Patio Heater Propane Cost Calculator

A patio heater burns propane to throw radiant heat.
The cost per hour comes down to two numbers: BTU per hour and price per pound of propane.

The math:

lb_per_hour = BTU_per_hour / 21,600 cost_per_hour = lb_per_hour × price_per_lb

Propane delivers about 21,600 BTU per pound.
That figure is a property of the fuel, so it does not vary by brand or by season.
What varies wildly is the price per pound, and the gap between the two ways of buying it is larger than most people realise.

The exchange-cage trap. A “20 lb” tank from a Blue Rhino or AmeriGas swap cage is not filled to 20 lbs. The industry moved to filling them to 15 lbs years ago while keeping the same bottle and roughly the same price. So $25 to $30 for a swap is $1.65 to $2.00 a pound, not the $1.30 the tank size suggests.
Take the same tank to a refill counter at U-Haul or Tractor Supply and you pay $15 to $20 for a genuine 20 lbs, which is $0.75 to $1.00 a pound. That is roughly half price for the identical fuel, and the only cost is that you wait ten minutes instead of grabbing a cage tank on the way out.
Bulk propane delivered to a stationary tank runs cheaper again, around 50 to 70 cents a pound.

A worked example.
A standard mushroom-style patio heater is rated 40,000 BTU/h.
That is 40,000 / 21,600 = 1.85 lb of propane per hour at full output.
At $1.30 a pound that is $2.41 an hour. At swap-cage prices nearer $2.00 a pound it is $3.70 an hour, which is the same heater costing half as much again.
A properly filled 20 lb tank lasts about 11 hours at full output. A 15 lb swap tank lasts about 8.
Two or three evenings of patio dinners and you are buying another one.

Wall-mounted infrared heaters use less.
A 30,000 BTU/h infrared heater burns 1.39 lb an hour, which is $1.81 at $1.30 a pound, and it delivers most of its heat as direct radiation onto people rather than warming the air that then blows away.
That is why restaurants keep switching to infrared: the same warmth on the guest for noticeably less propane.

Output settings change consumption more than anything else you can control.
A 40,000 BTU/h heater on its lowest setting often runs at 15,000 to 20,000 BTU/h, which halves the hourly cost.
Most people leave the knob at full without thinking about it. The dial shows output, not consumption, so nothing on the heater tells you what turning it down is worth, and full output is rarely needed once the seating area has warmed up.

A few practical points.
Patio heaters are roughly 30 to 40% effective at warming people outdoors, because the rest of the heat rises and leaves.
Indoors they burn nearly all their fuel usefully, but they are also a carbon monoxide hazard with no flue, so never run one in a garage, a tent, or a covered porch without real ventilation.
Wind is the enemy. A 5 mph breeze can halve the warmth people actually feel, and no amount of extra propane buys that back. A windbreak on the exposed side is worth more than a second heater.
For a small patio, a 20,000 BTU/h tabletop unit burns 0.93 lb an hour, about $1.20 at $1.30 a pound, and it is plenty for a table of four.

The economics against a built-in propane fire pit.
A 60,000 BTU/h fire pit burns 2.78 lb an hour, which is $3.61 at $1.30 a pound. That is a serious burn rate, and it empties a 20 lb tank in about seven hours.
Fire pits also radiate into a single ring around them, while a patio heater spreads warmth over a wider area.
For a group, two patio heaters usually deliver more felt warmth than one fire pit, for less fuel.