⚡ Electrical Unit Converter
Two Values Unlock All Four
Ohm's law V = I × R plus the power formula P = V × I tie voltage, current, resistance, and power into one closed system: give any two and the other two are determined. The derived forms earn their keep too: P = I² × R (why undersized wires get hot) and P = V² ÷ R.
The Classic Use: Sizing an LED Resistor
Every electronics beginner meets this calculation. To run an LED (forward voltage ~2V, target current 15 mA) from a 5V Arduino pin:
- Voltage the resistor must drop: 5 − 2 = 3V
- R = V ÷ I = 3 ÷ 0.015 = 200Ω (use the next standard value up, 220Ω, for margin)
- Resistor power: P = V × I = 3 × 0.015 = 0.045W — a common 1/4 W resistor has plenty of headroom
Enter 3 (volts) and 0.015 (amps) above to verify. The same three steps size resistors for any voltage rail — 3.3V boards just drop less.
Everyday Numbers Worth Knowing
- USB charging: classic USB is 5V × 0.5A = 2.5W; a 30W fast charger pushes ~2A at 15V or 3A at 10V via USB-PD negotiation. Wattage = the honest speed rating of a charger.
- Wall power: a 1,500W kettle on a 120V US circuit draws 12.5A — most of a 15A household breaker by itself. The same kettle in a 230V country draws only 6.5A, which is why European kettles boil faster.
- Energy cost: a kilowatt-hour is 1,000W for one hour. A 60W-equivalent LED bulb (9W actual) burning 5 h/day uses ~16 kWh a year — power × hours ÷ 1000 prices any appliance.
- Wire heating: doubling current through the same wire quadruples the heat (P = I²R) — the physics behind fuse ratings and extension-cord fires.
Frequently Asked Questions
Why exactly two inputs?
One value underdetermines the system (110V pairs with any current); three can contradict each other. Two known values uniquely fix the remaining two — that's the algebra of these equations.
Does this work for AC circuits?
For resistive loads (heaters, incandescent bulbs), yes, using RMS values. Motors and switching supplies have a power factor below 1, so actual current runs higher than P÷V — budget ~20% margin, or use PF-aware formulas for precision.
What happens if I plug a 120V appliance into 230V?
For a resistive device, P = V²÷R means double the voltage gives four times the power — it burns out. Check the nameplate: "100–240V" switching adapters are safe worldwide; single-voltage devices need a transformer.
Volts, amps, watts — which one "kills"?
Physiologically it's current through the body that matters (tens of milliamps can be dangerous), but voltage is what pushes current through skin resistance. Treat anything above 50V with respect, and leave mains wiring to licensed electricians.