Watt Calculator
This calculator works out electrical power in watts using whichever two quantities you know: voltage and current (P = V x I), current and resistance (P = I²R), or voltage and resistance (P = V²/R). Select your formula, enter the values in volts, amps or ohms, then choose your circuit type - DC, AC single-phase or AC three-phase - and add a power factor if the load is inductive or capacitive, such as a motor or fluorescent light. The calculator returns the real power in watts, kilowatts, milliwatts and mechanical horsepower, alongside a full calculation breakdown and a worked example showing exactly how the figures were reached, plus a plain-English summary comparing your result to everyday appliances. A second tab converts power between watts, kilowatts, megawatts, milliwatts, horsepower, BTU per hour and calories per second, and includes a quick amps-and-volts converter that works out current and resistance from a given power and voltage. Use it to size a circuit, check an appliance's power draw against a New Zealand 230 V socket or 10 A breaker, convert a motor's horsepower rating to watts, or work through three-phase load calculations. These are standard electrical engineering formulas that give accurate results for the inputs supplied, but real-world wiring, circuit protection and power factor vary with equipment and installation, so always confirm sizing and safety with a licensed electrician.
1. Calculation Method
2. Circuit Type
Calculation Breakdown
Worked Example
Enter a Power Value
Amps / Volts Quick-Convert
Enter watts and voltage to find current, or watts and current to find voltage.
Conversion Results
| Unit | Symbol | Value |
|---|
How the Watt Calculator Works
A watt (W) is the SI unit of power. It measures the rate at which energy is transferred or converted. The fundamental relationship between electrical power, voltage, current, and resistance is described by Ohm's Law combined with the power formula:
- P = V x I (power equals voltage times current). Use this when you know the voltage (in volts) and the current (in amps).
- P = I²R (power equals current squared times resistance). Use this when you know current and resistance.
- P = V² / R (power equals voltage squared divided by resistance). Use this when you know voltage and resistance.
All three formulas give the same result for resistive (DC) loads. For AC circuits, multiply by the power factor to get real (active) power. The power factor accounts for the phase difference between voltage and current in inductive or capacitive loads.
Power Unit Conversions
| Unit | Symbol | Equal to Watts |
|---|---|---|
| Milliwatt | mW | 0.001 W |
| Watt | W | 1 W |
| Kilowatt | kW | 1,000 W |
| Megawatt | MW | 1,000,000 W |
| Horsepower (mechanical) | hp | 745.69987 W |
| BTU per hour | BTU/h | 0.29307107 W |
| Calorie per second | cal/s | 4.1868 W |
Common Appliance Wattages
New Zealand mains supply is 230 V AC at 50 Hz. The following typical wattages help put the calculator results in context:
| Appliance | Typical Wattage | Current at 230 V |
|---|---|---|
| LED light bulb | 8 to 15 W | 0.03 to 0.07 A |
| Laptop | 45 to 90 W | 0.20 to 0.39 A |
| Television (55") | 80 to 150 W | 0.35 to 0.65 A |
| Refrigerator | 100 to 250 W | 0.43 to 1.09 A |
| Microwave | 700 to 1,200 W | 3.0 to 5.2 A |
| Electric jug / kettle | 2,000 to 2,400 W | 8.7 to 10.4 A |
| Heat pump (heating) | 800 to 2,000 W | 3.5 to 8.7 A |
| Electric shower | 7,000 to 10,500 W | 30 to 46 A |
| EV charger (home, 7 kW) | 7,000 W | 30 A |
Worked Example
A standard NZ wall socket is rated at 230 V with a 10 A circuit breaker. The maximum power available is:
P = V x I = 230 x 10 = 2,300 W (2.3 kW)
An electric kettle rated at 2,400 W at 230 V draws:
I = P / V = 2,400 / 230 = 10.43 A
This slightly exceeds a standard 10 A circuit, which is why kettles in NZ typically have a 13 A rated element or are plugged into a 15 A circuit. The calculator above produces exactly these results with its default values (230 V, 5 A giving 1,150 W).
AC Power and Power Factor
For AC circuits with inductive or capacitive loads (such as motors, fluorescent lights, and air conditioners), the apparent power (volt-amperes, VA) differs from the real power (watts). The power factor (pf) is the ratio of real power to apparent power:
Real Power (W) = Apparent Power (VA) x Power Factor
A power factor of 1.0 means all the apparent power does useful work (resistive loads such as heaters and incandescent bulbs). Motors typically have a power factor of 0.7 to 0.9. The calculator accounts for power factor when you select an AC circuit type.
Three-Phase Power
For a balanced three-phase AC circuit the real power formula is:
P = sqrt(3) x VL x IL x pf
Where VL is the line-to-line voltage and IL is the line current. In NZ, three-phase supply is 400 V line-to-line (230 V phase-to-neutral). A 400 V, 32 A, pf 0.85 load draws 400 x 32 x 1.732 x 0.85 = approximately 18,844 W (18.84 kW).
Related Calculators
- Essential Calculators: the full converter and utility collection.
- Electrical Power Calculator (P = VI): focused on the basic power formula.
- Appliance Running Cost Calculator: turn watts into electricity cost over time.
- Amp Hour Battery Life Calculator: battery capacity and runtime from power draw.
- Bandwidth Calculator: data-rate conversions and transfer time.
Sources and method: SI unit definitions for the watt (BIPM, 9th edition SI Brochure 2019). Ohm's Law (Georg Ohm, 1827). Mechanical horsepower defined as 550 ft lbf/s = 745.69987158227022 W (IEEE Std 100-2000). BTU/h conversion per NIST Special Publication 1038.
This calculator uses the standard electrical power formulas for DC and AC circuits. For AC circuits, real power depends on power factor, which varies with the load type. Consult a licensed electrician for wiring and circuit sizing decisions.