Depreciation is an allowance for wear and tear on the equipment used in your business. As time passes, your equipment will usually lose value, and this can be considered a cost to your business. You need to think about how long you expect your assets to last. For example, if you purchase a computer system, you may forecast that in 5 years it will be obsolete. That means the depreciation rate is 20% per year. If you determine it to be 2 years, then it will be 50% per year. This does not have any effect on cashflow, just on how profits are calculated. Deprecation is an accounting cost that must be included to give a Profit & Loss account more relevance.