Article — Power Conversion Calculator
Power Conversion: Watts to kW, MW, hp, BTU/h, Refrigeration Tons
Power conversion translates watts (1 W = 1 J/s) into nine practical units: kilowatt, megawatt, three horsepower variants (mechanical, metric, electrical), BTU per hour, calorie per second, foot-pound per second, and refrigeration ton. 1 mechanical hp = 745.7 W, 1 metric hp = 735.5 W, and 1 RT = 3517 W. Each factor comes from an SI-traceable definition.
Power is the rate of energy transfer — joules per second, watts. Engineers, electricians, mechanics, and HVAC technicians all measure power, but each profession defaults to a different unit. Power conversion bridges the gap when the same physical quantity appears in mechanical-engineering papers, electric-motor catalogs, and air-conditioner brochures.
What power conversion does
The watt is the SI base unit for power. One watt is one joule of energy delivered per second. Every other power unit reduces to watts through an exact factor. Power conversion is a single multiplication: target = W × factor.
The factor varies by unit category. For SI prefixes (kW, MW), the factor is a power of ten. For thermal units (BTU/h, cal/s), it derives from the energy definition of the BTU or calorie. For mechanical units (hp, ft·lbf/s), it derives from historical engine-rating conventions. For refrigeration tons, the factor comes from the latent heat of ice formation.
James Watt invented horsepower in 1782 to sell steam engines to coal-mine operators. He measured a strong brewery horse turning a mill wheel and calculated 33,000 ft·lbf per minute of work output. That figure, 550 ft·lbf/s, became 1 mechanical horsepower. Watt's marketing strategy was so successful that the unit still exists 240 years later, alongside the SI watt named in his honor.
Watts to horsepower — three variants
The most common power conversion takes watts to horsepower. But "horsepower" has three definitions. Mechanical horsepower (the U.S. automotive standard) is 745.7 W. Metric horsepower or PS (the European standard) is 735.5 W. Electrical horsepower (used for motor nameplates) is 746 W rounded for convenience.
The three differ by about 1.5%. A 500 hp (mechanical) engine rates 503 hp (metric) or 500 hp (electrical) — close enough that casual usage treats them as identical, but precise engine specs must declare which definition applies. Power conversion factors built into this calculator use the exact figures, so the small differences appear when needed.
Power conversion for thermal units
Thermal power units measure heat-transfer rates. BTU per hour is the U.S. standard for heating and cooling capacity. 1 W = 3.41214 BTU/h. A 5 kW electric heater dissipates 17,061 BTU/h. A 12,000 BTU/h window air conditioner removes about 3.5 kW of heat from the room.
Calorie per second appears in old physiology and chemistry literature. 1 W = 0.2390 cal/s. The food calorie used on nutrition labels is actually a kilocalorie (kcal = 1000 cal), so a 2000-kcal daily intake corresponds to an average metabolic rate of 97 W — about the power of a 100 W incandescent light bulb.
Power conversion in HVAC and refrigeration
The refrigeration ton (RT) is a power unit specific to HVAC and chiller systems. 1 RT = 12,000 BTU/h = 3516.853 W. It originated as the cooling power needed to freeze one short ton of water (2000 lb at 32°F) in 24 hours. The latent heat of fusion (144 BTU/lb) times 2000 lb gives 288,000 BTU per 24 hours, or 12,000 BTU/h.
Commercial chillers are rated in RT: a 100-ton chiller delivers 352 kW of cooling. Air conditioner SEER ratings convert directly: a SEER 15 unit produces 15 BTU/h of cooling per 1 watt of electricity consumed. Power conversion translates the RT rating to kW for electrical-load calculations.
1 kW = 1.341 hp (mech) 1 kW = 1.36 PS (metric)1 kW = 3412 BTU/h 1 hp = 745.7 W1 RT = 3517 W = 12,000 BTU/h 1 W = 0.7376 ft·lbf/s1 W = 0.239 cal/s 1 MW = 1000 kWPower conversion for electric appliances
Domestic electric appliances are rated in watts. An LED bulb draws 5–15 W; a laptop draws 30–90 W; a microwave oven 800–1200 W; a hair dryer 1500–2000 W; an electric oven 2500–3500 W; a Level 2 EV charger 7400–11,500 W. The watt scale fits everyday consumer products cleanly.
Appliance energy consumption (kWh) and power (kW) are different things. A 1000-W microwave used for one hour consumes 1 kWh of energy. A 1000-W microwave used for five minutes consumes 1/12 of a kWh = 0.083 kWh. Power conversion gives the rate; energy is rate times time.
Power conversion at industrial scale
Industrial and utility power crosses into megawatts. A diesel locomotive produces 3–5 MW. A medium wind turbine outputs 2–8 MW. A natural-gas-fired plant unit generates 200–500 MW. A typical nuclear reactor produces 1000–1500 MW. The watt is still the base unit, but MW is the natural reading.
Power conversion to BTU/h is useful for thermodynamic-cycle analysis. A 500-MW plant produces 1.7 × 10^9 BTU/h of electrical output. With a 40% efficiency, the plant's fuel-input rate is about 4.3 × 10^9 BTU/h — equivalent to 43,000 therms or 43 GJ per hour. Power conversion to RT (refrigeration tons) sometimes appears for industrial chillers: a 1000-RT chiller is a 3.5-MW thermal load.
Watts and joules per second describe rate (power). Watt-hours and joules describe quantity (energy). A 100-W bulb running one hour consumes 100 watt-hours = 0.1 kWh of energy. The same bulb running ten hours consumes 1 kWh. Power conversion translates rates; energy units (joules, kWh, BTU) translate quantities. Mixing them is a frequent error in load and consumption calculations.
Common power conversion mistakes
The first mistake is using the wrong horsepower variant. A 200 PS European car is 197 hp in the U.S. mechanical scale, while a 200-hp American car is 203 PS in metric. Spec sheets that quote "200 hp" without specifying which standard leave a 1.5% ambiguity.
The second mistake is mixing BTU and BTU/h. BTU is energy; BTU/h is power (rate). A 12,000-BTU air conditioner is shorthand for 12,000 BTU/h, a power rating. The energy delivered over time depends on how long it runs.
The third mistake is treating refrigeration ton as a mass unit. The "ton" in RT refers to the historical 1 short ton of ice melted per day, but RT is now strictly a power unit (3517 W). It has no relationship to the kilogram or pound except through that obsolete derivation.
- 1 W = 1 J/s (SI base)
- 1 kW = 1000 W = 1.341 hp (mech)
- 1 MW = 1000 kW = 10^6 W
- 1 hp (mech) = 745.7 W
- 1 PS (metric) = 735.5 W
- 1 RT = 3516.853 W = 12,000 BTU/h
- 1 W = 3.41214 BTU/h
- Avg U.S. home peak ≈ 4 kW
For quick mental conversion, 1 kW is about 1.34 hp and 1 hp is about 0.75 kW. For HVAC, 1 ton of refrigeration is about 3.5 kW. For thermal, 1 kW is about 3400 BTU/h. These three rules of thumb cover most everyday power-conversion questions and stay accurate to within a couple of percent.