A British Thermal Unit, or BTU, is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In the International System of Units (SI), commonly known as metric, heat is expressed not in BTUs, but in Joules. One Joule is defined as the amount of energy required to accelerate a mass of one kilogram at a rate of one meter per second squared, over a distance of one meter. The two are related by a factor of about a thousand:
1 BTU is 1,055.06 Joules (J), or slightly more than 1 kilojoule (kJ)
Why are we bothering with this? Because these terms are important when comparing the relative powers of heat sources that brewers use to boil water and wort:
- Kitchen stoves
- Propane burners
- Natural gas burners
- Electric heating elements
For most of us, the first time we ever think about heat output is when we step up from partial (concentrated) boils to full-volume boils. And that often coincides with our initial forays into all-grain brewing because one normally collects and boils the full wort volume in all-grain brewing.
If you’ve ever tried to boil 6.5 gallons of wort on your kitchen stove, you’ve either given up, waited a really long time, or straddled your kettle across multiple burners. That’s because most residential kitchen stoves just aren’t designed to deliver that kind of heat. That’s why so many of us rely on outdoor turkey fryers to give the extra oomph we need.
Now, when manufacturers talk about this many or that many BTUs, what they really mean is BTUs per hour, which is power, though the “per hour” part is usually dropped, which can cause some confusion. The SI unit of power is the Watt, which is simply one Joule per second. It’s easy to convert between BTUs per hour and Watts:
1 BTU per hour is 0.2931 Watts
This relationship lets us directly compare different kinds of heat sources. The following table offers approximate power ratings for burners and elements commonly used in homebrewing. All figures are for one burner or element, so if your kitchen stove has four burners, the number expresses one quarter of your stove’s total potential output.
These numbers can be misleading because of efficiency ratings. If you’ve ever stood next to a turkey fryer propane burner, you know that a great deal of heat doesn’t actually go into the pot, but rather into the air surrounding it (and, consequently, to the knees of those standing nearby).
In fact, a gas burner can lose as much as 60 percent of its heat to the surrounding environment, but if you’re starting with, say, 100,000 BTU/hour, that still gives you 40,000 BTU/hour to work with, significantly more than you can get on a kitchen stove. So we use turkey fryers despite their inefficiencies because a small fraction of a gigantic number is still a very large number.
But this is also one reason why many brewers opt to build or purchase electric brew rigs, as we’ve done in the Craft Beer & Brewing office. Because the electric element is in direct contact with the water or wort, virtually all of the heat is directed where we want it and not into the environment. Of course, electric heating brings a whole host of other issues, not the least of which is the inherent risk of mixing electricity and water. But that’s an issue for another day.