Revenue Recognition Renewable Energy System Sales Renewable energy system sales involving real estate We recognize revenue for sales of renewable energy systems that involve the concurrent sale or the concurrent lease of the underlying land, whether explicit or implicit in the transaction, in accordance with ASC 360-20, Real Estate Sales . For these transactions, we evaluate the renewable energy system to determine whether the equipment is integral equipment to the real estate. If the equipment is determined to be integral to the real estate, the entire transaction represents the sale of real estate and is therefore subject to the revenue recognition guidance applicable to real estate. A renewable energy system is determined to be integral equipment when the cost to remove the equipment from its existing location, ship and reinstall at a new site, including any diminution in fair value, exceeds ten percent of the fair value of the equipment at the time of original installation. For those transactions subject to real estate accounting, we recognize revenue and profit using the full accrual method once the sale is consummated, the buyer's initial and continuing investments are adequate to demonstrate its commitment to pay, our receivable is not subject to any future subordination, and we have transferred the usual risk and rewards of ownership to the buyer. If these criteria are met and we execute a sales agreement prior to the delivery of the renewable energy system and have an original construction period of three months or longer, we recognize revenue and gross profit using the percentage of completion method of accounting applicable to real estate sales when we can reasonably estimate progress towards completion. During the year ended December 31, 2014 , 2013 and 2012 we recognized $10.6 million , $32.5 million and $72.8 million , respectively, of revenue using the percentage of completion method for the sale of renewable energy systems involving real estate. If the criteria for recognition under the full accru