Power dissipation calculator

What "power" means on a breadboard

Every component that lets current through at a cost — a resistor, an LED, a motor coil — turns some of that electrical energy into another kind: heat, light, motion. Power is the rate of that conversion, measured in watts (W). One watt is one joule per second. For a resistor, every bit of that power comes out as heat.

Three formulas, same answer, each useful in a different story:

• P = V × I — "the voltage across it times the current through it". Natural when you already know both.
• P = I² × R — "the current squared, times the resistance". Natural when the current is set by the rest of the circuit (e.g. an LED driver).
• P = V² ÷ R — "the voltage squared, divided by the resistance". Natural when the voltage is pinned (e.g. battery straight across a resistor, which is what this calculator shows).

Ohm's law (V = I × R) is the glue between the three. They all give the same wattage; pick the one with the two numbers already in your hand.

Standard resistor wattages

Through-hole resistors come in a short ladder of power ratings:

• ⅛ W (125 mW) — tiny, usually in SMD or dense kits.
• ¼ W (250 mW) — the beginner default. Everything on 3–6 V with sensible resistors sits inside this.
• ½ W (500 mW) — a step up when 9 V meets a smaller resistor.
• 1 W (1 000 mW) — motor drives, LED strings, voltage-divider current sinks.
• 2 W and 5 W — physically larger, often ceramic. Rare on a hobby breadboard; past this, the circuit wants a different approach (a MOSFET, a resistor in parallel, or a lower voltage).

The "2× headroom" rule above means you look up the dissipated power, double it, and pick the smallest standard size that still comes out on top. A resistor rated at ¼ W will work at 240 mW, but it will run hot — the paint discolours and the value drifts. Doubling buys quiet, long-lived parts.

Rough numbers to keep in your head

• 4.5 V × 220 Ω → 20 mA → 90 mW — safe for ⅛ W, comfortable for ¼ W.
• 9 V × 220 Ω → 41 mA → 370 mW — needs ½ W.
• 9 V × 100 Ω → 90 mA → 810 mW — needs 1 W.
• 9 V × 10 Ω → 900 mA → 8.1 W — past normal breadboard parts; the resistor smokes and the battery heats, too.

The pattern: as voltage goes up, power rises with the square of voltage, not just linearly. Doubling the battery quadruples the heat for a fixed resistor. That's why "9 V popped my LED" is usually also "9 V smoked my resistor".

What this calculator leaves out

A real breadboard has more parts than a single resistor. In series with an LED, the LED keeps about two volts for itself, so the resistor only sees the remainder. Pop the LED's forward voltage off first, then feed the leftover into this calculator (or use the LED resistor calculator, which does both steps for you). For a voltage divider under load, the two resistors each dissipate their own share; the series/parallel calculator prints per-part power.

Common questions

Why does my resistor get hot? Current through resistance makes heat (P = V × I). A ¼ W resistor sheds about 250 mW before it gets too hot to touch. Push 500 mW and the paint darkens; push 1 W and it smokes. Fix with a bigger wattage, a bigger resistance, or a smaller battery.

What wattage do I need? Compute the power, double it, pick the smallest standard size that still comes out on top. 100 mW → ¼ W. 300 mW → ½ W. 700 mW → 1 W.

Can I use a bigger wattage than I need? Always. A 1 W where ⅛ W would do is cooler and bigger; the circuit is identical. The resistance is what matters electrically.

Do LEDs have a wattage? Yes, but you size them by current (mA). A 5 mm LED wants about 20 mA; use the LED resistor calculator to pick a series resistor that holds it there.

For the equation this calculator rests on, read the Ohm's law calculator or the Ohm's law skill page. Build the real loop in First Light.

Rules of thumb verified against: SparkFun — Resistors · AAC — Power in Electric Circuits · Wikipedia — Resistor power dissipation