Gera Tools

Power Unit Conversion Reference

Convert watt, kilowatt, horsepower, and BTU/h instantly.

Convert between watts, kilowatts, megawatts, metric and mechanical horsepower, BTU per hour, kcal/h, and ft-lb/s using exact SI factors, all in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

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What is the base unit used?

The watt (W), the SI unit of power equal to one joule per second. Every input is converted to watts first, then out to every other unit, so chained rounding cannot accumulate.

One place for every power unit

Power is the rate of energy transfer — joules per second, expressed in whatever unit the industry historically settled on. Engineers use watts and kilowatts, car makers quote horsepower (in two conflicting definitions), heating and cooling engineers use BTU per hour, and European technical documents still use kcal/h or PS. This reference converts between all of them from a single input so you never have to memorise the cross-unit factors.

How it works

Each unit has an exact factor to the SI base unit, the watt. Your value is converted to watts first, then divided out to each target:

1 kW      = 1,000       W
1 MW      = 1,000,000   W
1 hp      = 745.6999    W   (mechanical / imperial)
1 PS      = 735.49875   W   (metric horsepower, PS or CV)
1 BTU/h   = 0.293071    W
1 kcal/h  = 1.163111    W
1 ft·lb/s = 1.355818    W

Because everything routes through one watt base, no rounding error accumulates the way it does when you chain unit-to-unit multiplications.

The two horsepower definitions — why it matters

The single most common source of confusion in this domain is the word “horsepower”. There are two definitions in everyday use, and they are not interchangeable:

  • Mechanical horsepower (hp) — 745.6999 W. Used in the United States and the United Kingdom for engine ratings, compressor output, and electric motor nameplates. Originates from James Watt’s own measurement of draught-horse work.
  • Metric horsepower (PS or CV) — 735.49875 W. Used in continental Europe, Japan, and South Korea for vehicle power ratings (the “PS” on a German car spec sheet). PS stands for Pferdestärke, CV for cheval-vapeur.

The difference is about 1.4%. A 150 PS engine produces 150 × 735.5 = 110,325 W, while “150 hp” (mechanical) produces 150 × 745.7 = 111,855 W — a meaningful gap in a performance comparison.

A quick mental conversion: 1 kW ≈ 1.341 hp (mechanical) or 1.360 PS (metric). Going the other way, 100 hp ≈ 74.6 kW and 100 PS ≈ 73.5 kW.

HVAC context — BTU/h and tons of cooling

The BTU per hour appears wherever heating and air-conditioning equipment is sold in the US. The key reference point most people know is the ton of refrigeration: 12,000 BTU/h, which equals approximately 3,517 W or 3.52 kW. Window air conditioners are often rated in BTU/h (a “12,000 BTU unit” is one ton of cooling), while commercial chillers are rated in tons.

For quick estimation: divide BTU/h by 3,412 to get kilowatts, or divide by 3.412 to get watts. For example, an 18,000 BTU/h mini-split is roughly 5.3 kW.

Worked examples

  • Car engine comparison: A car rated at 200 PS is producing 200 × 735.499 = 147,100 W = 147.1 kW. In mechanical horsepower, that is 147,100 / 745.7 ≈ 197 hp — slightly less than the PS figure.
  • Electric motor: A 3 kW electric motor delivers 3,000 / 745.7 ≈ 4.02 hp. Multiply by 1.341 for a quicker estimate: 3 × 1.341 ≈ 4.02 hp.
  • Boiler rating: A 50,000 BTU/h boiler outputs 50,000 × 0.293071 ≈ 14,654 W ≈ 14.7 kW.

Practical tips

  • Electrical heaters labelled in kW deliver that power directly as heat — 1 kW in = 1 kW of heat out. They are 100% efficient by definition.
  • Heat pumps, by contrast, deliver 2–5 kW of heat per kW of electricity consumed (the coefficient of performance), so their rated BTU/h output is not equal to their electrical input in watts.
  • In engineering datasheets, always note whether the power is continuous-duty or peak — the same unit applies to both, but they describe very different operating scenarios.