Power Calculator: Ohm’s Law Power and Dissipation
Quick Start
- Select which two values you know: voltage (V), current (A), or resistance (Ω).
- Enter values and units. SI prefixes are supported where relevant.
- Read the power in watts; large values may be shown in kW.
- Adjust inputs to compare scenarios.
Input Formats
- Accepts integers and decimals; unit symbols are optional.
- Supports SI prefixes: m, k, M where applicable.
- Empty or invalid fields are ignored until a valid pair exists.
Choose the Equation
- P = V × I
- P = I² × R
- P = V² ÷ R
The calculator picks the appropriate form based on the provided inputs.
How It Works
- Internally normalizes values to base SI units.
- Displays power with sensible rounding.
- Notes when power approaches common rating thresholds.
Worked Examples
- 5 V and 20 mA → 0.10 W (P = V × I)
- 12 V across 330 Ω → ~0.44 W (P = V² ÷ R)
- 100 mA through 100 Ω → 1.00 W (P = I² × R)
Common Mistakes
- Mixing mA with A, or kΩ with Ω.
- Using P = V²R instead of P = V² ÷ R.
- Using supply voltage instead of the voltage across the component.
- Ignoring that current changes when resistance changes.
Design Tips
- Pick a resistor with a rating ≥ 2× the calculated power (derating).
- Account for ambient temperature, airflow, and enclosure.
- For pulsed loads, use RMS or duty‑cycle averaged power.
- Check package limits and thermal paths.
FAQ
Which formula does the tool use?
It selects among P=VI, P=I²R, or P=V²/R based on the inputs you provide.
Why is my result slightly rounded?
For readability; calculations use full precision internally.
Can I enter mA or kΩ?
Yes. SI prefixes like m, k, M are supported where appropriate.
Does this pick a wattage rating for me?
It reports power only. Choose a rating with margin, e.g., ≥ 2×.