The general rule of thumb is that every liter of digester volume yields its same volume in gas after 24 hours at body temperature. That is an ideal that is rarely met. But theoretically the typical 1000 liter (275 gallon) home digester should make 1 cubic meter (1000 liters, 275 gallons) of biogas in a day or so after being fed. The amount of food that usually yields that volume of gas is about a bucket's worth, a bucket being between 20 and 25 liters (5 gallons or so). We use buckets because people have them and most people were instructed to dislike math. We gotta deal with that reality. Bucketfulls are easier to remember.
So we say a bucket of ground up food waste (mixed with warm water) poured in a 1 cubic meter digester (IBC tanks are cubes so they make it easy to visualize) should be able to fill another IBC with gas.
The problem is usually temperature. Most biodigesters seem to have slurry in them normalizing at about 20 C. These only produce about 100 to 300 liters of gas a day. At 25 you start topping 300. At 30 C you may be looking at 500 liters of gas. At 35 to 37 (98.6 Fahrenheit -- body temperature) you may see the theoretical 1000 liters that will enable you to cook for about 2 hours on a single burner.
Look, the microbes came from the guts of an animal and are optimized at mammalian body temp. So if you make them happy they will produce. The curve is nonlinear though -- they produce a LOT more at 37 C than they do at 30 C. At 15 C they get really sluggish. Warren Weisman at Hestia Home Biogas gets production down at 13 C. But the higher the temp the faster the gas comes out, that is for sure.
Our rule of thumb is: "every 100 liters of gas should give you 15 minutes cooking time on a single burner on medium flame" -- the kind most of us cook on. So don't sweat it if you are at 20 C and you are only getting 100 liters a day -- you can still cook breakfast until you figure out your best way of keeping the tank warm.
Meanwhile, a bucket of food waste isn't that hard to come up with. Just don't overfeed. If you do it will go acid (go sour) with indigestion. Then you will need to feed it an antacid (Sodium carbonate is good, Sodium hydroxide faster) and maybe reinnoculate with fresh bacteria. Try RidX or go back to fresh manure. Then start feeding again slowly.
How does temperature affect all this?
Here is a chart that David House, author of the Biogas Handbook, came up with, featured in his blog at Mother Earth News. It does a great job of breaking it down for you based on temperature:
"95 degrees Fahrenheit/35 degrees Celsius: 100 percent
• 85 degrees Fahrenheit/30 degrees Celsius: 68 percent
• 75 degrees Fahrenheit/24 degrees Celsius: 46 percent
• 65 degrees Fahrenheit/18 degrees Celsius: 32 percent
• 55 degrees Fahrenheit/13 degrees Celsius: 21 percent
• Colder than 55 degrees Fahrenheit: zero percent"
He says, "Just to keep it simple so as far as temperature is concerned, we’ll call the rate of biogas production at 95 degrees “100 percent,” and compare other (lower) temperatures to that. Every time Mother Nature drops the temperature by as little as 10 degrees, the rate of the production of biogas also drops, pretty steeply, by about a third. (See the table above)"
How much food waste do I have to add to make my digester useful and how long do I have to wait? I'm a busy guy, and I eat out a lot!