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Watts, BTU, and the 1,500 watt ceiling

Every plug-in electric patio heater sold in the United States is 1,500 watts. That is not a coincidence, a coordination, or a coincidence of marketing. It is your wiring.

By Scooter M.

Published September 1, 2026

Last reviewed September 1, 2026

Measuring tools and a notebook on a wooden surface

1,500 watts is 5,118 BTU per hour, at 3,412 BTU per kWh. Every plug-in electric patio heater in the U.S. stops there because electrical code caps a continuous load at 80 percent of the circuit rating: 15 A x 120 V x 0.8 = 1,440 W.

The conversion

Watts measure power. BTU per hour measure power too — they are the same kind of quantity in different units, which is why one converts cleanly into the other. One kilowatt-hour is 3,412 BTU, so:

WattsBTU per hourTypical device
750 W2,559 BTU/hA small electric heater on its low setting
900 W3,071 BTU/hThe low setting on most 1,500 W patio heaters
1,200 W4,095 BTU/hThe middle setting
1,500 W5,118 BTU/hEvery plug-in patio heater at full output
2,000 W6,824 BTU/hNeeds a 20 A circuit
3,000 W10,237 BTU/hHardwired 240 V territory
14,065 W48,000 BTU/hWhat a propane tower produces

Computed at 3,412 BTU per kWh. That last row is the one worth staring at: matching a mid-range propane tower electrically would take about 14 kW, which is roughly ten household circuits.

Where the 1,500 W ceiling comes from

A standard U.S. wall outlet is on a 120-volt, 15-amp branch circuit. That is 1,800 watts on paper. But electrical code treats any load running for three hours or more as a continuous load and limits it to 80 percent of the circuit rating, which gives 1,440 watts.

A patio heater run for an evening is unambiguously a continuous load. So manufacturers build to 1,500 W as the conventional maximum for a plug-in appliance and no further, and the figure is set by your house rather than by their engineering. It is the same reason every hair dryer, every space heater and every electric kettle sold in the U.S. lands at the same number.

The consequence people meet the hard way

Two 1,500 W heaters on one circuit is 3,000 W against a 1,440 W continuous limit. The breaker will trip, and repeatedly resetting it is the wrong response — that is a protective device doing exactly its job. Two heaters need two circuits.

What to do when your patio needs more

If the calculator returns more than about 5,000 BTU/h, one plug-in electric heater will not do it, and you have four options rather than one.

  1. Two units on two circuits. Perfectly workable, and it also solves the coverage-shape problem on a long patio. Requires two genuinely separate circuits, not two outlets on the same one.
  2. Reduce the requirement. A windbreak cuts the need by about a third on our model; a roof cuts it by another third. Cheaper than an electrician and it lowers every future bill too.
  3. Go propane. A tower carries its own energy supply and produces about ten times what an outlet can. This is the whole case for propane on an open patio.
  4. Hardwire at 240 V. A 240 V circuit doubles the available power for the same current, which is how commercial patio heaters reach 3 kW and beyond. It is an electrician's job and a permanent installation.

Reading a quartz heater's wattage

Listings for quartz and carbon infrared heaters often quote 2,000 W or a similar figure. Read those carefully, because they are sometimes an element rating rather than the unit's actual draw, and sometimes a figure from a market where 230 V is standard.

The number to trust is the one on the rating plate and in the electrical specification: volts, amps and watts. If a unit genuinely draws 2,000 W at 120 V, that is 16.7 amps, which exceeds a 15-amp circuit outright and needs a 20-amp one. That is a real constraint and not a detail to discover after mounting.

Why propane numbers look so much bigger

Because they are, and the reason is simply that a propane heater carries its own fuel rather than drawing through a wire. 48,000 BTU/h is 14 kW-equivalent. No plug delivers that.

What the big number does not tell you is how much of that output reaches a person, and that is where the electric versus propane comparison actually resolves. Ten times the output is not ten times the warmth when most of it is warming air that then leaves.

Common questions

How many BTU is 1,500 watts?

5,118 BTU per hour. The conversion is 3,412 BTU per kWh, so 1.5 kW x 3,412 = 5,118. Heat Storm publishes 5,200 for its 1,500 W unit, which is the same figure rounded.

Why are all electric patio heaters 1,500 watts?

Because a standard 15-amp, 120-volt circuit allows 1,440 watts of continuous load once code's 80 percent derating is applied. 1,500 W is the conventional maximum for a plug-in appliance, and no manufacturer can sell past your wiring.

Can I run a 2,000 W patio heater on a normal outlet?

Not on a 15-amp circuit. 2,000 W at 120 V is 16.7 amps, which exceeds the circuit rating outright and is well past the 12-amp continuous limit. It needs a 20-amp circuit, and you should check the rating plate rather than the listing headline.

How many watts would match a 48,000 BTU propane heater?

About 14,065 W, which is roughly ten household circuits. That is why no plug-in electric heater competes with a propane tower on output, and why the comparison between them has to be about where the heat goes rather than how much there is.

Sources

Every figure on this page either comes from one of these sources or is computed on the page from a figure that does. If you think one of them is wrong, tell us — we publish corrections.

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