BTU/h to Watts - Electric Heater Ratings and Amps at 240 V

Turning a Heater's BTU Rating into Watts and Amps

A heater arrives on site with 17,060 BTU/h stamped on the plate and nothing else useful for the person wiring it. Before a breaker can be chosen or a cable pulled, that thermal rating has to become watts, and watts have to become amps at the supply voltage. The first step is a single multiplication, and everything else on the circuit follows from it.

Conversion factor: 1 BTU/h = 0.29307107 W, so multiply the plate figure by 0.293. That 17,060 BTU/h heater is 5,000 W; on a 240 V supply it draws 20.8 A, and after the 125 % allowance for a continuous load that is a 30 A breaker on 10 AWG copper.

Why the Thermal Number Comes First

Heating is specified thermally

A room's heat loss is worked out in BTU/h, so the equipment that answers it gets labelled the same way — even when the energy arrives down a cable rather than a gas line.

Circuits are specified electrically

Breakers, conductors and load schedules only understand amps. Nothing on the electrical side of the job can be sized until the BTU figure has become watts.

No efficiency term in between

A resistance element turns every watt it takes into heat in the room, so the conversion is pure arithmetic with nothing to derate for.

The panel schedule needs a number

Every heater on the job adds to the connected load, and the total has to be recorded in watts or VA before the service can be checked.

From Nameplate to Breaker Size on Site

The sequence below is the one that gets used standing in front of an open panel with the appliance instructions in the other hand.

Enter the BTU/h from the rating plate

Type it as printed — 17060, 34 121, 5,118. Spaces are ignored, and a comma is read as a decimal separator, which matters when the plate came off a machine built for a European market.

Divide the watts by the supply voltage

Resistance heat has a power factor of 1, so amps are simply watts over volts and VA equals watts. 5,000 W is 20.8 A at 240 V, or 41.7 A if someone insists on putting it on 120 V.

Add the continuous-load allowance

Fixed space heating runs for hours at a time, so the overcurrent device and the conductor are sized at 125 % of the current. 20.8 A becomes 26 A, which lands on the next standard breaker up.

Reverse it to label an element in BTU/h

Given a 3,000 W element and a customer expecting a thermal figure, press the swap button (↔) for W → BTU/h and read 10,236. By hand it is a multiplication by 3.412.

Check which voltage the plate assumes: an element built for 240 V and fed 120 V produces a quarter of its rated heat, not half, because power falls with the square of the voltage. The BTU/h figure only holds at the rated supply.

Electric Heater Ratings and the Current They Draw

Common resistance heating equipment with its thermal rating, the electrical power behind it, and the current at each of the two usual supply voltages.

Heater	Rating (BTU/h)	Power (W)	At 120 V	At 240 V
Baseboard heater, 3 ft	2,559 BTU/h	750 W	6.25 A	3.13 A
Plug-in fan heater	5,118 BTU/h	1,500 W	12.5 A	6.25 A
Oil-filled column radiator	6,824 BTU/h	2,000 W	16.7 A	8.33 A
Infrared patio heater	10,236 BTU/h	3,000 W	240 V only	12.5 A
Wall convector, hard-wired	13,649 BTU/h	4,000 W	240 V only	16.7 A
Garage unit heater	17,061 BTU/h	5,000 W	240 V only	20.8 A
Workshop unit heater	25,591 BTU/h	7,500 W	240 V only	31.3 A
Air handler strip heat	34,121 BTU/h	10,000 W	240 V only	41.7 A

The 120 V column runs out quickly, and that is the whole reason plug-in heaters stop at 1,500 W: 12.5 A is already 83 % of a 15 A circuit, above the 80 % a continuous load should occupy, so the heater wants a 20 A circuit and no other significant load on it. Everything above about 2,000 W moves to 240 V, where the same heat costs half the current and a smaller conductor.

Handy Behaviour When You Are Up a Ladder

Watts appear while the plate is still in front of you

Both boxes update on every keystroke, so a plate with two ratings — high and low setting — can be checked one after the other without clearing anything.

Label an element back in thermal units

The swap arrows run W → BTU/h, which is the direction you need when a heating contractor asks what your 4 kW convector is worth against a room's heat loss.

kW on either side for panel schedules

Pick kW from the searchable dropdown when the load schedule is kept in kilowatts, or BTU/min if a plate happens to be marked per minute.

Bare figures for the load calculation sheet

Copy puts the digits alone on the clipboard, ready for a load schedule column or a message to the office, with no unit symbol to strip out.

Wiring Questions About Heater Ratings

The plate gives watts — how many amps will the heater actually draw?

Divide watts by the supply voltage. A resistance element is a pure resistive load with a power factor of 1, so there is no distinction between watts and VA and no reactive component to allow for: 4,000 W at 240 V is 16.7 A, the same 4,000 W at 120 V is 33.3 A. Nameplate voltage matters — check whether the plate says 208, 240 or 277 V before you divide.

Why is electric resistance heat treated as 100 % efficient?

At the appliance, every watt pushed through the element ends up as heat in the space. There is no flue carrying combustion products away and no unburnt fuel, so nothing is lost between the terminals and the room. That is why the conversion needs no efficiency factor: 1,500 W of element is 5,118 BTU/h delivered, not an input figure to be discounted.

What breaker and cable does a 10 kW strip heater need?

10,000 W at 240 V is 41.7 A. Space heating counts as a continuous load, so the branch circuit is sized at 125 % — 52 A — giving a 60 A breaker with 6 AWG copper at 75 °C, subject to any temperature or bundling derating on the run. Very large strip heat is normally split into two circuits with separate contactors, both because of conductor size and to allow staging.

Why is an imported electric heater labelled in BTU/h at all?

Because the buyer is comparing it against everything else that heats or cools the same room, and all of those are advertised in BTU/h. Manufacturers selling into North America print the thermal figure on the carton and leave the watts in small type on the plate. Anything shipped worldwide often carries both, and where they disagree slightly it is rounding, not a different rating.

Two heaters are both 1,500 W — why does one glow red and the other stay warm?

Total wattage sets the heat output; watt density sets the surface temperature. Spread 1,500 W along six feet of finned baseboard element — around 250 W per foot — and the sheath sits at a temperature you can nearly touch. Concentrate the same 1,500 W into a short quartz tube or an open coil and the surface glows, because the same energy leaves through a tiny fraction of the area. Both put 5,118 BTU/h into the room.

BTU per Hour to Watts