Under the units of production method, the amount of depreciation charged to expense varies in direct proportion to the amount of asset usage. Thus, a business may charge more depreciation in periods when there is more asset usage, and less depreciation in periods when there is less usage. It is the most accurate method for charging depreciation, since this method links closely to the wear and tear on assets. However, it also requires that someone track asset usage, which means that its use is generally limited to more expensive assets. Also, you need to be able to estimate total usage over the life of the asset in order to derive the amount of depreciation to recognize in each accounting period.
If the estimated number of hours of usage or units of production changes over time, incorporate these changes into the calculation of the depreciation cost per hour or unit of production. This will alter the depreciation expense on a go-forward basis. A change in the estimate does not impact depreciation that has already been recognized.