Power Converter Guide: Watts, Horsepower, and Kilowatts

Car brochures quote horsepower, electricity bills use kilowatt hours, air conditioners are rated in BTU per hour, and a kettle says 3,000 W on its base. Power is the rate at which energy is used, and every one of those units measures the same thing on a different scale.

Power Versus Energy

Getting this pair straight solves half the confusion. Power is a rate, measured in watts. Energy is power multiplied by time, measured in joules or kilowatt hours. A 2,000 W heater running for three hours uses 6 kWh of energy. Your electricity meter charges for energy, not power, which is why a high-wattage appliance used briefly can cost less than a modest one left on all day.

The Conversion Factors

• 1 kilowatt = 1,000 W = 1.341 mechanical horsepower
• 1 horsepower (mechanical) = 745.7 W
• 1 metric horsepower (PS) = 735.5 W
• 1 BTU per hour = 0.293 W, so 1 kW = 3,412 BTU/h
• 1 ton of refrigeration = 12,000 BTU/h = 3.517 kW

The gap between mechanical and metric horsepower is why the same engine is advertised as 197 hp in one market and 200 PS in another. It is a unit difference of about 1.4%, not a different engine.

Worked Conversions

A 150 kW electric motor is 150 x 1.341 = 201 hp. A 120 hp petrol engine is 120 x 745.7 = 89.5 kW. A 12,000 BTU/h air conditioner is 3.52 kW of cooling, though its electrical draw is much lower, typically around 1.1 kW, because a heat pump moves more heat than the energy it consumes. That ratio is the coefficient of performance, and mixing up cooling capacity with electrical input is the most common error in air conditioning specifications.

Electrical Power From Volts and Amps

For a resistive load, power equals voltage times current. A 230 V kettle drawing 13 A uses 2,990 W. On a 120 V supply the same 3 kW would need 25 A, which is why high-power appliances in North America often need a dedicated circuit. For three-phase supplies, multiply by the square root of 3 and the power factor, so 400 V at 20 A with a power factor of 0.9 gives 400 x 20 x 1.732 x 0.9 = 12.5 kW.

Power factor matters on motors and older lighting. Apparent power is measured in kilovolt-amperes, and real power in kilowatts is the apparent power times the power factor. A generator rated 10 kVA delivers only 8 kW to a load with a 0.8 power factor.

Working Out Running Costs

Multiply appliance power in kilowatts by hours used, then by the unit price. A 2 kW electric heater run four hours a day for thirty days uses 240 kWh. At 0.28 per kWh that is 67.20 a month. The same arithmetic settles most household energy arguments quickly: a 10 W LED bulb left on all month uses 7.2 kWh, about 2, while a 3 kW immersion heater reaches that in under three hours.

Human and Vehicle Power for Scale

These anchors help you smell out a wrong conversion. If a calculation tells you a hair dryer needs 20 kW, the factor went the wrong way.

Heating and Cooling Ratings

Boilers are usually quoted in kilowatts in Europe and BTU per hour in North America, so a 24 kW combi boiler is about 82,000 BTU/h. Radiator output is given in watts at a stated temperature difference, and that qualifier matters: the same radiator delivers much less heat in a system running at 45 degrees flow than at 70 degrees, which catches people out when converting to a heat pump.

Mistakes to Avoid

Confusing kW with kWh is the big one, since the first is a rate and the second an amount. Mixing metric and mechanical horsepower introduces a small but real error in engine comparisons. Ignoring power factor makes generator sizing optimistic. And treating an air conditioner BTU rating as electrical consumption overstates running cost by a factor of three or more.

Convert Power Units Free

The power converter on SimpleCounter moves between watts, kilowatts, horsepower, metric horsepower, BTU per hour, and tons of refrigeration in a single field, and our energy tools handle the kWh side for running costs. Both are free, work offline once loaded, and need no signup.