How to Convert Watts to BTU

Most electrical gear is labeled in watts. Most HVAC and gas gear is labeled in BTU. When the two worlds collide — sizing a heat load, comparing a space heater to a furnace, picking a backup generator — you need to flip watts into BTU per hour.

The math is one multiplication. This guide covers the formula, worked examples, and the cases where the number means something different than you’d expect.

BTU/h = Watts × 3.412

Multiply the wattage by 3.412 and you have BTU per hour. The factor is the same one used for the reverse conversion — it comes from the ratio of joules in a BTU (1,055.06) to seconds in an hour (3,600).

For higher precision, use 3.41214. For quick estimates, 3.4 is close.

The reverse:

Watts = BTU/h ÷ 3.412

A 1,500 W space heater. What’s the BTU rating?

• Start with the watt value: 1,500
• Multiply by 3.412: 1,500 × 3.412 = 5,118
• Round: 5,118 BTU/h

A 1,500 W heater outputs 5,118 BTU per hour. That’s the heat delivered to the room, assuming standard resistance heating (close to 100% efficient at converting electricity to heat).

A few shortcuts:

Triple it and add 14%.1,500 × 3 = 4,500. Add 14% (630) to get 5,130. Exact: 5,118. Close enough.

Round to 3.4 for mental math.750 × 3.4 = 2,550 BTU/h. The exact answer is 2,559. The 9 BTU/h error doesn’t matter.

Use 1 kW = 3,412 BTU/h as the anchor. Scale up or down from there. 2 kW = 6,824. Half a kW = 1,706.

Watts come in three common scales:

• Watt (W) — small appliances, light bulbs
• Kilowatt (kW) — heaters, AC units, EV chargers (1 kW = 1,000 W)
• Megawatt (MW) — power plants, industrial sites (1 MW = 1,000,000 W)

Convert kW to BTU/h: multiply by 3,412. The BTU/hr to kW converter and BTU/hr to MW converter handle the math both ways.

• 5 kW = 17,060 BTU/h
• 10 kW = 34,120 BTU/h
• 50 kW = 170,600 BTU/h

For MW, the numbers get big:

• 1 MW = 3,412,000 BTU/h = 3,412 kBTU/h
• 10 MW = 34.12 million BTU/h

The result depends on what kind of device you’re converting:

Electric heater (resistance): the BTU number matches the heat output almost exactly. Every watt of electricity becomes a watt of heat. A 1,500 W heater produces about 5,118 BTU/h of room heating.

Heat pump: the watt rating on the nameplate is electrical input. The heat output is 2 to 4 times higher. A 1,500 W heat pump might deliver 15,000 to 20,000 BTU/h, not 5,118. The COP (Coefficient of Performance) sets the multiplier.

Air conditioner: same story in reverse. A 1,000 W AC pulls 1,000 W from the wall but moves heat at 3 to 4 times that rate. The BTU label on the box is the cooling output, not the electrical draw multiplied by 3.412.

Light bulb or computer: almost all the watts end up as heat eventually. A 100 W bulb adds 341 BTU/h to the room. A 65 W laptop adds 222 BTU/h. Useful when sizing AC for offices with lots of equipment.

Designers use watts-to-BTU conversion when sizing cooling for spaces full of electronics. The steps:

• Add up the wattage of every device that runs at peak
• Multiply by 3.412 to get BTU/h
• Add human body heat (about 400 BTU/h per person)
• Add solar gain through windows
• Total = cooling capacity needed

Example: a small office with five computers (200 W each), ten LED lights (10 W each), and three people.

• Computers: 5 × 200 = 1,000 W → 3,412 BTU/h
• Lights: 10 × 10 = 100 W → 341 BTU/h
• People: 3 × 400 = 1,200 BTU/h
• Subtotal: 4,953 BTU/h
• Solar gain (estimate): 2,000 BTU/h
• Total: 6,953 BTU/h

A 9,000 BTU/h mini-split would handle it with room to spare. For KVA loads (UPS, generators) use the KVA to BTU/hr converter since power factor changes the heat-output math.

The conversion is exact for energy quantities. It treats every watt as identical. Real-world equipment doesn’t always cooperate:

Motors with reactive power.A motor rated at 1 kW might draw 1.2 kVA from the wall due to power factor. The “real” power is still 1 kW, so the BTU conversion uses 1 kW. The apparent power matters for wiring and breakers, not heat output.

Variable-speed drives. A heat pump cycling between 30% and 100% output never sits at one wattage. The nameplate value is peak. Real-world heat delivery is lower on average.

Standby losses. A 1 kW heater rated for continuous operation might pull 1,005 W including controls. The extra 5 W shows up as heat anyway, so the BTU conversion is still accurate within rounding.

Print it, tape it to your toolbox, never reach for a calculator again.

Multiply watts by 3.412 to get BTU per hour. That’s the entire procedure. Keep 1 kW = 3,412 BTU/h in your head as the anchor and you’ll handle every other value by scaling. Watch out for heat pumps and AC units — their nameplate watts aren’t the same thing as their heat output.