Final answer:
The units of production method is the depreciation method that varies expense based on usage and is useful for assets where depreciation aligns more with production levels than time.
Step-by-step explanation:
The method of depreciation that charges a varying amount to depreciation expense for each period depending on its usage is called the units of production method. This method is particularly useful for assets whose wear and tear is more closely related to the number of units they produce or the hours they operate rather than the amount of time they have been in use. For example, a manufacturing machine might depreciate more in years of heavy production than in years where it is used less frequently.