Introduction
Power is the rate at which energy is used or transferred. Whether you are working with a simple DC circuit, an AC system with phase angles, or just trying to figure out how much an appliance costs to run, knowing how to calculate power is an important skill. The basic formula is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. From this one relationship and Ohm's Law (V = I × R), you can solve for any unknown value in a circuit as long as you know at least two of the four quantities: voltage, current, resistance, and power.
This Power Calculator gives you five tools in one place. The DC Power tab lets you enter any two known values and instantly solves for the other two. The AC Power tab handles phasor-based calculations, giving you real power, reactive power, apparent power, and power factor. The Energy Cost tab estimates how much it costs to run a device over time based on its wattage, daily usage, and your electricity rate — similar to our dedicated Electricity Cost Calculator. The Voltage Drop tab helps you pick the right wire gauge for a cable run by calculating how much voltage is lost over distance. Finally, the Reference tab provides a handy Ohm's Law power wheel, a copper and aluminum wire ampacity table, and common unit conversions. Just enter your numbers, hit Calculate, and get clear results right away.
How to Use Our Power Calculator
Enter your known electrical values into any of the five tabs below, and the calculator will solve for the missing values and display detailed results.
DC Power Tab: Enter exactly 2 of the 4 fields — Voltage (V), Current (I), Resistance (R), or Power (P) — and the calculator will solve for the other 2 using Ohm's Law and the power formula. If you need to work specifically with Ohm's Law relationships, you can also try our Ohms Law Calculator. You can pick different units like millivolts, kilowatts, or megaohms from the dropdown next to each field.
AC Power Tab: Enter 2 complete phasor quantities by filling in both the magnitude and the angle (in degrees) for any 2 of the 4 fields — Voltage (V), Current (I), Impedance (Z), or Complex Power (S). The calculator will find the remaining 2 phasors and also show you real power, reactive power, apparent power, and power factor. For dedicated impedance analysis, see our Impedance Calculator.
Energy Cost Tab: Enter the power consumption of your device in watts or kilowatts, the number of hours you use it each day, your electricity rate in dollars per kilowatt-hour, and the total number of days. The calculator will show you the energy used and the cost broken down by day, week, month, and year.
Voltage Drop Tab: Enter your source voltage, choose single phase or three phase, enter the load current in amps, type in the one-way wire length in feet or meters, pick your wire material (copper or aluminum), and select the wire gauge (AWG). The calculator will show the voltage drop, the percentage of drop, the voltage at the load, the power lost in the wire, and whether the result meets NEC recommendations. For a more detailed analysis, you can also use our Voltage Drop Calculator or our Wire Size Calculator.
Reference Tab: Browse the Ohm's Law power wheel diagram, a copper and aluminum wire ampacity table based on NEC 310.16, and a quick chart of metric unit prefixes for easy conversions.
What Is Power in Physics?
Power is the rate at which energy is used, produced, or transferred over time. In simple terms, it tells you how fast work gets done. A 100-watt light bulb uses energy twice as fast as a 50-watt bulb. The standard unit of power is the watt (W), named after James Watt. One watt equals one joule of energy per second. Since power is fundamentally about how quickly energy changes, it is closely related to concepts like kinetic energy and potential energy — the difference is that power measures the rate of energy transfer rather than the total amount.
The Key Power Formulas
In electrical circuits, power connects four basic quantities: voltage (V), current (I), resistance (R), and power (P) itself. The most common formula is P = V × I, which says power equals voltage times current. From Ohm's Law (V = I × R), you can rearrange things to get other useful versions: P = I² × R and P = V² / R. If you know any two of the four values, you can always solve for the other two. Need to work directly with current calculations? Our Amp Calculator and Amps to Watts Calculator can help with those specific conversions.
DC Power vs. AC Power
In a DC (direct current) circuit, current flows in one direction and the math is straightforward. You multiply voltage by current and you get power. Batteries and solar panels are common DC sources.
AC (alternating current) is what comes out of wall outlets. Because AC voltage and current rise and fall in wave patterns, they can get out of sync with each other. This creates three types of power:
- Real power (P) – measured in watts (W), this is the power that actually does useful work, like heating a room or spinning a motor.
- Reactive power (Q) – measured in volt-amperes reactive (VAR), this is power that bounces back and forth between the source and components like motors or capacitors. It does no useful work but is needed to maintain electric and magnetic fields. Components like capacitors play a key role here — our Capacitor Calculator can help with those calculations.
- Apparent power (S) – measured in volt-amperes (VA), this is the total combination of real and reactive power. It represents what the power source must actually deliver.
The power factor tells you how much of the apparent power is doing real work. A power factor of 1.0 (unity) means all the power is useful. A lower number means more energy is wasted going back and forth. Power factor is either "lagging" (common with motors and inductors) or "leading" (common with capacitors).
Energy, Cost, and Kilowatt-Hours
Power tells you the rate of energy use, but your electric bill charges you for total energy consumed. Energy equals power multiplied by time. The unit on your bill is the kilowatt-hour (kWh) — that is 1,000 watts running for one hour. For example, a 100-watt device running for 10 hours uses 1 kWh. To find the cost, you multiply the total kWh by your electricity rate (price per kWh). For a deeper look at managing your energy expenses, our Electricity Cost Calculator provides additional breakdowns, and if you're considering renewable energy, our Solar Panel Calculator can help you estimate potential savings.
Voltage Drop in Wires
Every wire has some resistance. When current flows through a wire, some voltage is lost as heat along the way. This is called voltage drop. Longer wires, thinner wires, and higher currents all increase voltage drop. If the drop is too large, your equipment may not work properly. The National Electrical Code (NEC) recommends keeping voltage drop below 3% for branch circuits and below 5% total for the whole system. Choosing the right wire gauge (thickness) and material — copper has less resistance than aluminum — keeps voltage drop in a safe range. For related circuit design tasks, our Voltage Divider Calculator, Parallel Resistor Calculator, and Resistor Color Code Calculator are also useful tools. If you're sizing a backup power source for your system, check out our Generator Sizing Calculator as well. And for mechanical power applications like engines, our Horsepower Calculator handles those conversions.