Subtracting accumulated depreciation from an asset's cost results in the asset's book value or carrying value. Hence, the credit balance in the account Accumulated Depreciation cannot exceed the debit balance in the related asset account. Composite (Group) Depreciation is a method for calculating depreciation expense for groups of realated assets as a single entity.For asset owners, this approach reduces labor requirements for asset tracking and asset reporting.

The accumulated depreciation is the difference between this resulting NBV and the original cost. Excel has a POWER formula in its formulae toolbar (or at least Excel 2007 does) to create the formula above. I hope that helps you.