Counting the water
Food at 85 percent water is mostly ice. The model keeps the dry solids, lets the finished product hold 2 percent moisture, and removes the rest: water removed = load x water fraction minus the small amount left behind.
Drying time is that water divided by an average sublimation rate, a simplified assumption. Real rates fall as the dry layer thickens and vapor must escape through it, so the end of a batch is the slowest part.
Try it: load 4 kg of 85 percent water food at 0.12 kg/h and the batch takes about 28 hours, in line with the 20 to 40 hours extension services quote for home units. Now try 10 kg and see why big loads run overnight twice.
Why it takes energy
Turning ice into vapor needs about 2.83 megajoules per kilogram, the latent heat of sublimation, which is more than boiling the same water from room temperature. The energy readout covers only that heat; pumps and refrigeration add much more in practice.