BTU/hour to Horsepower (Electric)

The classic rule: 20 BTU/h per square foot. A 300 sq ft bedroom needs about 6,000 BTU/h; a 500 sq ft living room about 10,000 BTU/h. But this varies wildly with sun exposure (+10% for south-facing), ceiling height, insulation quality, number of occupants (+600 BTU per person), and climate zone. A room above a pizza oven in Phoenix needs more than a basement in Seattle. When in doubt, oversize slightly — an undersized unit runs constantly and never reaches setpoint.

Undersizing is obvious — the unit runs constantly and never reaches the thermostat setpoint on hot days. But oversizing is worse in subtle ways. An oversized AC cools the air quickly then shuts off before removing enough humidity, leaving you with a clammy 72°F house. The short cycles also wear the compressor faster (startup is the hardest moment) and waste energy. A 1-ton oversize in a humid climate like Florida can raise indoor humidity from a comfortable 45% to a muggy 60%. Proper Manual J load calculations matter more than most homeowners realize.

Exactly 12,000 BTU/h. One ton of AC is the cooling effect of melting one short ton (2,000 lbs) of ice over 24 hours. The ice absorbs 288,000 BTU of heat as it melts (2,000 lbs × 144 BTU/lb latent heat), divided by 24 hours = 12,000 BTU/h. Residential systems run 1.5–5 tons; commercial buildings 10–500 tons. The "ton" unit persists because HVAC contractors think in tons — "that house needs a 3-ton unit" is faster than "that house needs 10.5 kW of cooling."

Modern units achieve 12–25 BTU/h per watt of electricity (SEER 12–25). A SEER 20 unit removes 20 BTU/h of heat for every watt consumed — effectively a 3:1 heat pump ratio. That 12,000 BTU/h window unit draws 500–1,000 W of electricity depending on efficiency. The best mini-splits achieve SEER 30+, removing 30 BTU/h per watt, making them cheaper to run than resistive electric heaters even in heating mode.

A gas furnace's BTU/h rating is its thermal output after combustion efficiency losses (typically 80–96% of fuel input). A heat pump's BTU/h rating is the heat delivered including energy moved from outside — at COP 3, a heat pump delivering 36,000 BTU/h uses only 12,000 BTU/h worth of electricity. This makes direct BTU/h comparisons misleading: a 60,000 BTU/h furnace and a 60,000 BTU/h heat pump deliver the same heat, but the heat pump uses one-third the energy.

It's a deliberate rounding for simplicity. The mechanical horsepower is 745.69987... watts — an ugly number for electrical calculations. The electric motor industry rounded to 746 W for clean nameplate ratings and straightforward calculations. The 0.04% difference is far below any motor's manufacturing tolerance. Some standards even use 750 W as an approximation. In practice, the distinction between 745.7 and 746 matters only to standards lawyers and unit conversion pedants.

Divide the hp rating by the motor efficiency and power factor. A 5 hp motor at 90% efficiency draws: 5 × 746 / 0.90 = 4,144 W of electrical power to deliver 3,730 W of shaft power. For three-phase motors, also divide by (√3 × voltage × power factor) to get amperage. Real-world efficiency ranges from 75% for small motors to 96% for large premium-efficiency ones. The nameplate always shows shaft output, not electrical input — a common source of confusion.

Because American homeowners and pool contractors have decades of experience sizing pumps in hp: "a 20,000-gallon pool needs a 1.5 hp pump" is ingrained knowledge. The Department of Energy actually mandated variable-speed pool pumps in 2021 and encouraged watt-based efficiency ratings, but manufacturers still prominently display hp because it drives purchasing decisions. A customer choosing between a 1 hp and 1.5 hp pump understands the difference intuitively; 746 W vs 1,119 W means nothing to them.

NEMA (National Electrical Manufacturers Association) defines standard motor sizes using electric hp: 1/4, 1/3, 1/2, 3/4, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100 hp and up. These are standardized frame sizes — a 5 hp motor from any manufacturer fits the same mounting bolts. The hp(E) rating is the shaft output; NEMA also specifies efficiency classes (IE1 through IE4) that determine how much electrical power the motor actually consumes.

Not usually. EV manufacturers rate motors in kilowatts because the electrical connection is obvious and kW is internationally understood. A Tesla Model 3 motor is listed as 211 kW, not "283 hp(E)." However, marketing materials often convert to hp for American consumers: "283 horsepower" sounds sportier. Interestingly, EV motors are far more efficient (90–97%) than combustion engines (25–40%), so a 200 hp EV motor consumes far less total energy than a 200 hp gasoline engine.