How Long Do Patio Heaters Burn: Runtime, Calculations

Most standing propane patio heaters rated at 40,000–48,000 BTU/hr will burn for roughly 8 to 10 hours on a standard 20-lb (9 kg) propane cylinder. Smaller tabletop gas heaters running at 10,000–15,000 BTU/hr can stretch the same tank to 25–40 hours. Electric patio heaters don't 'burn' fuel at all, their runtime is limited only by your power supply, or by a battery pack's kWh capacity if you're running a cordless unit. The exact number for any heater comes down to three things: how much fuel is in the cylinder, how energy-dense that fuel is, and how hard the burner is working.

Typical patio heater burn times at a glance

Before we get into the math, here are ballpark runtimes you can use for planning. These assume a full cylinder at roughly 80% liquid fill (the safe legal maximum under NFPA 58) and a heater running at its rated input continuously. Real-world burn time is usually a little longer because most people dial back the heat during warmer parts of the evening.

* The 48,000 BTU/hr figure on a 20 lb cylinder looks longer than the 40,000 BTU/hr figure because many 48,000 BTU models (like the HeatMaxx SRPH33A and similar mushroom-head heaters) use a larger 20 lb tank with a higher usable BTU capacity that offsets the higher burn rate. The full worked calculation is in the dedicated section below.

What 'burn time' actually means, and why electric is different

For gas heaters, burn time is how many hours the flame keeps going before the cylinder runs out of usable fuel. It's a consumption question: you're converting stored chemical energy in propane or butane into heat, and once the fuel is gone, the heater stops. The rated BTU/hr figure on the label tells you how fast that energy is being released, it's the burn rate, not a measure of how long anything lasts.

For electric patio heaters, there's no combustion, so 'burn time' is a bit of a misnomer. A mains-powered electric heater runs as long as you leave it plugged in. The relevant number is power consumption in watts or kilowatts, which tells you how much electricity you're using per hour. If you're running a portable electric heater from a battery pack, then runtime is simply the pack's energy capacity (kWh) divided by the heater's draw (kW). A 2 kW infrared heater on a 5 kWh portable power station will run for 2.5 hours before the battery dies.

The practical takeaway: when people ask how long a patio heater burns, they almost always mean a propane or LPG gas model. Gas heaters dominate the outdoor heating market for freestanding use because they don't need an extension cord and put out serious heat. The rest of this article focuses on gas runtime math, with electric notes where useful.

What determines how long a gas patio heater burns

Four variables control everything. Get these right and you can estimate runtime for any heater-and-cylinder combination before you buy so much as a new regulator hose.

• Fuel type: Propane (LPG, C3H8) is the most common fuel for patio heaters in North America and Australia. Butane and propane-butane blends (sold as 'patio gas' in the UK and Europe) have slightly different energy densities. Propane has a lower heating value (LHV) of roughly 46 MJ/kg; butane is slightly higher at around 45.7 MJ/kg — they're close enough that the same formula works for both with minimal error.
• Heater output (BTU/hr or kW): This is the burn rate stamped on the data plate. A 48,000 BTU/hr heater consumes fuel roughly 4.8 times faster than a 10,000 BTU/hr tabletop unit. Higher output means more heat but shorter runtime from the same cylinder.
• Cylinder size: Bigger cylinders hold more fuel, so they last longer. Common sizes include 1 lb disposable cans, standard 20 lb (9 kg) BBQ cylinders, 13 kg refillable bottles (popular in Europe and the UK), and larger 20 kg or 47 kg domestic bottles.
• Usable fill percentage: You never get 100% of a cylinder's theoretical capacity as usable fuel. Under NFPA 58, refillable LPG cylinders are filled to a maximum of about 80% of their liquid volume to leave expansion space. This is why a '20-lb cylinder' is safe — filling a standard BBQ tank (water capacity ~47.6 lb) to only about 42% of that water weight gives you the nominal 20 lb of propane, which corresponds to roughly 80% of liquid fill capacity.

Standard conversion values you need for the calculation

These are the fixed constants I use every time I work through a runtime estimate. They come from NIST, the EPA's AP-42 compilation, and standard engineering references. Write them down, you'll use them repeatedly.

For simplicity in worked examples, I round propane energy to 21,600 BTU/lb when working in US units and 46 MJ/kg (which converts to about 12.78 kWh/kg) when working in metric. Those rounded figures are accurate enough for planning purposes, manufacturer tolerances and real-world combustion efficiency will introduce more variation than rounding does.

How to calculate runtime: the formula and a worked example

The formula is the same regardless of cylinder size or heater model. All you're doing is dividing the usable energy stored in the cylinder by the rate at which the heater consumes that energy.

The formula

Runtime (hours) = Usable fuel energy (BTU) ÷ Heater rated input (BTU/hr)

Usable fuel energy (BTU) = Cylinder fuel mass (lb) × 21,600 BTU/lb

Or in metric: Runtime (hours) = (Cylinder fuel mass (kg) × 46,000 kJ/kg) ÷ (Heater kW × 3,600 kJ/kWh)

Step-by-step worked example: 30,000 BTU/hr heater on a 20 lb cylinder

1. Identify the usable fuel mass. A nominal 20 lb cylinder holds 20 lb of propane when properly filled (the NFPA weight method accounts for the 80% fill rule automatically — the 20 lb figure is already the usable amount stamped on the tare weight label).
2. Calculate usable energy: 20 lb × 21,600 BTU/lb = 432,000 BTU.
3. Identify the heater's rated input from the data plate: 30,000 BTU/hr.
4. Divide: 432,000 BTU ÷ 30,000 BTU/hr = 14.4 hours.
5. Result: expect roughly 14 hours of continuous full-heat runtime. In practice, you might get slightly more if you run on a lower flame setting for part of the session.

Same example in metric (9 kg cylinder)

1. Cylinder fuel mass: 9 kg (a common European/Australian refillable bottle; this is the usable propane mass, not the total cylinder weight).
2. Usable energy: 9 kg × 46,000 kJ/kg = 414,000 kJ.
3. Convert heater output to kJ/hr: 30,000 BTU/hr ÷ 3,412 BTU/kWh × 3,600 kJ/kWh = 31,648 kJ/hr. Or more directly: 30,000 BTU/hr × 1.055 kJ/BTU = 31,650 kJ/hr.
4. Divide: 414,000 kJ ÷ 31,650 kJ/hr ≈ 13.1 hours.
5. The slight difference from the US-unit answer reflects the fact that a 9 kg cylinder holds slightly less propane than a US 20 lb cylinder (20 lb = 9.07 kg), so 9 kg gives a fraction fewer hours.

Runtime table: common heater ratings vs common cylinder sizes

These figures use 21,600 BTU/lb (US) and 46 MJ/kg (metric) as the propane energy constant. Liquid propane density is about 0.493 kg/L (≈4.11 lb per U.S. liquid gallon), a conversion commonly used to relate gallons, kilograms and pounds for cylinder mass calculations (see Propane, Wikipedia (liquid density and lb/gal conversions)) blank" rel="noopener noreferrer">Propane — Wikipedia (liquid density and lb/gal conversions). Cylinder fuel masses used: 1 lb disposable = 1 lb propane; 20 lb US cylinder = 20 lb propane; 9 kg cylinder = 9 kg propane; 13 kg cylinder = 13 kg (28.7 lb) propane. All assume the heater runs continuously at its rated input.

A few things worth noting in that table. The 13 kg cylinder (popular in the UK and Europe as a standard 'patio gas' bottle) gives you roughly 35–40% more runtime than a US 20 lb cylinder on any given heater, simply because it holds more fuel by mass. If you're buying a gas bottle for a season's worth of entertaining, the 13 kg option is worth considering where available. For guidance on which bottle size suits your heater, the topic of what size gas bottle for a patio heater goes deeper on the sizing decision. For guidance on which bottle size suits your heater, see our guide on what size gas bottle for a patio heater for detailed sizing advice.

How long does a propane or gas bottle actually last?

This is really the same question as runtime, just framed from the bottle's perspective. See the dedicated guide on how long does patio heater propane tank last for bottle-specific runtimes and refill planning. The answer depends entirely on your heater's output, not on the bottle itself. The bottle doesn't 'last' a fixed number of hours, it gets emptied at a rate determined by your burner. See our detailed guide on how long does a patio gas bottle last for typical bottle sizes and heater outputs.

For a typical US 20 lb propane cylinder (the standard BBQ-style tank), real-world patio heater runtimes look like this: a full-size 40,000–48,000 BTU/hr standing heater will empty a 20 lb cylinder in roughly 9–11 hours of continuous use. A mid-size 30,000 BTU/hr model stretches it to about 14 hours. A small 10,000–12,000 BTU/hr tabletop heater could run for 36–40 hours from the same tank, basically four or five typical evenings before you need a refill.

For UK and European users, the standard 13 kg patio gas bottle (typically a propane-butane blend or pure propane depending on the brand) adds roughly 40% more burn time compared to a 9 kg cylinder at any given heater output. A 13 kg bottle on a 48,000 BTU/hr (14 kW) heater gives you about 12 hours of runtime. On a more typical 10–12 kW UK patio heater, expect 17–20 hours. How long a 13 kg gas bottle lasts on a patio heater is covered in more detail in the dedicated article on that topic, but the math here gives you a solid estimate.

Disposable 1 lb (454 g) propane canisters are really only suitable for small tabletop units. They hold about 9,744 BTU of usable energy. At 10,000 BTU/hr, that's roughly 58 minutes of heat, barely enough for a single evening. They're convenient for camping but poor value for regular patio use.

How long will a 48,000 BTU patio heater run?

This is one of the most searched questions about full-size patio heaters, and the answer surprises most people. A 48,000 BTU/hr heater on a 20 lb US propane cylinder runs for approximately 9 hours of continuous full-power use. For a deeper breakdown of runtimes and cylinder choices for 48,000 BTU heaters, see the 48000 BTU patio heater how long does it last guide. EPA's Compilation of Air Pollutant Emission Factors (AP‑42) provides LPG heating values used in fuel‑energy calculations Compilation of Air Pollutant Emission Factors (AP‑42) — LPG heating values (EPA). Here's how that number breaks down.

1. Cylinder fuel mass: 20 lb of usable propane (standard fill on a properly certified 20 lb BBQ-style cylinder).
2. Energy content: 20 lb × 21,600 BTU/lb = 432,000 BTU total usable energy.
3. Heater burn rate: 48,000 BTU/hr (rated input, as shown on the data plate of models like the HeatMaxx SRPH33A and similar mushroom-head units).
4. Runtime: 432,000 ÷ 48,000 = 9.0 hours exactly at full rated input.
5. Real-world adjustment: If you dial the heat back during mild parts of the evening — say to 35,000 BTU/hr for a few hours — the actual runtime stretches to 10–11 hours. Most users get comfortably 9–10 hours from a single fill.

On a 13 kg cylinder (28.66 lb of propane), the same 48,000 BTU/hr heater calculates as: 28.66 lb × 21,600 BTU/lb = 618,500 BTU ÷ 48,000 BTU/hr = approximately 12.9 hours. Round that to 12–13 hours of full-power runtime. If you host regularly, a 13 kg bottle paired with a 48,000 BTU heater means you can run three evenings of 4-hour sessions without changing the bottle. For step-by-step safety checks and a clear procedure, see how to connect a gas bottle to a patio heater.

A note on efficiency: the BTU/hr rating on a patio heater label is the rated input, meaning the heat energy in the fuel being consumed. Not all of that becomes radiated warmth, some is lost as convection and exhaust gases. Combustion efficiency on a well-maintained burner is typically 85–95%, so the heat you actually feel is slightly less than the input BTU number suggests. This doesn't change the runtime calculation (the fuel is still consumed at the rated rate), but it's worth knowing when comparing heaters by 'output' vs 'input' ratings.

How to connect a gas bottle to a patio heater

Connecting a cylinder correctly is the single most important safety step you'll take with a gas patio heater. A leaking fitting is a serious hazard, not a nuisance. Take your time here and never skip the leak test.

1. Check the hose and regulator before connecting anything. Look for cracks, kinks, or discoloration on the rubber hose. Check the regulator body for corrosion or impact damage. If the hose is more than 5 years old or shows any wear, replace it before proceeding. Most manufacturers recommend hose replacement every 5 years regardless of appearance.
2. Confirm you have the correct regulator for your cylinder type and your heater's required pressure. US propane patio heaters typically require a regulator set to approximately 11 inches water column (W.C.) output pressure. UK/European heaters using butane or propane have different fittings and pressure requirements — always match regulator to heater spec.
3. Close the heater's control valve (turn to OFF) and make sure the cylinder valve is also closed before connecting.
4. Attach the regulator to the cylinder valve. Most US propane regulators use a POL fitting (left-hand thread) — turn it clockwise (which is 'tighten' for reverse threads). UK patio gas cylinders often use a clip-on bayonet fitting. Don't force it. It should seat cleanly by hand.
5. Tighten the regulator connection with a wrench (for POL fittings) — firm, not excessive. For clip-on fittings, ensure the locking clip snaps fully into place.
6. Perform a leak test before lighting: slowly open the cylinder valve a quarter turn, then apply soapy water (or dedicated leak detection solution) to all connection points — the regulator-to-cylinder join, every hose connection, and the hose-to-heater fitting. Watch for bubbles. Any bubbling means a leak. Close the cylinder valve immediately, disconnect, inspect the fitting, and re-test.
7. If no bubbles appear after 30 seconds of observation, fully open the cylinder valve and proceed to light the heater per its ignition instructions.
8. After your session, always close the cylinder valve first, then allow the heater to burn off the residual gas in the line before shutting the control valve. This prevents stale gas sitting in the regulator and hose.

If you're having trouble with the heater igniting after following these steps, the issue is almost always at the igniter, thermocouple, or pilot assembly, not the gas connection itself. Check the thermocouple flame contact and the igniter electrode gap before assuming a gas supply problem.

Getting more burn time from the same cylinder

You can't change the laws of thermodynamics, but you can make practical choices that stretch a cylinder further without buying a bigger tank.

• Use a wind guard or position the heater where it isn't fighting a crosswind. Wind strips heat from the area around the heater, making it feel colder even when the burner is working at full output. This causes people to crank the heat up unnecessarily.
• Run the heater at 70–80% of full output in mild weather. The difference in perceived warmth is modest but the fuel saving is proportional — running at 35,000 BTU/hr instead of 48,000 BTU/hr extends your runtime by roughly 37% from the same cylinder.
• Insulate your outdoor seating area. Screens, pergolas, and even outdoor curtains dramatically reduce the heat volume you need to warm. A 48,000 BTU heater heating a semi-enclosed patio feels much stronger than the same heater in open air.
• Keep the burner and reflector clean. A dirty reflector reduces radiant heat output; a clogged burner port creates incomplete combustion and wastes fuel. Wipe the reflector with a damp cloth and blow out the burner ports with compressed air at the start of each season.
• Store cylinders at moderate temperature. Cold cylinders have lower internal pressure, which can cause regulators to deliver slightly less gas. Don't store cylinders in very cold conditions before use — although this is more of a flame quality issue than a major runtime issue.
• Check your regulator output pressure with a manometer once a year. A regulator drifting out of spec (common on older units) can cause the heater to run lean or rich, reducing efficiency. For most propane patio heaters, the target manifold pressure is around 11 inches W.C.

Safety checks that affect both runtime and reliability

A patio heater that's poorly maintained doesn't just waste gas, it can become dangerous. These checks take five minutes and should happen at the start of every season and after any period of storage.

• Tilt switch: Most standing patio heaters have a tilt/tip-over switch that cuts the gas if the unit falls over. If your heater shuts off randomly on level ground, a faulty tilt switch is a common culprit. This is a known failure point — the switch corrodes or the contact wears. It's a repairable component, not a reason to replace the whole heater.
• Thermocouple: The thermocouple is the safety device that keeps the gas valve open when the pilot flame is lit. If your heater lights but won't stay lit when you release the control knob after 30–45 seconds, a weak or failed thermocouple is almost always the cause. This is one of the most common and easiest repairs on a gas patio heater.
• Regulator and hose: Replace the hose every 5 years as a baseline. Look for brittleness, surface cracking, and discoloration. A restrictor or partially failed regulator can cause a weak or lazy flame that wastes fuel without delivering usable heat.
• Burner head and ports: Spider webs and debris in the burner orifice are a very real and very common problem with patio heaters stored outdoors or in sheds. Partial blockages cause uneven combustion and can trigger safety valve shutoff. Remove the burner assembly and clear ports with a fine wire at the start of each season.
• Anti-tilt base and wheel locks: Check that the base is sitting level before each use. Many 'random shutoff' complaints are actually the tilt switch doing exactly what it's supposed to — the base has settled unevenly.

When your heater won't stay lit or burns poorly, quick DIY signposts

If your heater isn't burning correctly, the runtime calculation above becomes meaningless because you're not getting rated output from the fuel you're consuming. Here are the most common fault symptoms and where to look first.

The thermocouple and tilt switch are the two components responsible for the vast majority of 'won't stay lit' and 'shuts off randomly' complaints. Both are replaceable parts available for most heater models, and neither repair requires more than basic hand tools. If your heater shuts off after 3–4 seconds every time, that's a thermocouple problem. If it shuts off after a few minutes of normal operation, suspect the tilt switch or a low-pressure issue at the regulator.

How long does a patio heater last? (service life, not burn time)

This is a different question from runtime, but it comes up constantly alongside it. For a deeper look at expected service life and maintenance tips, see our guide on how long does a patio heater last. A well-maintained gas patio heater should give you 5–10 years of reliable service, and many well-built models last considerably longer if you replace consumable parts as they fail rather than binning the whole unit. The components that limit service life are the burner assembly (corrosion and clogging), the regulator (pressure drift after 5–8 years), the hose (5-year replacement cycle regardless), and the finish on the reflector and housing (cosmetic, not functional).

The single biggest waste I see is homeowners replacing a perfectly good heater because the igniter failed or the thermocouple gave out. These parts cost $10–$30 and take 20 minutes to swap. Treating a patio heater as a repairable appliance rather than a disposable product means a $150 heater can realistically serve you for a decade with two or three minor part replacements along the way.