Power Converter

Power units in electrical and mechanical systems

Power is the rate at which energy is transferred or converted. The SI unit is the watt (W), defined as one joule per second (J/s). A 60 W light bulb consumes 60 joules of electrical energy every second. Power scales quickly: a kilowatt (kW) is 1,000 W, a megawatt (MW) is 1,000,000 W, and a gigawatt (GW) is 1,000,000,000 W. Large power stations are rated in gigawatts.

Electricity billing uses the kilowatt-hour (kWh), which is a unit of energy, not power. One kWh is the energy consumed by a 1 kW device running for one hour (3,600,000 J). A typical UK home uses about 3,500 kWh per year; a typical US home uses about 10,500 kWh per year. The power converter above handles kW; kWh requires multiplying kW by hours.

Horsepower (hp) is the unit used for engine and motor output in the United States and in automotive contexts worldwide. One mechanical horsepower equals exactly 745.69987 W. Car engines are commonly rated in horsepower in the US (and in metric horsepower in Europe, where 1 PS = 735.499 W). A 200 hp engine produces about 149 kW of mechanical power.

BTU per hour (BTU/hr) is the standard unit for heating and cooling capacity in the United States. HVAC systems, air conditioners, and boilers are rated in BTU/hr. One BTU/hr equals approximately 0.29307 W. A 12,000 BTU/hr air conditioner (marketed as "1 ton of cooling") provides about 3,517 W of cooling capacity.

Power in motor selection and generator sizing

Electric motor nameplates list power in both kW and hp, along with voltage, current, and efficiency. A motor labelled 7.5 kW (about 10 hp) running at 90% efficiency draws about 8.33 kW from the supply. Generator sizing starts with the total load in watts and adds a margin (typically 20-25%) for startup surges from motors and compressors.

Solar panel output is rated in watts peak (Wp), measured under standard test conditions. A 400 Wp panel will produce roughly 400 Wh of energy in one peak sun-hour. A household array of 10 panels at 400 Wp each has a 4 kWp rating, which might generate 3,500-5,000 kWh per year depending on location and orientation.

Frequently asked questions