Electric Power Calculator
Find electrical power and resistance from voltage and current.
Results update as you type.
How it works
Electrical power is the rate at which a circuit converts energy into heat, light, or motion, and this electric power calculator finds it directly from the voltage across a component and the current flowing through it. It also returns the resistance implied by those two values, giving a fuller picture of how the component behaves. Power is what your electricity meter measures, what determines whether a wire runs cool or hot, and what a device's rating on its label actually describes. Understanding power matters whenever you size a supply, choose a fuse, or estimate running costs. A phone charger, a kettle, and an electric motor differ mainly in how much power they draw, and getting that figure wrong can mean an overloaded circuit or a component that overheats. Because power is simply voltage multiplied by current, the calculator removes the arithmetic and lets you compare devices at a glance. The relationship holds because each coulomb of charge carries an amount of energy set by the voltage, and current counts how many coulombs pass every second, so their product is energy per second — power in watts. Electricians, hobbyists, and students use an electric power calculator to check ratings, plan wiring, and understand exactly how much energy their circuits consume.
power = voltage × current
Power equals voltage multiplied by current because voltage is the energy carried per unit of charge and current is the charge flowing per second, so multiplying them gives energy delivered per second, which is power in watts. Resistance follows from Ohm's law as voltage divided by current, revealing how strongly the component opposes the flow. Together these two results describe both the energy use and the electrical character of the component.
Source:Standard physics (electrical power)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter voltage and current.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Power updates instantly as you type — no button to press.
Frequently asked questions
What units does this calculator use?
Enter voltage in volts and current in amperes, and the power comes out in watts, the SI unit of power. The resistance is returned in ohms. If your current is measured in milliamps, divide by one thousand to convert it to amperes first. Keeping voltage in volts and current in amperes ensures the power reads correctly in watts without any additional scaling on your part.
How is electrical power different from energy?
Power is the rate at which energy is used, measured in watts, while energy is the total amount consumed over time, measured in joules or kilowatt-hours. A 100-watt bulb draws energy at a steady rate, and running it for longer uses more energy without changing its power. Your electricity bill charges for energy, so it multiplies each device's power by the hours you run it.
Why does the calculator also show resistance?
Resistance is closely tied to power in a circuit, so showing it gives you a fuller picture from the same two inputs. Using Ohm's law, resistance equals voltage divided by current, revealing how strongly the component limits the flow of charge. Knowing resistance alongside power helps you check whether a value looks reasonable and lets you cross-reference results against a resistor's marked rating or a datasheet figure.
Can I calculate power without knowing current?
This calculator needs both voltage and current, but if you know voltage and resistance instead, you can find power with the equivalent formula voltage squared divided by resistance. Likewise, current squared times resistance gives power when you know current and resistance. These forms all come from combining the power equation with Ohm's law, so any two of voltage, current, and resistance are enough to determine the power.
What happens if the current is zero?
If the current is zero, no charge is flowing, so the power is zero regardless of the voltage present — an open switch delivers no energy. The resistance in that case is undefined, because dividing voltage by zero current has no finite value, so the calculator returns a dash for resistance. A zero-current reading simply means the circuit is open or the component is not conducting.