Electrical Power Calculator - P = V x I Watts and kWh

An electrical power calculator turns voltage and current into watts using the foundational P = V * I formula, so you can size wiring, compare batteries, or estimate runtime without re-deriving the units each time.

• • Sizing a battery-powered load: Pick the right battery and runtime by reading the pack voltage and load current and getting watts and kilowatt-hours back.
• • Checking a USB or low-voltage DC rail: Convert a 5 V, 9 V, or 12 V DC supply and the current a device pulls into watts for thermal or fuse checks.
• • Verifying an appliance nameplate: Confirm that the watts printed on a nameplate match P = V * I for the rated voltage and current.
• • Estimating solar panel output: Multiply panel voltage by measured current to find DC watts before inverter losses are added.

The watt is the SI unit of power, defined as one joule per second, so the electrical power calculator output doubles as an energy rate you can multiply by run time to get joules or kilowatt-hours.

For DC circuits and purely resistive AC loads the simple P = V * I equation is exact. For reactive AC loads the power factor correction is handled separately.

If you only have voltage and resistance, the equivalent forms P = V^2 / R and P = I^2 * R give the same answer for an ohmic load, and the calculator shows both as a cross-check.

When the resistance is the unknown and you want to back it out from V and I before computing power, Ohm's Law & Basic Circuit Calculator returns R alongside the rest of the V-I-R-P picture.

The calculator reads voltage and current, multiplies them to get watts, then divides by 1000 for kilowatts and multiplies by hours for kilowatt-hours, while using the optional resistance to cross-check the result with the V^2/R and I^2*R forms.

• V (voltage): DC potential difference across the load, in volts. SI symbol V.
• I (current): Steady DC current through the load, in amperes. SI symbol A.
• R (resistance): Optional DC resistance of the load, in ohms. SI symbol Ω.
• t (run time): Hours the load runs at the calculated power. Used only for the kWh output.
• P (power): Real power delivered to the load, in watts (W). The main result.
• kW: Power expressed in kilowatts (1 kW = 1000 W). Useful for utility billing and generator sizing.
• kWh: Energy used if the load runs for t hours. One kWh equals 3.6 million joules exactly.

All three forms are mathematically equivalent for an ohmic load where V equals I times R, so the electrical power calculator shows them side by side when resistance is supplied.

The kilowatt and kilowatt-hour outputs are direct SI conversions: 1 kW = 1000 W, and 1 kWh = 1000 W running for one hour = 3.6 million joules exactly.

According to OpenStax University Physics Volume 2, electrical power delivered to a device is the product of the voltage across it and the current through it, P = V * I

According to BIPM SI Brochure (9th edition), the watt is the unit of power and equals one joule per second, with 1 W = 1 V * 1 A exactly in SI base units

When the load is AC and the voltage and current waveforms do not peak together, AC Wattage Calculator adds the power factor that turns apparent VA into real watts.

Four ideas cover most DC and resistive electrical power calculator problems in physics homework, hobby electronics, and battery sizing.

Once the SI definitions are in hand the formulas become three algebraic rearrangements of the same Ohm's law relationship.

To convert the watt output into joules or kilojoules for a mechanical-work problem, Work, Energy and Power Calculator handles the W-to-J and kWh-to-J bookkeeping.

Five steps turn a voltage and a current reading into watts, kilowatts, and an energy estimate.

1. 1 Read the voltage across the load: Use a multimeter on DC volts, or read the supply rating from the battery, USB port, or DC rail label.
2. 2 Read the current through the load: Use a multimeter in series on DC amps, a clamp meter, or the device's rated current from its nameplate or datasheet.
3. 3 Add the resistance if you know it: Measure with an ohmmeter or look it up. Leave at zero if you are unsure and the V*I result is all you need.
4. 4 Set the run time in hours: Pick the duration you want the energy estimate for, from minutes (0.1 h) up to a full year (8760 h).
5. 5 Read watts, kilowatts, and kilowatt-hours: The black P card shows real watts. The next rows give the I^2*R and V^2/R cross-checks, then kilowatts for utility sizing and kilowatt-hours for energy billing.

A USB-C laptop charger is rated 20 V and 3.25 A. Enter those, leave resistance at zero, and set time to 1 hour. The electrical power calculator returns 65 W, 0.065 kW, and 0.065 kWh, matching the 65 W nameplate.

If the watts you just computed need to be turned into milliwatts, BTU per hour, or horsepower, Watt Converter handles the unit conversion without re-entering the original circuit values.

Six practical wins when you replace hand arithmetic with this electrical power calculator.

• • Skip the unit conversions by hand: Watts, kilowatts, and kilowatt-hours all come out of the same inputs, so there is no separate 1000-divider step in your head.
• • Cross-check ohmic measurements: P = V * I, P = V^2 / R, and P = I^2 * R are computed in parallel; matching numbers confirm the load is ohmic and the meters are accurate.
• • Plan battery runtime in one pass: Combined with the run time input the kWh output goes straight into battery-capacity comparisons without a second calculator.
• • Size fuses and wiring quickly: Knowing the DC watts and the supply voltage lets you read the current off a fuse or wire-ampacity chart without re-deriving I = P/V.
• • Estimate utility costs: kWh output times your local electricity rate gives the running cost of a DC appliance, server, or solar array at a glance.
• • Teach DC power without a lab: The three formula views and the cross-check make it useful for physics homework, where showing the work in multiple forms is often part of the grade.

The trade-off is that the formula assumes a DC or purely resistive load; reactive AC loads need a power factor correction that this page does not compute.

If your circuit is AC and you also need to know what fraction of the apparent power is doing real work, Power Factor Calculator returns the power factor and reactive var.

Five reasons the calculated watts may differ from what a meter shows in practice.

• • The calculator assumes DC or a unity-power-factor resistive AC load; inductive or capacitive loads need the AC wattage workflow.
• • If voltage, current, and resistance do not satisfy V = I * R, the cross-check rows disagree with P = V * I; that signals a non-ohmic load, not a bug.
• • The run-time estimate is linear and does not account for battery discharge curves where voltage and current both drop as the cell empties.

According to NIST's Engineering Division, measurement uncertainty characterizes the dispersion of values attributed to a measurand, so a meter reading 12.00 V sits in a tolerance band rather than at an exact value

Once the wattage is known, 220V Wire Size Calculator helps pick the copper gauge that carries the resulting current without exceeding the voltage drop you allowed.