Watts to BTU Calculator
Convert watts to BTU per hour — why a 1,500 W heater is also a 5,118 BTU/h heater.
Updated
In BTU per hour
5,118 BTU/hof heat
0.43 tons · 1.500 kW
- BTU per hour
- 5,118 BTU/h
- Tons of refrigeration
- 0.43
- Kilowatts
- 1.500 kW
- Over 24 hours
- 36.0 kWh
An electric heater converts essentially all of its input to heat, so its watt rating and its BTU/h output are the same quantity in two units.
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In short
How many BTU is 1500 watts?
5,118 BTU per hour. Multiply watts by 3.41214. For an electric heater that figure is both the electricity it draws and the heat it gives out, because a resistive element converts essentially all of its input into heat rather than moving heat around.
True for resistive heaters only. A heat pump outputs several times its input.
How to use the watts to BTU calculator
Multiply by 3.41214 and read the BTU per hour. This is the direction that turns an electrical rating into a heating one, and for one class of appliance the two figures are the same physical quantity in different clothes.
That class is resistive heating: bar heaters, fan heaters, oil-filled radiators, immersion elements, toasters. Electricity passes through a resistance and essentially all of it becomes heat. There is nowhere else for the energy to go, so a 1,500 W heater puts out 1,500 W of heat, which is 5,118 BTU per hour.
3.41214
BTU per hour in one watt
the power factor
5,118
BTU/h from a 1,500 W heater
input equals output
1,500 W
The standard portable heater
a 15 A circuit at 120 V
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The 1,500 watt figure is not arbitrary. A standard 15-amp circuit at 120 volts carries 1,800 watts, and the eighty percent rule for continuous loads puts the safe working limit at 1,440. Manufacturers settled just above it at 1,500, which is why almost every plug-in heater sold in North America has the same rating.
Where the equivalence breaks is heat pumps, and it breaks in the useful direction. A heat pump does not make heat; it moves heat that already exists from outside to inside. That lets it deliver three or four units of heat per unit of electricity, so its BTU output is several times what this conversion would suggest from its wattage.
- 100 W bulb, old style
- 341 BTU/h
- 500 W
- 1,706 BTU/h
- 750 W, low setting
- 2,559 BTU/h
- 1,000 W
- 3,412 BTU/h
- 1,500 W, standard heater
- 5,118 BTU/h
- 2,000 W
- 6,824 BTU/h
- 3,000 W, hardwired
- 10,236 BTU/h
The old incandescent bulb belongs on this list for a reason: it was a 341 BTU/h heater that happened to emit some light. Replacing a room of them with LEDs removes that heat, which is why the change is felt more in summer than in winter.
The same factor, the other way
BTU to watts converts an equipment rating into power, which is the direction you want when reading an air conditioner or furnace label.
Open BTU to watts →Electrical power in watts converted to heat output in BTU per hour, with the current each would draw on a 120-volt circuit.
| Watts | BTU per hour | Amps at 120 V | Tons |
|---|---|---|---|
| 100 | 341 | 0.83 | 0.03 |
| 250 | 853 | 2.08 | 0.07 |
| 500 | 1,706 | 4.17 | 0.14 |
| 750 | 2,559 | 6.25 | 0.21 |
| 1,000 | 3,412 | 8.33 | 0.28 |
| 1,200 | 4,095 | 10.00 | 0.34 |
| 1,500 | 5,118 | 12.50 | 0.43 |
| 2,000 | 6,824 | 16.67 | 0.57 |
| 3,000 | 10,236 | 25.00 | 0.85 |
| 5,000 | 17,061 | 41.67 | 1.42 |
Why resistive heating is the honest baseline
A resistive heater is the only heating appliance whose output you can state with certainty from its label alone. Everything else — heat pumps, gas boilers, wood stoves — has an efficiency or a coefficient of performance sitting between the input and the output, and that figure varies with conditions.
- Resistive heater
- 5,118 BTU/h out
- Heat pump at COP 3
- about 15,355 BTU/h out
- Heat pump at COP 4
- about 20,473 BTU/h out
- Electricity consumed
- identical in all three
- Cost per BTU delivered
- three to four times lower
- Same input
- very different output
The heat pump is not breaking any law: it is moving heat from outdoors, not creating it. Its advantage shrinks as the outside temperature falls, because there is less heat out there to move, which is why cold-climate performance is quoted separately.
The practical consequence is that comparing heating options by wattage alone only works within the resistive class. Across classes you need the efficiency figure, and the honest comparison is cost per delivered BTU rather than cost per unit of fuel.
Sizing a heater from the BTU figure
The rough rule for a moderately insulated room is around ten watts per square foot, or about thirty-four BTU per hour. A 150 square foot bedroom therefore wants roughly 1,500 watts, which is exactly what a standard portable heater delivers and is not a coincidence.
Do
- Read a heater wattage as its heat output directly
- Use about 10 watts per square foot for a first estimate
- Check the circuit can carry the load continuously
- Compare heating options on cost per delivered BTU
Don't
- Believe an electric heater is more efficient than another of the same wattage
- Run two 1,500 W heaters on one 15 A circuit
- Apply this conversion to a heat pump output
- Size from floor area alone in a poorly insulated room
The circuit warning is the practical one. Two 1,500 watt heaters on the same 15-amp circuit draw 25 amps, which will trip the breaker if you are lucky and overheat the wiring if you are not. One such heater per circuit is the rule, and it is why hardwired units above 1,500 watts exist at all.
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The formula, worked line by line
One multiplication by 3.41214. For a resistive heater the result is not an estimate of anything — it is the same quantity of power written in the unit the heating trade uses.
“An electric heater is the one appliance whose output you can read straight off the label, because there is nowhere else for the energy to go.”
That certainty is worth more than it sounds. Every other heating comparison involves an efficiency figure somebody has measured under conditions that may not be yours.
BTU per hour = watts x 3.41214
watts = (BTU per hour) / 3.41214
1,500 x 3.41214 = 5,118 BTU/h
tons = (BTU per hour) / 12,000
amps at 120 V = watts / 120- Power drawn
- 1,500 W = 1.5 kW
- Heat delivered
- 5,118 BTU/h
- Running 8 hours
- 12 kWh
- At $0.17 per kWh
- $2.04 a day
- Over a 30-day month
- $61.20
- Cost per 1,000 BTU
- about 5.0 cents
Sixty dollars a month for one room is why electric resistance heating is the expensive option almost everywhere. A heat pump delivering the same heat at a coefficient of three would cost around twenty, and gas is usually cheaper still per delivered BTU.
Why 1,500 watts is the standard
A 15-amp circuit at 120 volts carries 1,800 watts. The eighty percent rule for continuous loads brings the safe working figure to 1,440, and manufacturers sit just above it at 1,500. Anything larger has to be hardwired or run on a 240-volt circuit, which is why plug-in heaters cluster at one number.
Heat output per unit of electricity
- Resistive electric heater
- 3,412 BTU
- Heat pump, COP 2
- about 6,824 BTU
- Heat pump, COP 3
- about 10,236 BTU
- Heat pump, COP 4
- about 13,649 BTU
Every row consumes exactly one kilowatt-hour. The resistive row is the floor and cannot be beaten by any other electric heater, which is the useful thing to know when a product claims otherwise.
One habit to carry away: when a heating appliance quotes watts, that is what it costs to run. When it quotes BTU output, ask what it consumes, because the two are only the same for the simplest kind of heater.
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Questions people ask
How many BTU is 1500 watts?
5,118 BTU per hour, multiplying by 3.41214. For a resistive electric heater that is both what it draws and what it delivers, because the element converts essentially all of its electricity into heat. There is nowhere else for the energy to go.
Are some electric heaters more efficient than others?
Not for resistive heaters, which is nearly all portable ones. Every watt becomes heat regardless of the design, so two 1,500 W heaters produce identical heat. What differs is how that heat is distributed and how quickly a room feels warm, which is a comfort question rather than an efficiency one.
Why are portable heaters always 1,500 watts?
Because a 15-amp circuit at 120 volts carries 1,800 watts, and the eighty percent rule for continuous loads brings the safe working figure to 1,440. Manufacturers sit just above it. Anything larger needs hardwiring or a 240-volt supply, so plug-in heaters cluster at the same rating.
Does this apply to a heat pump?
No. A heat pump moves existing heat rather than creating it, so it delivers three or four times its electrical input as heat. Converting its wattage with this factor would badly understate its output. Use its coefficient of performance, and note that the figure falls as outdoor temperatures drop.
Can I run two 1,500 watt heaters on one circuit?
No. Two of them draw 25 amps at 120 volts, well over a 15-amp circuit rating and over a 20-amp one too. At best the breaker trips; at worst the wiring heats up without tripping anything. One 1,500 watt heater per circuit is the working rule.
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