Electrical Power Calculator

Calculate electrical power in watts from voltage, current, and resistance using P = V × I, P = I²R, and P = V²/R — with resistor wattage and heat-loss guidance.

Power Calculator

Compute power dissipation from current and resistance (P = I²R) or voltage and resistance (P = V²/R), with heat-loss and resistor-rating guidance.

Fill any two — leave the rest blank

VVoltage

ICurrent

RResistance

PPower

What Is Electrical Power?

Electrical power is the rate at which a circuit converts electrical energy into another form — heat, light, motion, or sound — measured in watts (W). One watt is exactly one joule of energy delivered every second, or equivalently one volt pushing one amp. Whenever you size a resistor, pick a power supply, choose a heat sink, or read a utility bill, you are working with the power equation.

This power calculator solves wattage from any two of voltage, current, and resistance using the three equivalent forms P = V × I, P = I²R, and P = V²/R. It also returns the full set of Ohm's Law quantities so you can immediately check the current a load draws or the resistance it presents, and it flags whether a standard 1/8 W, 1/4 W, 1 W, or 5 W resistor can safely dissipate the heat.

This is one mode of the full Ohm's Law Calculator — you can also jump to the voltage drop calculator or the all-in-one unit converter for related electrical work.

How Electrical Power Is Calculated

Power = volts × amps

The most direct form: P = V × I. A 12 V supply delivering 2 A puts out 24 watts. This is the form your utility meter uses and the easiest to reason about for a complete load.

Power = current squared × resistance

P = I²R is the heat-loss form. Because current is squared, doubling the current quadruples the heat — which is why undersized wire and resistors fail thermally long before they fail electrically.

Power = voltage squared ÷ resistance

P = V²/R is handy when you know the supply voltage and the resistor value. A 100 Ω resistor across 10 V dissipates 1 W; across 20 V it dissipates 4 W — voltage is squared, so headroom matters.

Always check the wattage rating

A resistor that has the right ohm value can still burn out if it can't dissipate the heat. Pick a part rated for at least twice the calculated power to survive tolerance, ambient heat, and surge.

The Three Power Formulas

All three forms are algebraically identical given Ohm's Law (V = I × R). Use whichever matches the two quantities you already know — this calculator picks automatically.

From V and I

P = V × I

Power equals voltage times current. The canonical form, in watts.

From I and R

P = I² × R

Power equals current squared times resistance. The resistor heat-loss form.

From V and R

P = V² ÷ R

Power equals voltage squared divided by resistance. Used in heat-sink and resistor sizing.

How to Use the Power Calculator

1
Enter any two known values
Type any two of voltage, current, and resistance. The calculator solves the third and computes power using the matching formula.
2
Pick the right unit prefix
Each field has its own selector — millivolts to kilovolts, microamps to amps, ohms to megaohms, milliwatts to kilowatts — so you avoid scientific-notation mistakes.
3
Read the wattage and the heat guidance
The result card shows power in watts plus a recommended minimum resistor rating, so you know whether a 1/4 W part is fine or you need a 5 W ceramic.
4
Review the step-by-step working
Every formula used and every substituted number is shown, so you can learn the relationship and verify the answer by hand.

Key Power Concepts

Watt (W)

The SI unit of power: one joule per second. A 60 W bulb consumes 60 joules every second it is on. Kilowatts (1,000 W) and milliwatts (0.001 W) scale the same unit.

Power dissipation

The portion of electrical power that a resistor or wire turns into heat. It is wasted energy in a signal circuit but the entire point of a heater or incandescent lamp.

Resistor power rating

The maximum heat a resistor can shed before its value drifts or it fails. Common ratings: 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W, 5 W. Always derate for high ambient temperature.

Energy vs power

Power is the instantaneous rate (watts); energy is power accumulated over time (watt-hours). Your utility bills energy in kilowatt-hours — power multiplied by the hours it ran.

Real-World Power Calculations

💡

LED current resistor

Dropping 1.8 V at 20 mA through a series resistor dissipates only 36 mW — a 1/8 W resistor is plenty, with huge margin.

🔥

Resistive heater

A 120 V heating element drawing 12.5 A delivers 1,500 W of heat — the basis of space heaters, kettles, and toasters.

🎚️

Amplifier into a speaker

100 W into a 4 Ω speaker means √(P × R) = 20 V at 5 A. Used to size speaker wire and protect tweeters from over-power.

🔋

Battery discharge rate

A 100 W load on a 12 V battery pulls 8.3 A; on a 24 V battery only 4.2 A. Higher voltage at the same power means lower current and thinner wire.

🌡️

Heat-sink sizing

A voltage regulator dropping 5 V at 1 A dissipates 5 W — enough to need a heat sink, since a bare TO-220 can only shed about 2 W in still air.

⚙️

Motor power

A DC motor drawing 4 A at 24 V consumes 96 W of electrical power; mechanical output is that minus copper, iron, and friction losses.

Best Practices for Power Calculations

✓Size resistors for 2× the calculated power. Tolerance, ambient heat, and surge all eat into the nameplate rating. A resistor dissipating 0.4 W belongs on a 1 W part, not a 1/2 W one.
✓Watch the square terms. Because P = I²R and P = V²/R both square a variable, a 40% over-voltage nearly doubles the heat. Small input errors become large wattage errors.
✓Convert to consistent units first. Mixing milliamps with amps is the most common power-calculation error — it lands you off by a factor of a million. Let the calculator handle the prefixes.
✓Account for duty cycle. A resistor that pulses 5 W for 10% of the time averages 0.5 W, but the peak still has to survive 5 W instantaneously. Rate for the peak, budget energy for the average.
✓Derate at high temperature. Most resistors are rated at 70°C and must be derated linearly to zero by 155°C. In a hot enclosure, a 1 W part may only safely dissipate half that.

Common Power-Calculation Mistakes

Ignoring the wattage check

Choosing a resistor by ohm value alone and forgetting the I²R heat is the fastest way to cook a 1/8 W part. Always compute the power too.

Forgetting current is squared

Doubling current quadruples heat. People expect a linear rise and undersize conductors and resistors as a result.

Confusing watts and watt-hours

Watts measure rate; watt-hours measure energy. A 100 W panel does not store 100 Wh — it produces 100 W while the sun shines.

Using nameplate voltage under load

Real voltage sags under load, and since P = V²/R the dissipation changes with it. Measure the actual voltage at the component when it matters.

Why Power Calculations Matter

Power is what turns into heat, and heat is what destroys electronics. Get the wattage wrong and resistors char, regulators shut down, wires melt their insulation, and battery packs vent. Getting it right is the difference between a circuit that runs cool for years and one that fails in minutes.

Power also drives cost and capacity: it sizes your power supply, your heat sinks, your fuses, your wiring, and ultimately your energy bill. Every watt you save in dissipation is a watt you don't have to remove with a fan or pay for at the meter, which is why power budgeting is one of the first things any electrical designer does.

Built for electronics hobbyists, electrical engineers, technicians, and students sizing resistors, power supplies, and heat sinks.

Formulas cross-checked against standard electrical engineering references — see our methodology and editorial policy. Educational only — always confirm critical designs with a licensed electrician and your local electrical code.

Power Calculator FAQs

Electrical power in watts is calculated with P = V × I (voltage times current). If you only know current and resistance, use P = I² × R; if you know voltage and resistance, use P = V² ÷ R. All three give the same answer because they are derived from Ohm's Law. Enter any two of voltage, current, and resistance above and the calculator returns the power automatically.

The three equivalent power formulas are P = V × I, P = I² × R, and P = V² ÷ R, where P is power in watts, V is voltage in volts, I is current in amperes, and R is resistance in ohms. Pick the form that matches the two quantities you already know — for example, use P = I²R when you know the current through a resistor and its value.

12 volts at 2 amps is 24 watts, because P = V × I = 12 × 2 = 24 W. The same 24 W could also come from 24 V at 1 A or 6 V at 4 A — power is the product of voltage and current, so you can trade one for the other at constant wattage.

Use P = I² × R if you know the current through the resistor, or P = V² ÷ R if you know the voltage across it. For example, 0.1 A through a 100 Ω resistor dissipates 0.1² × 100 = 1 W, so you would choose a resistor rated at 2 W or more to be safe.

Because power dissipated in a resistance is P = I²R — the current appears twice (once for the charge flow and once for the voltage that current creates across the resistance, V = IR). That squaring is why doubling the current quadruples the heat, and why high-current circuits need disproportionately thicker wire.

Calculate the power the resistor will dissipate (P = I²R or V²/R), then choose a standard rating at least twice that value to allow for tolerance, ambient temperature, and surge. A resistor dissipating 0.4 W should use a 1 W part; one dissipating 2 W should use a 5 W part.

A kilowatt is 1,000 watts. Small electronics are rated in watts or milliwatts (thousandths of a watt), household appliances in watts or kilowatts, and industrial or grid equipment in kilowatts or megawatts. The converter in this tool switches between milliwatts, watts, and kilowatts instantly.

The calculator uses exact algebra, so its precision far exceeds any practical need (about 15 significant digits). Real-world accuracy is limited only by your input values and unmodeled effects like wire resistance and temperature drift — verify critical designs with a meter under load.