A deeper look at where each power unit on this page comes from — the SI-prefixed units used in everyday electrical work, the industrial units used in mechanical and thermal engineering, and the two specialized units used in HVAC and RF/telecom work.
SI-Prefixed Units (Microwatts through Gigawatts)
Watts, Milliwatts, Microwatts, Kilowatts, Megawatts, and Gigawatts are all the same underlying unit — the watt — scaled by a power of ten. Power on a nameplate is almost always written in these SI prefixes. A milliwatt is a thousandth of the watt you typed, and a kilowatt is a thousand of them. Moving between those prefixes does not round, because each step is an exact power of ten.
Horsepower — Mechanical (745.699872 W) vs. Metric / PS (735.49875 W)
Mechanical horsepower, standard in the US, is defined as 550 foot-pounds per second. Metric horsepower — written PS in Germany, CV in Spain, cv in Italy, ch in France — is defined as 75 kilogram-force meters per second. Metric horsepower is about 1.4% smaller than mechanical horsepower, so a 100 hp engine is roughly 101.4 PS. Always check which horsepower type a spec sheet is using before comparing figures.
BTU per Hour (≈ 0.29307107 W)
British Thermal Units per hour is a thermal power unit standard in US HVAC engineering, describing the rate of heat transfer. Building heating and cooling load calculations, and most US air conditioner and furnace nameplates, are still rated in BTU/h.
Foot-pound-force per Second (≈ 1.35582 W)
A mechanical power unit from the imperial system, describing torque-driven work done per second. It appears in legacy machinery specs and some US industrial rotating-equipment calculations, and is historically related to horsepower's original 33,000 ft·lbf/min definition.
Ton of Refrigeration Specialized · HVAC (3,516.85284 W)
A Ton of Refrigeration (TR) is the power required to melt one short ton (2,000 lb) of ice at 0°C in 24 hours. It's the standard unit for HVAC chiller and air conditioner cooling-capacity ratings. The conversion is an exact linear multiplication, same as any other unit here — it's set apart in the Specialized row simply because it's only meaningful for HVAC and refrigeration work, not general electrical conversions.
Decibel-Milliwatts (dBm) Specialized · RF/Log
dBm measures power on a logarithmic scale, referenced to 1 milliwatt (1 mW = 0 dBm). It's the standard way RF and telecom engineers express signal power, because it compresses a huge dynamic range — from picowatts to kilowatts — into small, easy-to-compare numbers. Unlike every other unit on this page, dBm does not convert by simple multiplication: mW = 10^(dBm ÷ 10), and conversely dBm = 10 × log10(mW). Because power in dBm is a ratio expressed logarithmically, negative dBm values are normal (they represent power below 1 mW) — but a power of exactly zero watts has no dBm equivalent, since the logarithm of zero is undefined.
| Unit |
Symbol |
Value in Watts |
Common Use |
| Microwatt | µW | 1 × 10⁻⁶ | RF power budgets, sensor sleep current |
| Milliwatt | mW | 1 × 10⁻³ | Laser diodes, small electronics |
| Watt | W | 1 | SI base unit — household electronics |
| Kilowatt | kW | 1 × 10³ | Appliances, EV chargers, small generators |
| Megawatt | MW | 1 × 10⁶ | Industrial plants, utility-scale generation |
| Gigawatt | GW | 1 × 10⁹ | National grid capacity, large power plants |
| Horsepower (Mechanical) | hp | ≈ 745.699872 | US engine and motor ratings |
| Horsepower (Metric / PS) | PS | ≈ 735.49875 | European / Asian vehicle ratings |
| BTU per Hour | BTU/h | ≈ 0.293071 | HVAC heating / cooling capacity |
| Foot-pound/Second | ft·lb/s | ≈ 1.355818 | Legacy mechanical power specs |
| Ton of Refrigeration HVAC | TR | ≈ 3,516.85284 | HVAC chiller / AC capacity |
| Decibel-Milliwatt RF / Log | dBm | logarithmic | RF / telecom signal power |
This guide is provided for educational and reference purposes. Always confirm measured power values against a properly calibrated meter, nameplate, or instrument before relying on them for any electrical, mechanical, or HVAC work.
