Choose the calculation mode (power, voltage or current), enter the two known values in the corresponding fields and press Calculate. The calculator instantly displays the result rounded to four significant figures, along with the power in kilowatts and, where applicable, the circuit resistance.
Indicative result
The calculator is for educational and informational purposes. Results are computed using P = U × I, U = P / I and I = P / U. Actual device parameters may differ. Always consult a qualified electrician before working on electrical installations.
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