Power Converters — watts, kilowatts, horsepower, BTU/h
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Power conversions span four dominant units that together cover everyday electrical and consumer-electronics power ratings, US-customary automotive engine specifications, residential and industrial HVAC heating-and-cooling capacity, and SI-canonical scientific and engineering documentation. The watt (W) is the SI-derived primary power unit specified by ISO 80000-6 for technical writing across electrical engineering, mechanical engineering, electronics, lighting, audio, and consumer-product power-rating contexts globally. The kilowatt (kW) is the everyday-engineering scale unit for residential and small-commercial electrical applications, mandated by EU Ecodesign Directive 2009/125/EC on every EU-jurisdiction electrical product power-rating label. The horsepower (hp) is the dominant US-customary automotive engine-power unit, with the parallel metric horsepower (PS, "Pferdestärke") used in continental European automotive specs alongside kW as the regulatory primary. The BTU per hour (BTU/h) is the universal US-customary heating-and-cooling-capacity unit on every US-domestic air conditioner, gas furnace, heat pump and tankless water heater, with the iconic "ton of refrigeration" at exactly 12,000 BTU/h as the standard HVAC capacity reference. The four units coexist across cross-jurisdictional contexts where electrical-power, automotive-engine, HVAC-capacity, and SI-canonical scientific-engineering documentation all need parallel power-unit reference frameworks. The watt is the SI-derived primary for new technical writing, kilowatt the everyday-engineering scale for residential and small-commercial contexts, horsepower the US-customary automotive-engine convention, and BTU/h the universal US-customary HVAC-capacity unit for heating-and-cooling equipment ratings.
Units in this category
Watts (W)
The watt (W) is the SI-derived unit of power, equal to one joule of energy delivered per second of time (1 W = 1 J/s = 1 N·m/s = 1 V·A in electrical engineering). The watt is anchored to the SI base units kilogram, metre and second through the relationship 1 W = 1 kg·m²·s⁻³. Since the 2019 SI redefinition the watt is fixed via the Planck constant h = 6.62607015 × 10⁻³⁴ J·s exactly, with the kilogram derived from this anchoring rather than the historical artifact-based definition.
Kilowatts (kW)
The kilowatt (kW) is exactly 1000 watts by SI prefix definition. The relationship is fixed and exact, with the kilo- prefix denoting 1000 of the underlying unit. One kW equals 1000 J/s of energy delivery, or 1000 W of power output, or 3.6 MJ delivered per hour (the basis of the kWh energy-billing unit).
Horsepower (hp)
The horsepower (hp) under the US-customary mechanical-horsepower definition is exactly 745.69987158227022 W (typically rounded to 745.7 W). The metric horsepower (PS, sometimes "hp" in continental European contexts) is exactly 735.49875 W — about 1.4% smaller than the mechanical hp. The recognised symbols are "hp" (mechanical, US-customary) and "PS" or "ch" (metric, continental European), with both widely used and the disambiguation context-dependent.
BTUs per hour (BTU/h)
The BTU per hour (BTU/h) under the modern IT-BTU convention is exactly 0.29307107 W (typically rounded to 0.293 W per BTU/h). The factor follows from 1 BTU = 1055.06 J ÷ 3600 s = 0.293 W. Equivalently, 1 W = 3.412 BTU/h.
History of power measurement
Power measurement traces from James Watt's late-eighteenth-century steam-engine improvements through twentieth-century unit standardisation. James Watt patented the separate-condenser steam-engine improvement in 1769 and defined the "horsepower" around 1782 as a marketing tool for selling steam engines against horse-team capacity, with the figure rounded to 33,000 foot-pounds per minute (550 foot-pounds per second) for convenience. The unit "watt" was formally adopted by the British Association for the Advancement of Science in 1882 to name the SI-derived power unit equal to one joule per second, and incorporated into the SI at the 11th CGPM in 1960. The BTU per hour emerged as the dominant US-customary heating-and-cooling-capacity unit through twentieth-century US-HVAC industry consolidation, with the IT-BTU formalised at 1055.06 J at the 1956 International Steam Table Conference and the "ton of refrigeration" at 12,000 BTU/h as the iconic US-HVAC-trained capacity reference. The 2019 SI redefinition fixed the watt via the kilogram-Planck-constant chain through the joule-per-second relationship.
Where power conversions matter
Power conversions appear across every modern electrical, automotive, HVAC, and aerospace discipline. Electrical engineering and consumer-electronics power ratings: every electrical and electronic device denominates power consumption in watts on the consumer-facing rating label, with EU Ecodesign Directive mandating watt-based product power-rating on every EU-jurisdiction electrical product. Typical residential lighting at 5-15 W LED, kitchen appliances at 800-3000 W, computer power supplies at 350-1500 W, residential solar-PV inverters at 3000-10000 W (3-10 kW). US-customary automotive engine power specs use horsepower universally on every US-domestic car, truck, motorcycle, and ATV (typical US passenger-car 150-300 hp, sports-car 400-700 hp, supercar 700-1000 hp). Continental European automotive specs preserve metric horsepower (PS) alongside kW regulatory primary under EU vehicle-type-approval rules. US residential HVAC and gas furnace ratings universally use BTU/h (typical residential window AC 5000-12,000 BTU/h, central AC 24,000-60,000 BTU/h or 2-5 tons of refrigeration). Industrial-electrical and mechanical-equipment ratings use kW globally (industrial pneumatic compressors 5-50 kW, industrial heat pumps 5-500 kW, commercial HVAC 10-1000 kW). EV-battery and EV-charging systems use kW for charging-rate and kWh for capacity (Tesla Model 3 at 250 kW peak DC fast-charge, 75 kWh battery). Renewable-energy generation: residential solar-PV systems at 3-10 kW peak rated capacity, commercial-rooftop solar at 50-500 kW, utility-scale solar farms at 10-500 MW with each MW = 1000 kW.
How to convert power units
Power-unit conversion runs against the watt as the SI-canonical reference, with each non-SI unit related to the watt by an exact (or near-exact) numerical factor: 1 kW = 1000 W exactly, 1 hp (mechanical, US-customary) = 745.7 W (specifically 745.69987158227022 W), 1 PS (metric horsepower, continental European) = 735.49875 W, 1 BTU/h = 0.29307107 W (specifically derived from 1 BTU = 1055.06 J ÷ 3600 s). Cross-conversion between non-SI units typically goes through the watt reference: kW-to-hp = kW × 1.341, hp-to-kW = hp × 0.7457, BTU/h-to-kW = BTU/h × 0.000293, kW-to-BTU/h = kW × 3412.14. The "ton of refrigeration" at exactly 12,000 BTU/h equals 3.517 kW, and the conversion runs at every cross-jurisdictional HVAC-capacity reference. Mechanical horsepower (US-customary at 745.7 W) and metric horsepower (PS at 735.5 W) differ by about 1.4%, with the disambiguation context-dependent — US automotive specs use mechanical hp, continental European specs use metric PS alongside kW regulatory primary.
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Frequently asked questions
How many watts in a kW?
One kilowatt equals exactly 1000 watts by SI prefix definition. The kilo- prefix means 1000, so 1 kW = 1000 W. The relationship is exact and unchanged across every metric measurement context. Higher-power multiples use megawatts (MW, 10⁶ W) for grid-scale power-generation and gigawatts (GW, 10⁹ W) for nuclear-power-station references.
How many kW is 1 hp?
One mechanical horsepower (US-customary) equals exactly 0.7457 kW (specifically 745.69987158227022 W). One metric horsepower (PS, continental European) equals 0.7355 kW (735.49875 W). The two definitions differ by about 1.4% and the disambiguation is context-dependent — US automotive specs use mechanical hp, continental European automotive specs use metric PS alongside kW regulatory primary.
How many BTU/h is 1 kW?
One kilowatt equals 3412.14 BTU/h under the IT-BTU definition. Equivalently, 1 BTU/h = 0.293 W. The conversion is exact derived from 1 BTU = 1055.06 J at the IT-BTU definition divided by 3600 s per hour. The "1 kW = 3412 BTU/h" reference is the canonical cross-jurisdictional HVAC-capacity conversion factor.
What is a "ton of refrigeration"?
One ton of refrigeration equals exactly 12,000 BTU/h or 3.517 kW. The unit is the iconic US-HVAC-trained capacity reference, derived historically from the cooling-power required to melt one ton of ice in 24 hours (1 ton × 144 BTU/lb × 2000 lb/ton ÷ 24 h = 12,000 BTU/h). A typical residential central AC delivers 2-5 tons of refrigeration; a typical commercial rooftop unit delivers 5-25 tons.
Why does the US use hp but most countries use kW?
The mechanical horsepower emerged in late-eighteenth-century US-and-British engineering practice and persisted through the twentieth-century US-customary engineering ecosystem, with US automotive consumer-marketing universally using hp on every US-domestic car, truck, motorcycle. Most other countries adopted metric kW as part of broader metrication transitions in the 1960s-1970s, with EU vehicle-type-approval mandating kW as the regulatory primary. The continental European market preserves metric horsepower (PS) alongside kW for consumer-recognition reference.
How many kW is a typical car engine?
A typical US passenger-car engine delivers 150-300 hp = 112-224 kW. A typical European compact-car engine delivers 90-150 PS = 66-110 kW. A typical sports-car engine delivers 400-700 hp = 298-522 kW. A typical Tesla Model S Plaid delivers 1020 hp = 760 kW peak motor power across three motors. The kW figure is the regulatory primary on EU vehicle-type-approval documentation; the hp or PS figure is the consumer-recognition reference.
How many BTU/h does a typical air conditioner need?
A typical residential window air conditioner delivers 5000-12,000 BTU/h cooling capacity, sufficient for a 150-450 ft² room (15-42 m²). A typical central residential AC delivers 24,000-60,000 BTU/h (2-5 tons of refrigeration), sufficient for a 1500-3000 ft² home depending on insulation and climate. AC-sizing rules-of-thumb suggest 20 BTU/h per square foot of conditioned space.