Power Calculator
Power is the rate at which a circuit uses energy — and there are three equivalent ways to compute it: P = V×I, P = V²/R, and P = I²×R. Enter any two of voltage, current and resistance; the calculator returns power plus the missing electrical quantity.
Electrical power
172.34 m W
Enter any two values — the third is solved with Ohm's law
- Power
- 172.34 m W
- Voltage
- 9 V
- Current
- 19.15 m A
- Resistance
- 470 Ω
P = V × I = V² ÷ R = I² × R. Works with DC or RMS AC values for resistive loads.
How it works
- Enter any two of: voltage (V), current (I), resistance (R).
- Unit prefixes (k, M, m, µ, n) are supported — 4.7 kΩ and 25 mA work directly.
- The missing quantity is solved with Ohm's law.
- Power is derived: P = V × I.
- The result shows watts with an engineering-style label (mW, W, kW).
The formula
Power
P = V × I = V² ÷ R = I² × R
All three forms are equivalent through Ohm's law — use whichever two quantities you have.
Ohm's law
V = I × R
Used to solve the missing value before computing power.
Worked example
Worked example
- A 9 V battery driving a 470 Ω resistor
- Current = 9 ÷ 470 = 19.1 mA
- Power = 9 × 0.0191 = 0.172 W
172 mW — comfortably within a ¼ W resistor's rating. Rule of thumb: choose a resistor rated at least 2× the dissipated power.
Common mistakes
- Using peak values for AC power — use RMS values for resistive loads.
- Forgetting prefix conversions — 25 mA is 0.025 A, and mixing units gives results off by 1000×.
- Choosing an under-rated resistor — 0.5 W through a ¼ W part will smoke it (literally).
Frequently asked questions
What's the difference between power and energy?
Power is the rate (watts); energy is the amount over time (watt-hours or joules). Use the electrical energy calculator for P × t.
Does this work for AC circuits?
For purely resistive AC circuits, yes — use RMS values. For reactive loads (capacitors, inductors), you need the power factor and apparent power.
Why do resistors have wattage ratings?
Because dissipated power becomes heat. Exceeding the rating can change the value or destroy the component.
What is a good rule for resistor ratings?
Pick a resistor rated at least double the calculated dissipation — e.g. use a ½ W resistor when the circuit dissipates 250 mW.
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