Wattage Calculator
Power Is What You Pay For
Utility bills are calculated in kilowatt-hours, which makes power — not voltage or current alone — the number that actually matters for cost and capacity planning. This calculator computes electrical power three different ways depending on which two quantities you already know, using the same underlying relationship each time.
The Formula
P = I² × R
P = V² / R
where P is power in watts, V is voltage in volts, I is current in amps, and R is resistance in ohms. All three forms come from combining Ohm's Law with the basic power relationship P = V × I.
Typical Household Loads
Comparing a calculated wattage against familiar appliances gives a useful sanity check:
| Device | Typical Wattage |
|---|---|
| LED light bulb | 8–12 W |
| Laptop computer | 30–65 W |
| Refrigerator (running) | 150–400 W |
| Microwave oven | 600–1200 W |
| Toaster | 800–1500 W |
| Hair dryer | 1200–1875 W |
| Portable space heater | 1500 W |
| Electric clothes dryer | 1800–5000 W |
Ranges reflect typical manufacturer specifications; check a specific appliance's nameplate for its actual rating.
Where This Calculation Matters
- Circuit capacity checks — adding up device wattage against a circuit's amperage rating (at a known voltage) before plugging in more equipment.
- Generator and inverter sizing — backup power equipment is rated in watts, so converting device current draw to wattage tells you what a generator needs to supply.
- Motor and heating element specification — the I²R form is standard for estimating power dissipated by a resistive load such as a heating coil.
- Energy cost estimates — wattage multiplied by hours of use and divided by 1000 gives kilowatt-hours, the unit on a utility bill.
How to Use This Calculator
- Select the calculation mode based on which two values you know: Voltage × Current, Current² × Resistance, or Voltage² ÷ Resistance.
- Enter the corresponding values in the input fields.
- Select Calculate to see the resulting power in watts along with the formula substitution.
Related Calculations
Work backward from voltage and current with the Ohm's Law Calculator, or check whether a wire can safely carry the resulting current with the Wire Size Calculator.