Solar panels currently have a 20-year depreciation period. Depreciation is the annual depreciation. The total depreciation amount is the difference between the purchase price and the residual value. It is not allowed to deduct all costs in the year of purchase. Instead, the cost is spread over a 20-year period. This means you are allowed to deduct a portion of the cost each year. In the case of solar panels, this can be advantageous: you can deduct twenty years of costs, which reduces your (future) profit. As a result, you have to pay less tax.