The Reality Of BTUs In Outdoor Fire Pits And Outdoor Grills
BTU ratings are one of the most common ways outdoor cooking and fire products are compared, yet they are also one of the most misunderstood. On paper, BTUs appear to offer a clear, objective measure of performance. In practice, a BTU is simply a unit of energy, not a description of how well that energy gets transferred to its target.
This distinction becomes important when evaluating systems like the Zutto SP1 compared to other appliances for outdoor cooking. In this article, we'll mainly focus on BTUs and their relationship to cookware.
What a BTU Rating Actually Represents?
A BTU, British Thermal Unit, is a unit of energy. Technically, it is the amount of heat needed to raise 1 lb of water by 1ºF:
- 1 lb of propane = ~21,500 BTUs
- one 20 lb tank of propane = ~430,000 BTUs
In propane devices, BTU ratings typically reflect the rate of energy released from fuel, which corresponds to how much fuel is burned over time. What it does not represent is:
- How much of that heat reaches your cookware
- How evenly that heat is distributed
- How usable that heat is in real conditions
This is where most confusion starts.
How Are BTUs Measured?
There are a few approaches to evaluate the BTUs of a device.
Input Capacity
This method is quite simple and tends to be the industry standard. It effectively measures the fuel consumed by the device over a period of time. For natural gas, input flow is used to calculate BTUs; for propane, you simply weigh the tank:
- Turn the fire pit on a desired setting
- Weigh the propane tank
- Wait a defined amount of time
- Weigh the propane tank again
- Convert the fuel consumed into BTUs (21,500 BTU per lb)
The method is actually quite precise at gauging how much a device can burn propane, but it's not great at evaluating how much of that heat makes it into the cookware, especially outside of a lab. In the industry, companies often list their device BTUs based on the highest flame the device can produce.
Theoretical or Calculated
This is less common, but engineers can calculate the maximum possible fuel flow and energy output based on a burner design and components. In a way, this is the theoretical version of the Input Capacity method. Neither really reflects real field use conditions.
Water Boiling
This is also a common method used for gas stove testing, specifically for cooking applications. Although simple on the surface, the method is very sensitive to variables, which is why it is usually performed in a controlled lab environment:
- Heat a known volume of water
- Measure time and temperature rise
- Calculate energy transfer
While the method attempts to measure efficiency instead of output, it becomes unreliable once real world variables are introduced. Indeed, wind will inevitably affect those measurements, outside temperature and humidity, cookware shape and materials, burner sizes and designs, lids, lava rocks, etc...
What About Real World Use?
With a cooking gas stove, heat is a little more controlled and directed since their only purpose is to heat cookware.
With the SP1, it's a little more complicated as the adapter is designed primarily to emulate a wood campfire, using propane instead. When used for cooking, heat will radiate outward as well as up, heat will also be absorbed and re-radiated via the lava rocks, wind and environment will be factors as well.
Ok, But What About BTU Ratings For The SP1?
If we were following industry standards, we could rate the SP1 as a 120,000+ BTU/hr device, but this feels misleading. We got this number with the weight method and the regulator set at 5 PSI* resulting in an excessively large flame that simply doesn't represent normal usage. At that rate, the SP1 would empty a 20 lb propane tank in about 3.5 hours (although the tank would freeze well before that happened).
The SP1 at 5 PSI
In practice, the results are different for the SP1:
- Large flame: ~50,000+ BTUs
- Medium flame: ~40,000 BTUs
- Small flame: ~30,000 BTUs
When it comes to cooking on the SP1, a small to medium flame is usually plenty. Although the SP1 is capable of delivering plenty of heat when cooking, results will be much more affected by proper wind shielding and lava rock placement.
One thing to note is that the SP1 is primarily designed to emulate a campfire, not a camp stove. In effect, the SP1 is a great fire pit that also allows you to cook! The caveat is that you are dealing with a yellow flame, instead of the small, efficient blue flames, which means that some amount of soot is simply inevitable (as it would be cooking on fire). We'll share another blog post soon about soot and how to deal with it.
Conclusion
While BTUs are a useful metric, they usually only tell the story of how energy is being released from fuel, rather than how much heat is making it to its destination, whether it be people gathered around a fire, or cookware. According to our customers, the Zutto SP1 excels at both!
* The Zutto SP1 includes a 0 - 5PSI adjustable regulator hose. The reason it is this range is because there is no practical way to have a control valve on the fire pit, so it has to live on the regulator. The only certified adjustable regulators we could find were the 0 to 5 PSI kind. Most people use their SP1 in the 0 to ~1.5-2 PSI range.
