Do the math

How much heater, and what it costs to run

The two numbers that decide whether a patio heater was a good purchase, and the two that nobody in this category publishes the arithmetic for. Here is the arithmetic.

Last reviewed September 1, 2026

Start with the BTU calculator. It is the page this whole site is built around, and it takes about a minute.

Search for how many BTU a patio heater needs and you will get six answers, all confident, none showing its working, several contradicting each other. Some say 20 BTU per square foot. Some quote a coverage radius. Some publish a calculator that ends, remarkably, at their own catalog page.

None of that is dishonest exactly. It is that outdoor heating genuinely resists a clean formula. A room has a boundary and a heat-loss calculation; a patio has open sky on one side and a breeze that changes hourly. Anyone offering a single confident number is either simplifying or selling.

So the position we take is different: publish the rule, publish where it comes from, publish the corrections it needs, and show every multiplication so you can disagree with a specific step rather than with a conclusion. That is what the pages in this section do. It will not make outdoor heating exact. It will stop you buying a heater that was never going to work.

Read the covered column carefully, because it is the one that changes purchases. The same patio under a roof needs about a third less heat, which routinely moves a buyer from a 48,000 BTU propane tower down to a 1,500 W electric unit — a different fuel, a different price, no cylinder to swap and no flame to blow out. That single correction is worth more than any brand comparison on this site.

Cost per hour is simple arithmetic that almost nobody publishes with its inputs. For electricity it is watts divided by 1,000, times hours, times your rate. For propane it is rated BTU/h divided by the energy content of a gallon, times the price of a gallon. Both need a price, and a page that gives you a cost without telling you which price it assumed has told you nothing.

Two things fall out of those tables that are worth saying plainly. First, electric heating on a patio is genuinely cheap per hour — under thirty cents for most evenings — because the output is small. Second, propane is not expensive per unit of heat; it is expensive per evening because you are burning ten times as much of it. Neither fuel is being wasteful. They are answering different questions.

How long a 20 lb propane cylinder lasts is the most-asked question in this category and the one most often answered with a shrug and a range. It is not a mystery. A 20 lb cylinder holds 20 pounds of propane; propane weighs 4.24 pounds per gallon, so that is 4.72 gallons; a gallon carries 91,420 BTU, so the cylinder holds about 431,000 BTU. Divide by the burn rate and you have the answer.

At 48,000 BTU/h that is around nine hours — two or three evenings, not a season. Anyone who has been surprised by how fast a cylinder empties has just met that division for the first time. The full page walks through it, including what happens when you turn the heater down, which is the one lever that genuinely changes the answer.

• The BTU calculator — put your own patio in and see every multiplication.
• Coverage area — what a manufacturer's radius claim really covers, and how to check one.
• Cost to run — the full electric and propane tables with the assumptions stated.
• Propane tank runtime — the cylinder arithmetic in detail.
• Watts vs BTU — why 1,500 W is a hard ceiling on every plug-in heater sold in the U.S.
• Cheapest to run — the picks that come out best once the arithmetic is done.