Some of the new compact portable BBQs would have only one of two burners and the burners would range from around 7.5MJ to 11MJ. 10MJ would be typical.
Just to make it a bit more challenging, some BBQ manufacturers, like Weber, rate their burners in BTU. 1MJ equals 948BTU, so to get an equivalent number of Megajoules, just divide the BTU burner rating by 948.
One litre of NZ LPG (60:40 Propane-Butane mix) has approximately 26MJ of energy content. An 9kg gas bottle contains just a bit less than 17 litres or approximately 442MJ of energy.
How To Calculate How Long It Will Last
So, to determine how long the 9kg gas bottle will last, you just divide 442 by the MJ rating of the BBQ or patio heater.
For example, if you run a typical 4 burner BBQ, with 15MJ burners, on high, it will last approximately 7.4 Hours, as calculated: (442÷ (4×15) = 7.4)