Power Converter

How do you convert power units?

To convert power units, multiply by the starting unit's value in watts, then divide by the target unit's value in watts. This works because power is energy per unit of time, and every power unit has a fixed value in watts, where 1 W is 1 joule per second. Metric prefixes cover most everyday cases: 1 kW is 1,000 W, 1 MW is 1,000 kW and 1 GW is 1,000 MW.

A worked example

To convert 150 hp to kilowatts: 150 × 745.699872 = 111,854.98 W, then 111,854.98 ÷ 1,000 = 111.85 kW. The shortcuts are 1 hp ≈ 0.7457 kW and 1 kW ≈ 1.34102 hp, which the hp to kW converter and the kW to hp converter apply for you.

Mechanical horsepower versus metric horsepower

There are two common horsepowers, and car specifications mix them. Mechanical horsepower (hp), used in the United States and the United Kingdom, is defined as 550 foot-pounds-force per second, which is 745.699872 W. Metric horsepower, written PS in Germany, CV in France, Italy and Spain, and pk in the Netherlands, is 75 kilogram-force meters per second, which is 735.49875 W. Mechanical horsepower is about 1.4% larger, so a 100 PS engine makes about 98.6 hp, or 73.55 kW. This converter uses mechanical horsepower; European registration documents list engine output in kW, which avoids the ambiguity entirely.

What is the SI unit of power, and how do the other units compare?

The watt (W) is the SI unit of power, equal to one joule of energy per second. The BIPM SI Brochure lists it as a derived unit, kg·m²·s⁻³, and it links several fields at once: in electricity 1 W is one volt times one ampere, and in mechanics it is one newton-meter of work per second. Every other power unit is a fixed number of watts, set by definition rather than measurement.

Mechanical horsepower is defined as 550 foot-pounds-force per second, which works out to 745.699872 W from the exact foot, pound and standard gravity; NIST SP 811 lists the factor. Metric horsepower is 75 kilogram-force meters per second, or exactly 735.49875 W. US electric-motor nameplates use an electrical horsepower of exactly 746 W. The BTU per hour follows from the International Table BTU of 1,055.05585262 J, giving 0.293071 W per BTU/h.

For rotating machines, power is torque times rotational speed, so an engine's power curve and its torque curve describe the same output in two ways; the torque converter handles N·m and lbf·ft.

Where is each power unit used?

Watts and kilowatts are the standard almost everywhere, while horsepower survives for engines in the US and UK and BTU per hour for heating and cooling in North America. The scale of the job decides the prefix, from milliwatts in a sensor to gigawatts on a national grid.

• Milliwatts: laser pointers, radio transmitters, sensors and the standby draw of small electronics.
• Watts: light bulbs, phone chargers, kitchen appliances and computer power supplies worldwide. The watts to kW converter is handy for adding up appliance loads.
• Kilowatts: car and motorcycle engines in most countries, electric vehicle motors and chargers, home solar systems and heat pumps.
• Megawatts and gigawatts: power stations, wind turbines, data centers and national grids. A single large nuclear reactor produces on the order of one gigawatt of electricity.
• Horsepower: car, truck and boat engines in the United States and the UK, lawn equipment and electric motors on US nameplates.
• BTU per hour: air conditioners, furnaces, boilers, heat pumps and gas grills in North America.

What is the difference between watts and watt-hours?

A watt measures power, the rate at which energy flows at a given moment, while a watt-hour measures energy, the amount one watt delivers over one hour. Multiply power by time to get energy. A 100 W lamp left on for 10 hours uses 1,000 Wh, which is 1 kWh, and a 2 kW space heater running for 3 hours uses 6 kWh. Your electricity bill charges for kWh, an energy unit, so convert those with the energy converter. Appliance labels, engine ratings and solar panel specifications show power in W, kW or hp, which is what this page converts. If you know the voltage and current rather than the wattage, the electrical calculators will work out the watts.

The same distinction applies to air conditioning. A 12,000 BTU/h unit moves about 3.52 kW of heat out of a room, but depending on its efficiency it may draw only around a third of that from the wall socket. Always check the separate input wattage when sizing a circuit or estimating running costs.

How do you calculate watts from volts and amps?

Multiply volts by amps to get watts: P = V × I. A kettle that draws 10 A on a 120 V supply uses 1,200 W, and a 15 A household circuit at 120 V can deliver up to 1,800 W. On 230 V mains, a 13 A plug fuse allows about 2,990 W. The amps to watts calculator does the multiplication, and the watts to amps calculator reverses it when you know an appliance's wattage and need its current.

Watts versus volt-amperes

With alternating current, motors and power supplies can draw current that is out of step with the voltage. The product of volts and amps is then the apparent power, written in volt-amperes (VA), and the useful or real power in watts is smaller by a ratio called the power factor. A 1,000 VA uninterruptible power supply with a power factor of 0.8 can deliver about 800 W, which is why UPS and generator labels often show both numbers.

For circuit sizing, the US National Electrical Code limits continuous loads to 80% of a breaker's rating, about 1,440 W on a 15 A, 120 V circuit. Rules differ by country, so follow your local electrical code and use a licensed electrician for any wiring work. The electrical calculators cover ohms, volts and amps in more depth.