A drum roaster will usually keep roasting well down to about a third of its rated capacity, and below that it stops behaving like the machine you learned on. The charge you can defend thermally and the charge you can afford are two different numbers, and the second one moves first. If you are renting an hour on somebody else's drum, start with what the hour costs and work backward.
What a reduced charge is doing inside the drum
We have already said on this site that the trade cannot agree on a floor. Our piece on when to upgrade a roaster names the published minimums and then declines to run production against them. Here is what the smaller charge is doing while it runs.
Perfect Daily Grind and Roast Magazine both answer this well, and I would rather send you to them than restate it. Scott Rao's position is that a machine's stated capacity is not its optimal batch size, and that many produce their best coffee at 50 to 70 percent of nominal capacity. Carola Mapp puts the practical floor lower and is honest about the cost: much smaller batches are possible, in her experience down to about a third of total capacity, but the smaller the batch the harder the heat is to control. Three things go with it. Airflow comes down or beans get pulled out of the drum, drum speed comes down or they bounce out through the exhaust, and the bean probe stops telling you much once the pile is too shallow to cover it. Steve Cuevas gives the reason underneath all three: changing batch size changes how much convection you have, how much air touches the beans.
I learned that the expensive way, early in my roasting. A friend was over in my garage and I was teaching him the stages of the roast cycle, so I was watching him and not the coffee, and I had loaded a smaller amount than normal. The roast ran away and I took it to what seemed like "third crack" by accident, which is my phrase, not a stage anybody recognizes. The charge weight was the reason. The same heat into a smaller mass arrives faster than an instinct trained on a full drum expects.
The hour costs the same whether the drum is full
The question changes shape for anybody buying roast time rather than owning it. An hour of drum time costs what it costs, full charge or a third of one. So does the person standing in front of it, and the warm-up and cool-down around it. Those attach to the hour, not to the pounds.
So your minimum batch is one division, and you hold both inputs. Take what the hour costs, all in, and divide by the roasted pounds it produces. Then run it again at the smaller charge. Small batches are quicker, so the hour holds more of them and the yield does not fall as far as the charge does, but it does fall, because every batch still costs a charge, a drop, a cooling cycle and a reset. Hold that gap against the quality difference. A third-capacity batch is harder to steer and will spread wider between batches, but it will not usually give you a coffee nobody would buy. Cost per pound moves first and moves further.
Our own answer to this is one you probably cannot use
Ask me at the Roastery and I will tell you to use the other machine. Production here goes on a 15 kilo, charged full, which is a choice about that drum rather than a rule for anybody else's. For development work a 2 kilo stands beside it, which is where a profile gets settled before any production lot meets the big machine. That answer is only available because we happen to own both. A tenant who booked one hour on one drum has one drum, so it does not transfer, and I would rather say so than dress our floor plan up as a method.
What the co-roasting program systematized for me is the set of things every roaster goes through anyway: inspection and permitting with the city, learning the fundamentals, and getting over the sinking feeling in the pit of your stomach when you drop roasted coffee into the cooling tray with the door left open for the first time. Charge weight belongs on that list, and I would rather somebody find their floor on a small machine than on a production hour with a customer's order behind it. If the batch you settle on is not the one you developed on, the profile will not follow it, which is a separate job.
The floor for your machine is whatever charge you can still steer. The floor for your business is whatever charge still pays for the hour, and if you are renting the drum, that second one binds first.
Roasting at the Inheritance Coffee Lab
Roaster access at the Lab in Pensacola runs as a membership rather than a drop-in rental. The co-roasting application is short, and one thing worth knowing first: everyone who roasts here takes the basic class first, experience or not. Bring your monthly pounds rather than a batch size.