Watt Calculator
Solve volts, amps, ohms, watts, and optional watt-hours by entering any two core electrical values and letting the formulas fill in the rest.
Enter any two of voltage, current, resistance, or power. Add time if you also want energy use in watt-hours.
Watt Calculator
Solve volts, amps, ohms, watts, and optional watt-hours by entering any two core electrical values and letting the formulas fill in the rest.
Use exactly two of the four electrical fields. Time does not count toward the pair and is only used for watt-hour energy.
About the Watt Calculator
A watt calculator combines the two most common electrical relationships used in everyday circuit analysis: Ohm's law and the basic power equation. Ohm's law states that voltage equals current multiplied by resistance, or V = I × R. Electrical power is the rate at which energy is delivered, transferred, or consumed, and in simple circuits it is given by P = V × I. Once you know any two of the core quantities voltage, current, resistance, and power, you can usually solve the remaining pair. This tool automates that algebra so you can move from a pair of known values to a complete quick-check of the circuit.
The watt is a unit of power, not energy. One watt means one joule of energy per second. That distinction matters because many people casually say a device “uses 1,500 watts per hour,” when what they really mean is a 1,500-watt device running for one hour consumes 1,500 watt-hours of energy. The calculator therefore includes an optional time field. If you enter a usage duration in hours, the tool multiplies solved power by time to return watt-hours, giving you a direct bridge from circuit values to energy consumption. This is useful when comparing appliances, chargers, battery loads, or estimating electricity usage before converting to kilowatt-hours for billing.
Different input pairs tell different stories about the same circuit. Voltage and current immediately give power through P = V × I and resistance through R = V / I. Voltage and resistance tell you how much current will flow and what power will be dissipated. Current and resistance are common when you know the load and want to infer the supply requirements. Voltage and power are common for household or industrial equipment because many nameplates list a rated supply voltage and wattage. Resistance and power are helpful for heater elements, resistive loads, or lab exercises where current is not directly measured. By allowing any valid pair, the calculator mirrors the way electrical problems are actually posed in textbooks, bench work, and field troubleshooting.
This kind of calculation appears everywhere. A 120-volt circuit drawing 5 amps consumes 600 watts and implies a 24-ohm equivalent load. A 12-volt automotive accessory at 2 amps consumes 24 watts and has a 6-ohm resistance. A 240-volt heater rated at 1,500 watts draws 6.25 amps and has an effective resistance of 38.4 ohms. These are exactly the kinds of fast checks electricians, electronics hobbyists, students, and maintenance teams use when sizing wiring, selecting fuses, diagnosing loads, or estimating energy cost.
The calculator is meant for straightforward DC or single-value resistive-equivalent analysis. Real AC systems may involve phase angle, power factor, reactive power, harmonics, and inrush conditions that require a more advanced treatment. Even so, the solved values here are an excellent first-pass estimate and a strong teaching aid. If you need a quick, reliable way to move between volts, amps, ohms, watts, and watt-hours, this tool keeps the formulas visible and the arithmetic immediate.
Examples
These common electrical cases show how two known values determine the rest of the circuit and, when time is added, energy use as well.
| Input | Output | Notes |
|---|---|---|
| 120 V, 5 A | P = 600 W, R = 24 Ω | A standard outlet and current draw example. Once voltage and current are known, both power and equivalent resistance follow immediately. |
| 12 V, 2 A | P = 24 W, R = 6 Ω | Typical low-voltage battery load. This pair is useful for vehicles, portable electronics, and bench power supplies. |
| 240 V, 1500 W | I = 6.25 A, R = 38.4 Ω | A common heater-style resistive load. Rated voltage and rated power are often the only values printed on a nameplate. |
| 240 V, 1500 W, 3 h | Energy = 4500 Wh | Adding time turns a power calculation into an energy estimate, which is the starting point for cost calculations. |
How to Use
- Enter exactly two of the four electrical values: voltage, current, resistance, or power. Leave the other two blank so the calculator can solve them.
- Optionally enter a time in hours if you want the tool to convert solved power into watt-hours of energy use.
- Click Calculate to display the complete set of voltage, current, resistance, and power values together with energy when time is provided.
- Use the worked examples to load common household, low-voltage, and heater-style scenarios, then reset the form to try your own numbers.
Frequently Asked Questions
What is a watt?
A watt is a unit of power equal to one joule per second. It describes how quickly electrical energy is being transferred or consumed, not how much energy has been used over a long period.
Why do I need exactly two electrical inputs?
The relationships among voltage, current, resistance, and power are constrained, so two independent values are usually enough to solve the other two. If you enter fewer than two values there is not enough information, and if you enter more than two values they may conflict with one another.
What is the difference between watts and watt-hours?
Watts measure the rate of energy use at a moment in time. Watt-hours measure total energy over a duration, so they are found by multiplying watts by hours.
Can I use this for AC circuits?
You can use it as a first-pass estimate for resistive or effective single-value loads. For real AC systems with power factor, reactive power, or phase angle, a dedicated AC power calculation is more appropriate.
Why does the calculator show resistance for appliances?
Any steady-state electrical load can be expressed as an equivalent resistance at the operating point, even if the real device contains more complex components. That makes resistance a useful derived value for comparison, troubleshooting, and sanity checks.