Introduction
Power is the rate of energy transfer. The SI unit is the watt (W), one joule per second. Horsepower for engines, watts for electrical, BTU per hour for HVAC, and tons for refrigeration — all the same physical quantity, different unit per industry. The conversions are arithmetic; the naming is hostile.
Why power units exist and how they diverged
James Watt estimated 1 horsepower (1782) as the sustained work rate of a brewery horse lifting water — about 33,000 ft-lb/min, or 745.7 W. He picked the number to convince brewery owners that his steam engines were worth more than multiple horses. Mechanical horsepower (745.7 W) is US-standard; metric horsepower (735.5 W) is slightly less and used in European auto specs. The difference — 1.4% — occasionally matters in precision specs.
BTU/hour is US HVAC convention. One ton of air conditioning is 12,000 BTU/hr = 3.517 kW — originally the rate at which one ton of ice melts over 24 hours.
How to convert power
Horsepower to watts: multiply by 745.7. Watts to horsepower: divide by 745.7. BTU/hr to watts: multiply by 0.293. kW to BTU/hr: multiply by 3,412.
For appliance selection: a 1,500-W space heater puts out 5,118 BTU/hr. A window AC rated '8,000 BTU' (per hour, implied) delivers 2.34 kW of cooling. EV motor peak power: Tesla Model 3 Performance is 377 kW (506 hp).
Units supported by this power calculator
- Watts
- Kilowatts
- Megawatts
- Horsepower (mechanical)
- Horsepower (metric)
- BTU/hr
Common power conversion mistakes
- Mechanical vs metric horsepower. US = 745.7 W, metric = 735.5 W. About 1.4% gap. European car specs in 'PS' (Pferdestärke) use metric hp.
- Peak vs continuous. A motor rated '100 hp peak' may only sustain 50 hp continuously. Specs without 'peak/continuous/nominal' are marketing numbers.
- Watts vs watt-hours. Watts is rate; watt-hours is energy. A 100 W bulb draws 100 W; running for 1 hour uses 100 Wh of energy.
- BTU vs BTU/hr. AC/heater specs are per hour implicitly. '10,000 BTU' means 10,000 BTU/hour.
- Tons of cooling vs tons of weight. '5 tons of AC' is 60,000 BTU/hr = 17.6 kW of cooling. Nothing weighs 5 tons.
Real-world power examples
- 100 W incandescent bulb: 100 W.
- LED equivalent of 100 W bulb: ~10 W (90% more efficient).
- Hair dryer: 1,500-1,800 W.
- Microwave (cooking power): 700-1,200 W.
- Espresso machine boiler: 1,000-1,500 W.
- Small window AC (5,000 BTU): 1,465 W cooling, ~500 W power draw.
- Central AC (3 ton): 10.5 kW cooling, ~3.5 kW power draw.
- EV charging (Level 2, home): 7.2 kW.
- EV fast charger (Supercharger V3): 250 kW peak.
- Toyota Camry (2026): 149 kW (200 hp).
- Tesla Model 3 Performance: 377 kW (506 hp).
- Formula 1 car (2026 hybrid): ~735 kW (985 hp).
- Saturn V rocket first stage: 45 million kW (60 million hp) peak.
Tips for accurate power conversion
- For appliance sizing, match cooling capacity to room area: 20 BTU/hr per sq ft in shaded rooms, 30+ in sunny rooms.
- For electric bill estimates, power (W) × time (hours) ÷ 1000 = kWh. 1,500 W × 4 hours = 6 kWh = ~$1 at average US rates.
- Horsepower ratings vary. SAE net hp (modern US spec) is lower than SAE gross hp (old US spec) and different from metric PS. Comparing across eras or regions requires the same measurement standard.
Related: Energy Converter · Electric Current Converter · Torque Converter.