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Instant Calculation
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The real (average) power dissipated by a resistive AC load depends on the RMS (root-mean-square) voltage and current — the values a multimeter actually reads on an AC circuit, not the peak voltage. This tool calculates power (and any other missing value) from RMS voltage, RMS current, and resistance.
How it works
P = Vrms × Irms = Vrms² / R = Irms² × R. These are the same underlying relationships as Ohm's Law and P = VI, just framed specifically for RMS AC quantities. Enter any two of {Vrms, Irms, R, P} and the other two are solved for.
- Enter rMS Voltage (V) — leave blank to solve for this.
- Enter rMS Current (A) — leave blank to solve for this.
- Enter resistance (Ω) — leave blank to solve for this.
- Enter power (W) — leave blank to solve for this.
- Click Calculate to see your results.
Examples
A household AC circuit
120 Vrms across a 60 Ω resistive load draws 2 A RMS and dissipates 240 W of real power.
Who should use it
- Calculating power dissipation in a resistive AC circuit or heating element.
- Electronics and electrical engineering coursework.
Industry applications
- Electrical engineering
- Electronics design
- HVAC and appliance heating elements
Advantages
- Solve for power, voltage, current, or resistance — whichever two you know.
- Uses the RMS values that actually match multimeter readings on AC circuits.
Limitations
- Assumes a purely resistive load — does not account for power factor on reactive loads.
Common mistakes to avoid
- Applying these formulas to a reactive (inductive/capacitive) AC load without accounting for power factor — they're only exact for purely resistive loads.
Best practices
- For any load with a significant reactive component (motors, transformers, many appliances), use apparent and real power calculations that include power factor rather than this purely resistive formula.
Tips
- If your load has any significant inductive or capacitive component (like a motor), use a power-factor-aware apparent power calculation instead of assuming purely resistive behavior.