How do I calculate watts from amps and volts?

Multiply amps by volts: Watts = Amps x Volts (P = I x V). For example, a device drawing 5 amps at 230 V uses 1,150 watts (1.15 kW). This is the fundamental power formula and applies to both DC circuits and AC circuits (for resistive loads with a power factor of 1).

How many watts are in a kilowatt?

One kilowatt (kW) equals 1,000 watts. To convert watts to kilowatts, divide by 1,000. To convert kilowatts to watts, multiply by 1,000. For example, a 2.4 kW kettle uses 2,400 watts. New Zealand mains voltage is 230 V at 50 Hz, so a 10-amp circuit at 230 V can supply up to 2,300 watts (2.3 kW).

How do I convert watts to horsepower?

Divide watts by 745.7 to get mechanical horsepower (hp). So 1,000 W = 1.34 hp. To convert horsepower to watts, multiply by 745.7. For example, a 1.5 hp motor produces approximately 1,118 watts. This conversion uses the mechanical horsepower standard (1 hp = 745.699872 W), which is the most commonly used definition.

Watt Calculator

Calculate electrical power in watts from voltage and current, current and resistance, or voltage and resistance. Convert between watts, kilowatts, milliwatts, amps, volts, ohms, and horsepower. Results update instantly.

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Standard formula Ohm's Law and the Power Triangle: P = VI, P = I²R, P = V²/R (SI units).

1. Calculation Method

Formula to use

Voltage (V)

Current (A)

Resistance (Ω)

Ω

2. Circuit Type

Circuit type (AC or DC)

DC power: P = V x I (exact)

Power Result

Watts
1,150
W

Kilowatts

1.150

Milliwatts

1,150,000

Horsepower

1.542

hp (mechanical)

Calculation Breakdown

Formula usedP = V x I

Voltage (V)230 V

Current (I)5 A

Resistance (R)46 Ω

Circuit typeDC

Power factorN/A

Real power1,150 W

Worked Example

InputsV = 230 V, I = 5 A

FormulaP = V x I

Calculation230 x 5

Result= 1,150 W

A household circuit at 230 V drawing 5 A delivers 1,150 W (1.15 kW) of power.

Summary: Enter values above to calculate power.

Enter a Power Value

Value to convert

From unit

Amps / Volts Quick-Convert

Enter watts and voltage to find current, or watts and current to find voltage.

Power (W)

Voltage (V)

Current: 4.35 A | Resistance: 52.9 Ω

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 V_L x I_L x pf

Where V_L is the line-to-line voltage and I_L 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.