Kilowatt to BTU/hour

A typical Western household draws 1–5 kW on average, but peak demand can spike to 10–15 kW when the oven, dryer, AC, and water heater all run simultaneously. This peak is why electrical panels are sized at 100–200 amps (24–48 kW capacity). Adding an EV charger at 7–11 kW can push some older homes past their panel limits, requiring an upgrade.

EU directive 80/181/EEC mandated kilowatts as the official unit for engine power, making kW the legally required figure on vehicle documents since 2010. Manufacturers still advertise in PS (metric horsepower) because consumers are used to it, but the official registration papers always list kW. One kW equals about 1.36 PS, so a 100 kW engine is roughly 136 PS.

Home Level 2 chargers draw 7–22 kW, adding 30–130 km of range per hour. Public DC fast chargers range from 50 kW (older units) to 350 kW (latest ultra-rapid chargers). Tesla Superchargers V3 peak at 250 kW. A 350 kW charger can add 300 km of range in about 15 minutes on compatible vehicles — but your home wiring cannot deliver anywhere near that without industrial-grade supply.

When power returns after an outage, everything turns on simultaneously — fridges, AC compressors, water heaters, furnaces — creating an "inrush" spike 3–5× normal draw. A home that normally peaks at 10 kW might briefly pull 30–40 kW. This is why utilities restore grids in stages (rolling reconnection) rather than all at once: if an entire neighborhood surges simultaneously, transformers can overload and blow, causing a cascading failure that extends the blackout. Some smart thermostats now stagger restart to reduce this risk.

With modern 400 W residential panels, you need just 2.5 panels (so 3 in practice) for 1 kW of peak capacity. A decade ago, when panels were 250 W each, you needed 4. That 1 kW of panels produces roughly 1,000–1,600 kWh per year depending on location — enough to power a large refrigerator for a full year. A typical home installation is 4–10 kW (10–25 panels).

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.