# Electric Power > Electric power tells you how fast a device uses energy, and you get it by multiplying the volts by the amps to get watts. Page: https://formulabro.yunusemrevurgun.com/electricity/electric-power/ Category: Electricity Published: 2026-10-22 Updated: 2026-10-22 ## Formula $$ P = V I $$ Read it out loud: "P equals V times I. Power in watts equals volts times amps." ## What each symbol means - $P$: **Power**, how fast electrical energy is used or delivered, measured in watts (W). One watt is one joule of energy per second. - $V$: **Voltage**, the electrical push across the device, measured in volts (V). - $I$: **Current**, the flow of charge through the device, measured in amperes (A). ## What the formula is doing Power is a rate: it says how much energy a device uses or delivers each second. Its unit is the watt (W), named after James Watt. A device rated at 2000 W uses 2000 joules of energy every second while it runs. The formula $P = V I$ says power is the voltage times the current. Voltage $V$ is the push, in volts (V). Current $I$ is the flow, in amperes (A). A bigger push or a bigger flow gives more power, and multiplying the two gives watts. Ohm's law lets you rewrite the same formula with resistance $R$ in ohms ($\Omega$). Since $V = I R$, you get $P = I^2 R$ when you know the current, and $P = \frac{V^2}{R}$ when you know the voltage. All three versions give the same answer for a resistor, so use whichever pair of numbers you already have. Mains voltage is dangerous. It is 230 V in much of the world and 120 V in North America, and it can cause a lethal shock. Do not open mains devices such as kettles, heaters or chargers to test them, and get a qualified person to repair them. ## An everyday picture Think of a water wheel. The voltage is how high the water falls, the current is how much water flows past each second, and the power is how hard the wheel turns. A tall fall with a big flow turns the wheel hard. A tall fall with a trickle, or a big flow over a short drop, does much less. ## Worked example: A 2000 W kettle on 230 V and on 120 V A kettle is rated at $P = 2000\ \text{W}$. Mains supplies vary by region: first the kettle runs on a 230 V supply, then the same 2000 W is needed from a 120 V supply. How much current does it draw in each case? Do not open the kettle to check; the wiring is at mains voltage. 1. Rearrange $P = V I$ to get the current: $I = \frac{P}{V}$. 2. On 230 V: $I = \frac{2000\ \text{W}}{230\ \text{V}} \approx 8.70\ \text{A}$. 3. On 120 V: $I = \frac{2000\ \text{W}}{120\ \text{V}} \approx 16.7\ \text{A}$. 4. Check the 230 V case: $8.70\ \text{A} \times 230\ \text{V} = 2001\ \text{W}$, which is 2000 W with rounding. **Answer:** The kettle draws about **8.70 A** on 230 V. The same 2000 W on 120 V draws about **16.7 A**, nearly twice the current, so the wiring has to carry much more current for the same power. ## Common mistakes - Mixing up watts and watt-hours. A watt is a rate, while a watt-hour is an amount of energy. The page on energy and kilowatt-hours explains the difference. - Forgetting to convert units. 20 mA is 0.02 A, not 20 A, so convert milliamps to amps before you multiply. - Assuming the power stays the same when the voltage changes. For a fixed resistance, power grows with the square of the voltage, $P = V^2 / R$, so a small change in voltage makes a big difference. - Opening a mains device to measure its power. The voltage can cause a lethal shock, and the wiring stays dangerous even when the device is switched off. ## Where you will meet it - Appliance labels and electricity bills, where each device is rated in watts. - Choosing a fuse, circuit breaker or power supply that can carry the current a device will draw. - Rating solar panels, batteries and chargers, which list their output power in watts. - Heaters and electric motors, where the power rating shows how much heat or work the device can deliver. ## Questions people ask ### What is the difference between a volt and a watt? A volt measures the push (voltage). A watt measures the rate of energy use (power). A device can have a high voltage and still use little power if very little current flows. ### Can I use P = V I for a battery? Yes. For a battery, $V$ is the voltage it delivers and $I$ is the current it supplies, so $P$ is the power it hands to the device. ### How many watts are in a kilowatt? One kilowatt (kW) is 1000 W. A 2 kW heater uses 2000 W while it runs. ### Are $P = V I$ and $P = I^2 R$ different formulas? They are the same formula for a resistor, because Ohm's law links $V$ and $I$ through $R$. Pick whichever pair of numbers you have. ## Related formulas - [Ohm's Law](https://formulabro.yunusemrevurgun.com/electricity/ohms-law/) - [Energy and Kilowatt-Hours](https://formulabro.yunusemrevurgun.com/electricity/energy-and-kilowatt-hours/) - [Resistors in Series and Parallel](https://formulabro.yunusemrevurgun.com/electricity/resistors-in-series-and-parallel/) ## Sources - [Electric power, Wikipedia](https://en.wikipedia.org/wiki/Electric_power) - [Watt, Wikipedia](https://en.wikipedia.org/wiki/Watt) - [Volt, Wikipedia](https://en.wikipedia.org/wiki/Volt) From Formula Bro (https://formulabro.yunusemrevurgun.com/), a side project developed by yunusemrevurgun.com.