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W
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kWh
Enter values to see results
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W
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Wh
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kWh
Electrical power is the rate at which electrical energy is transferred by a circuit, measured in watts (W). This calculator computes power using three equivalent formulas derived from Ohm's Law and the fundamental power equation:
$$P = V \times I$$
$$P = I^2 \times R$$
$$P = \frac{V^2}{R}$$
where P is power in watts, V is voltage in volts, I is current in amperes, and R is resistance in ohms. Each formula is useful depending on which two quantities are known.
The first formula, P = VI, directly multiplies voltage and current. It is the most general form and applies to any circuit element. The second, P = I²R, is derived by substituting V = IR into P = VI and is especially useful when you know the current through a resistor but not the voltage across it. The third, P = V²/R, substitutes I = V/R and is ideal when voltage is known but current is not.
Power dissipation is critical for component selection. Every resistor, wire, and semiconductor has a maximum power rating. Exceeding it causes overheating, degraded performance, and potential fire hazards. A standard through-hole resistor is typically rated at 0.25 W or 0.5 W, while power resistors handle 1 W to hundreds of watts with appropriate heat sinking.
In household and industrial applications, power consumption determines energy costs. The kilowatt-hour (kWh), equal to 1000 watts consumed for one hour, is the standard billing unit. This calculator also estimates hourly and daily energy consumption to help with cost planning.
For AC circuits, power analysis involves apparent power (VA), real power (W), and reactive power (VAR), related by the power factor. This calculator provides accurate results for DC circuits and purely resistive AC loads where the power factor equals 1. For reactive loads, multiply the result by the power factor to obtain real power.
Understanding electrical power is essential for sizing power supplies, selecting fuses and circuit breakers, calculating heat dissipation requirements, and estimating operating costs of electrical equipment.
Enter the voltage, current, and resistance values. The calculator computes power using all three formulas simultaneously: P = VI, P = I²R, and P = V²/R. If your inputs are consistent (V = IR), all three results will be identical. If they differ, you can identify which pair of inputs corresponds to your actual circuit. Energy consumption is calculated as P × time for planning purposes.
When all three power values match, your inputs are self-consistent and describe a valid Ohm's Law relationship. If the values differ, the V/I pair result (P = VI) reflects the actual power if voltage and current were measured directly. Always ensure component power ratings exceed calculated dissipation by at least 50% for reliability.
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A USB device drawing 500 mA at 5 V consumes P = 5 × 0.5 = 2.5 W. With R = V/I = 10 Ω, all three power formulas confirm 2.5 W. Over 24 hours this uses 0.06 kWh.
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A 120 V space heater with a 9.6 Ω element draws 12.5 A and consumes 1500 W (1.5 kW). Running 8 hours per day uses 12 kWh, costing approximately $1.44 at $0.12/kWh.
All three are mathematically equivalent for a resistive circuit obeying Ohm's Law. They are derived from combining P = VI with V = IR. Use whichever formula matches the two quantities you have measured or know.
If the three outputs differ, the voltage, current, and resistance inputs are not consistent with Ohm's Law (V ≠ IR). This means at least one input does not correspond to the same operating point. Use the pair of values you measured directly for the most accurate power calculation.
A kilowatt-hour is a unit of energy equal to 1000 watts consumed for one hour (3.6 megajoules). It is the standard billing unit for electricity. To calculate cost, multiply kWh by your electricity rate (e.g., $0.12/kWh).
Calculate the power dissipation using P = I²R or P = V²/R, then select a resistor rated for at least 1.5 to 2 times that value. Common ratings are 0.125 W, 0.25 W, 0.5 W, 1 W, 2 W, 5 W, and higher for power resistors.
For purely resistive AC loads (heaters, incandescent lamps), use RMS voltage and current values, and the calculator gives accurate real power. For reactive loads (motors, capacitors), multiply by the power factor: P_real = P_apparent × cos(φ).
Maximum power is delivered to a load when the load resistance equals the source's internal resistance: R_load = R_source. At this point, exactly half the total power is dissipated in the load and half in the source, giving 50% efficiency.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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