I’ve worked a bit more on my stove calculator, and have added two more calculations to it. Download it here: updated stove calculator. The first new calculation figures out the thermal efficiency based off the temperature of combustion vs. the temperature of your exhaust.
The second is more useful, and figures out how efficient your stove is at converting wood BTUs into usable heat BTUs. This is where the rubber meets the road in your stove’s fuel efficiency. You can use the first calculator on the spreadsheet to figure your stove’s BTU output.
Next, you’ll need to know the BTU content for the wood you are burning. Here’s a great place to find that information. You’ll have to convert it into BTU per pound from that site. Also, keep in mind that the values here are for 20% moisture content. For more information on how these values are determined, read this Wikipedia article.
If your stove is burning smoke and odor free, you should see very high efficiencies. If your stove is smoke free, but produces a lot of soot deposition and odor, it’ll be high, but not up to its full potential. If your stove could be named Old Smokey, efficiencies will be fairly low.
So give it a try and see how your stove stacks up in the numbers. It is a very valuable tool in tweaking your stoves and measuring your progress. I hope you find these calculations as useful as I have over the years. I’ll be adding more in the near future. I’m also beginning development on a smartphone app that will allow you to take the calculator with you.