What Does Patio Heater BTU Mean?

BTU stands for British thermal unit. The EIA defines one BTU as the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. The patio heater BTU figure on a label is the fuel the burner uses per hour at its highest setting. It tells you how much heat the heater can release, not how much of that heat reaches your guests.

Electric heaters are rated in watts. To compare them with propane, use the EIA conversion of 3,412 BTU per kilowatt-hour: 1 watt equals about 3.4 BTU per hour.

These are our own conversions. As you will see below, a watt of well-aimed infrared heat usually does more for comfort than the same number of BTU from a propane tower, so do not compare the two types on BTU alone.

Why the “20 BTU per Square Foot” Rule Falls Short

Many buying guides, including those of large retailers, say you need about 20 BTU per square foot of patio. By that rule, a 48,000 BTU heater would warm 2,400 square feet, the size of a large house. No mainstream manufacturer we checked makes that claim for a single propane tower.

The rule appears to come from indoor heating, where walls and a roof keep warm air in. Outdoors, warm air rises and drifts away, so you need far more heat per square foot. At least one widely shared patio heater sizing table online is identical to the ENERGY STAR sizing chart for room air conditioners, which says a lot about how reliable these tables are.

Our advice: ignore BTU-per-square-foot rules that are not linked to real heaters. Start from the area manufacturers say their heater covers, then adjust for wind, roof and how warm you want to be.

What Patio Heaters Really Cover: Manufacturer Claims

We collected the coverage claims for common heater types. “Implied BTU per sq ft” is our own calculation from each claim.

Most patio heater BTU claims cluster around 180 to 260 BTU per square foot. The Mr. Heater claim is a clear outlier; its own low-profile model lists two different coverage figures, so treat very large claims with caution. Cuisinart even states 30 square feet on its U.S. site and 100 on its Canadian site for the same tabletop heater.

Patio Heater BTU Rules of Thumb

How many BTU do you need in practice? Based on the manufacturer claims above, these patio heater BTU rules give a realistic starting point for a propane or gas heater on a sheltered patio on a cool evening:

Rule of thumb: plan about 200 BTU per square foot of seating area for propane. Need more warmth because it is windy or near freezing? Plan on the higher end, or add a second heater instead of buying one with a higher rating.

Patio Heater Size Chart: How Many BTU for Your Patio?

To find the right patio heater size, use the area where people actually sit, not the size of the whole patio. A 20 by 20 foot patio with one dining table in the corner only needs heat around that table.

The electric column assumes 20 to 30 watts per square foot, based on the Solaira guidance explained in the next section. For an open, unsheltered patio, choose the upper end of each range.

Sizing an Electric Patio Heater in Watts

Electric infrared heaters are sized in watts per square foot. Solaira publishes a “recommended heat density” for a comfort increase of about 10°F:

For a 15°F increase, Solaira says to multiply these figures by 1.5; for 20°F, by 2.0. Manufacturer coverage claims point the same way:

• Infratech: a 1,500 W heater covers about 5 × 5 ft at a mounting height of 6–7 ft, while a 6,000 W heater covers about 11 × 11 ft at 8–12 ft.
• Bromic Tungsten Electric: 2,000, 3,000, 4,000 and 6,000 W models cover about 65, 100, 130 and 160 sq ft.
• Solaira: a 1.5 kW heater mounted at about 8 ft gives roughly 64 sq ft of effective heat.

A plug-in heater on a standard 120 V outlet is limited to 1,500 W. Infratech says a 120 V heater is fine for outdoor spaces no larger than about 5 by 5 feet. For more, you need 240 V models, installed by an electrician. Infratech also warns that a 240 V heater connected to 208 V gives only about 60% of its intended output.

How to Choose the Right Patio Heater BTU in 4 Steps

How Many Patio Heaters Do You Need?

Bromic suggests a simple method: divide the area you want to heat by the coverage of one heater. If one heater covers about 215 square feet and your seating area is 400 square feet, you need two.

When you work out how many patio heaters to buy, remember that two smaller heaters also spread the warmth more evenly than one big one. With a single tower in the middle of a long table, the people at the ends stay cold. Place heaters so that their circles overlap slightly over the seats, not over the walkways.

These counts are our own calculations from the manufacturer coverage claims. For electric heaters, larger 4,000 to 6,000 W models reduce the number you need.

What Changes How Warm a Patio Heater Feels

• Wind: several propane manuals say not to use the heater above 10 mph, and Bromic rates its portable heater for winds up to 8 mph. Even a light breeze carries warm air away, so add a windbreak where you can.
• Roof and walls: Infratech says sheltered spaces generally exceed the average coverage, and Bromic notes that covered terraces lose less heat. A covered patio often needs a smaller heater.
• Outdoor temperature: Bromic rates its gas heater coverage at about 60°F in a low-wind covered space. On a 40°F evening, plan for more heat or more heaters.
• Mounting height: wall and ceiling heaters need a minimum height for safety, but the higher you mount them, the weaker the heat at seat level. Infratech pairs higher wattages with higher mounting heights.
• Distance to the seats: radiant heat drops quickly with distance. Put the heater close to where people sit, within the clearances in the manual.

Propane.com says propane patio heaters can raise the outdoor temperature around them by up to 25°F. Treat that as a best case on a calm, sheltered evening.

Patio Heater BTU and Fuel Use

A higher patio heater BTU rating means more heat, but also more fuel. A full 20 lb propane cylinder holds about 4.7 gallons, or roughly 430,000 BTU by our calculation from the EIA figure of 91,452 BTU per gallon.

A 1 lb cylinder on a 10,000 to 11,000 BTU tabletop heater lasts about two to three hours, according to Napoleon, Fire Sense and Cuisinart. Our propane vs electric patio heater guide shows what this means for your running costs.

Worked Examples

Bistro set on a balcony

Two chairs and a small table take up about 30 square feet. If your building allows it, a 1,500 W electric heater aimed at the seats is enough. Propane is often not allowed on apartment balconies, so check the rules first.

Open deck with a dining table for six

How many BTU for this deck? The table and chairs cover about 12 × 14 feet, or 168 square feet. At about 200 BTU per square foot, that is roughly 34,000 BTU, so one 46,000 to 48,000 BTU tower in a sheltered spot works. On a windy deck, place two smaller heaters at the ends of the table.

Covered patio with a lounge set

A 10 × 12 foot seating area is 120 square feet. Under a roof, 15 to 20 W per square foot gives 1,800 to 2,400 W. Two 1,500 W heaters mounted on opposite sides, or one hardwired 2,000 to 3,000 W heater, would fit. Read our guide on using a patio heater under a covered patio for the mounting rules.

Common Patio Heater Sizing Mistakes

• Sizing for the whole patio instead of the seating area.
• Trusting the 20 BTU per square foot rule, which leads to heaters that are far too small.
• Expecting one 1,500 W plug-in heater to warm a whole dining area.
• Buying one very large heater for a long table instead of two smaller heaters at the ends.
• Forgetting that wind can make even a 48,000 BTU tower feel weak.
• Choosing a heater that is too powerful for the ceiling height of a covered patio.

Patio Heaters by Size

Matching patio heater size to your seating area is easier with real models. These models are part of our best patio heaters comparison, where you will find specifications and trade-offs for each one.

Coverage on Amazon listings is not always stated or consistent, so compare the output with the size chart above and check the manual for clearances before you buy.

Related Guides

• Best patio heaters: 8 models compared
• Propane vs electric patio heater: which is better?
• Can you use a patio heater under a covered patio?
• How to enjoy your patio during winter

Frequently Asked Questions