A Btu is an amount of energy, just as a calorie is an amount of energy. Both are most commonly used to measure amounts of heat.
The Btu, which stands for British thermal unit, was invented by engineers, so while it makes sense to the guys who design the stoves, it doesn’t mean much to us in the kitchen. But by sheer luck it turns out to be almost exactly one quarter of a nutritional calorie. So, for example, the 250 calories that it takes to boil off a pint of water is equal to 1,000 Btu.
Another example: The total amount of heat given off by the burning of an average candle is about 5,000 Btu.
That’s the amount of chemical energy the wax inherently contained, and the combustion process converts that chemical energy into heat energy. But a candle releases its heat slowly over a period of several hours, so it’s no good for cooking. In case you’ve been wondering, that’s why you can’t sauté a hamburger over a candle.
For cooking, we need a lot of heat delivered in a short period of time. Range burners are therefore rated according to how fast they can pump out heat, expressed as Btu per hour at their top settings. The confusion comes when people neglect to say “Btu per hour” and just say “Btu.” But the burners’ Btu ratings are not amounts of heat; they are the maximum rates at which they can pump out heat.
Most home gas or electric range burners produce from 9,000 to 12,000 Btu per hour. The gas burners in restaurant kitchens are capable of putting out heat twice as fast, because for one thing their gas-supply pipes are bigger and can feed in more gas per minute.
Also, restaurant ranges generally have several concentric burner rings instead of just one. Chinese restaurants that need to do high-temperature wok cooking have broad gas burners that spew out heat like a dragon with a mouthful of habanero peppers.
Remember that to boil off a pint of water from a stock requires 1,000 Btu of heat? Well, using your 12,000-Btu-per-hour burner, that should take one-twelfth of an hour or five minutes. But you know that it takes a lot longer than that. The reason is that most of the heat emitted by the burner is wasted. Rather than going directly into the liquid in the pan, most of it goes into heating up the pan itself and the surrounding air.
Put two different pots of food on two identical burners set at identical levels and they will heat and cook quite differently depending on their shapes and sizes, what materials they’re made of, how much and what kinds of foods they contain, and so on. That’s why you have to keep your eye on the pot and continually adjust the burner for every specific situation.
When shopping for a range, look for one that has at least one burner rated at 12,000 or preferably 15,000 Btu per hour. With that much heat output you’ll be able to boil water in no time, sear meats quickly, and stir-fry in your wok or stir-fryer like a Chinese chef.