How Many BTUs (or Watts) Does a Patio Heater Need?

There is no standard for how many BTUs a patio needs, so the working answer is what the makers publish. Divide rated input by stated heated area for the 31 heaters in our calculator and the gas models land between 140 and 267 BTU/h per square foot, while the electric models land between 20 and 64 watts per square foot. For a 200 square foot seating zone that is about 28,000 to 53,300 BTU/h of gas, or 4,000 to 12,800 W of electric. The spread is wide because every maker measures coverage its own way.

Firepit HQ has not tested any heater. This guide explains the method behind our patio heater BTU calculator, with its constants and sources, then lists published coverage by model. Every figure comes from a maker's document or from a standards or government source listed below.

How Many BTUs Does a Patio Heater Need per Square Foot?

The calculator uses one method. For every heater in its list, it divides the rated input by the heated area the maker states. The lowest and highest results for gas or for electric become the range, and your area multiplies both ends.

• Gas list: 10 models from Bromic (Tungsten and Platinum Gas) and Schwank (supremeSchwank and phantomSchwank), rated in BTU/h.
• Electric list: 21 models from Infratech, Bromic and Schwank, rated in watts.
• Gas range: 140 BTU/h per square foot at the low end, from Schwank's supremeSchwank 35K (35,000 BTU/h over about 250 sq ft), to 266.7 at the high end, from its phantomSchwank 80K (80,000 over about 300).
• Electric range: 20 W per square foot at the low end, from Schwank's eSchwank ES/ESD 2 kW (2,000 W over about 100 sq ft), to 64 at the high end, from Infratech's SL-16 (1,600 W over 25 sq ft).

The low end is the most generous coverage claim in each list and the high end is the most cautious. A heater rated near the low end is counting on its maker's best case. The exposure setting in the calculator changes the notes under the result and leaves the numbers alone. The calculator does not size a fire pit, because fire pit makers publish no heated area. Patio heater vs fire pit covers that comparison.

Patio Heater Size by Area: Gas BTU and Electric Watts

These are the calculator's ranges for common seating zones.

Each row is a spread of maker claims, and none is a requirement. At 200 sq ft the top of the gas range is 1.9 times the bottom, and the top of the electric range is 3.2 times the bottom. Other guides here quote different spans because they divide different model sets, and some convert electric watts to BTU/h first. This guide keeps gas and electric separate, as the calculator does.

Here is a worked example with invented numbers, so swap in yours. A seating zone of 12 by 12 feet is 144 sq ft. The calculator range is about 20,200 to 38,400 BTU/h of gas, or 2,900 to 9,200 W of electric. Now read what real models state. Bromic's Tungsten Gas 300 states 160 sq ft for 26,000 BTU/h, in ideal conditions. Schwank's supremeSchwank 35K states about 250 sq ft at a 10 ft mounting height. Infratech's CD-60 states 121 sq ft on average, 100 cold and 144 protected, per its installation manual, so one heater covers this zone only if the space is well protected.

How Many Watts Does a Patio Heater Need?

Electric heaters are sized in watts, and the published rates run 20 to 64 W per square foot. By maker:

Heatstrip is the one maker I read that states a rule as well as a model's area. Its Classic manual says a protected outdoor area requires a minimum of 500 W per square meter, which it prints as 46 W per square foot, and recommends a minimum of 37 W per square foot for indoor spot heating. Its coverage table gives the 3,200 W model 86 sq ft indoors, 69 outdoors enclosed and 57 outdoors exposed.

Watts also decide the circuit. Infratech's reference manual lists its 6,000 W CD6024 at 240 V and 25 A, which is a panel question for a licensed electrician. The infrared patio heater guide covers voltage, amps and mounting.

How to Convert Watts to BTU/h for a Patio Heater

Multiply watts by 3.412142 to get BTU/h. NIST's conversion table gives 1 BtuIT per hour as 0.2930711 W, which inverts to that factor. EIA's 3,412 Btu per kilowatt-hour and PERC's 3,413 agree, and Infratech's own table converts at 3.412, printing 6,824 BTU for 2,000 W, 13,648 for 4,000 W and 20,472 for 6,000 W. Infratech's row for the 1,500 W C1524 prints 3,412, which does not follow that factor, so convert that one yourself.

Converted, the electric range of 20 to 64 W per square foot is 68 to 218 BTU/h per square foot, which overlaps the gas range of 140 to 267 without being the same claim. Going the other way, multiply BTU/h by 0.2930711. Bromic's 43,000 BTU/h Tungsten Gas 500 is about 12,600 W of input, and Schwank's 115,000 BTU/h phantomSchwank is about 33,700 W.

The conversion changes the unit and nothing else. A gas heater and an electric heater with the same BTU/h input do not necessarily put the same warmth on a chair, and I found no maker document that compares them. That is why the calculator keeps gas and electric as separate ranges.

Patio Heater Coverage Area by Model

The gas heaters in the calculator, with the maker's stated area:

Gas heaters published outside the calculator's list:

Lava Heat Italia's comparison table gives its smallest tower, the 42,000 BTU/h Capri, more area than the 56,000 BTU/h Lava Lite, and our Lava Heat Italia profile found coverage figures that differ between the brand's own pages and catalog. Ask the maker which figure applies before you size from it. A selection of the electric heaters:

The calculator lists all 31 of its models with sources, and each rate above is the input divided by the area, with no adjustment.

Does Wind or Exposure Change the Number?

Makers say yes and mostly stop short of a figure. Infratech's chart and Schwank's page carry nearly the same note: indoor or well-protected environments will generally exceed average coverage, and heaters may (Schwank says can) provide less than average coverage in extremely cold or windy conditions. Infratech's FAQ adds that colder or draftier areas may require more heat and protected or warmer areas less. None of them gives a wind speed or a temperature.

Infratech's installation manual for its C and CD series does print cold and protected columns beside the average one, in square feet.

For the 4,000 W C-4024 that works out to 62.5 W per square foot in the cold column, 40 on average and 27.8 protected. The calculator uses the chart's average figure and does not apply these columns. Heatstrip publishes the same idea by exposure. Its 2,400 W Classic covers 65 sq ft indoors, 52 outdoors enclosed and 43 outdoors exposed. Heatstrip also says coverage drops by up to 40 percent when the Classic is mounted on a wall at an angle.

Some gas makers publish wind speeds instead. Bromic's Tungsten Gas manual says the heater is designed to function in winds up to 10 mph and is subject to reduced performance or failure in high wind or extreme saturation, while its product page says up to 8 mph. The two Bromic documents disagree. The calculator cites the manual, and Bromic can say which figure applies to your site. The Platinum Gas cut sheet says up to 12 mph. Heatstrip's brochure gives its gas heaters a wind protection rating of 16 km/h, which it prints as 10 m an hour. Schwank's tip for its phantomSchwank line is to choose the 80K or 115K BTU/h model for colder locations.

None of these documents says how many square feet a given wind takes away. Reasoning from them, I would size an open, breezy patio from the high end of the range and a walled one from the low end.

Mounting Height Changes Coverage Too

Every coverage figure assumes a mounting height. Infratech pairs 6 to 7 ft with its 25 sq ft heater and 8 to 12 ft with its 121 sq ft one. Schwank states its areas at 10 ft. Heatstrip says to follow its maximum mounting heights or performance may be reduced, and says that bringing a heater from above 2.7 m (8 ft 10 in.) down to between 2.3 and 2.5 m (7 ft 6 in. to 8 ft 2 in.) increases its effectiveness.

Sizing for the People on the Patio

Infratech's FAQ cites testing by Pacific Gas & Electric's Food Services Testing Center. A good patio installation, gas or electric, provides 3 to 7 degrees of perceived heat at 60°F, and more than 7 was too warm to sit in for long. Plan for that. A covered patio will not feel like indoors.

Two maker notes help with layout. Infratech says the best comfort comes from heating from two sides instead of one. Heatstrip says the most effective method for most outdoor layouts is to spot heat a table, and its table diagrams pick from one to four heaters for common residential table settings. For a large zone, plan more than one heater.

ASTM's F2644 is a test method for commercial gas and electric radiant patio heaters that measures energy input rate, preheat, temperature distribution and effective heated area. It rates a heater and says nothing about how large your patio is, and I read only its store summary.

What to Check Before You Pick a Size

• The zone: measure the seating area in square feet, not the whole patio.
• The mounting height: note what you can actually reach, because it decides which published areas apply.
• The exposure: choose the upper part of the range for open, windy sites and the lower part for walled ones.
• The basis: read whether the maker's area assumes ideal conditions, a mounting height or an exposure.
• The supply: a fixed gas heater needs a gas line sized for its input, so run the gas line sizing calculator before you buy, and have a licensed gas fitter do the line. An electric heater needs a circuit a licensed electrician sizes.
• The roof: gas heaters have open-air rules, which outdoor heaters for covered patios goes through, and how to choose a patio heater covers fuel and mounting.
• Local rules: check your local code and the installation manual for the model, starting with the fire code library.