offer our customers a global platform that reaches 24 countries with proven operational reliability, improved application performance, network choice and a highly scalable set of offerings.
Our cabinet utilization rates were approximately 80%, as of March 31, 2019, and 80%, excluding data centers acquired from Itconic and Zenium, as of March 31, 2018. Excluding the impact of our IBX data center expansion projects that have opened during the last 12 months, our cabinet utilization rate would have increased to approximately 83% as of March 31, 2019. Our cabinet utilization rate varies from market to market among our IBX data centers across the Americas, EMEA and Asia-Pacific regions. We continue to monitor the available capacity in each of our selected markets. To the extent we have limited capacity available in a given market, it may limit our ability for growth in that market. We perform demand studies on an ongoing basis to determine if future expansion is warranted in a market. In addition, power and cooling requirements for most customers are growing on a per unit basis. As a result, customers are consuming an increasing amount of power per cabinet. Although we generally do not control the amount of power our customers draw from installed circuits, we have negotiated power consumption limitations with certain high power-demand customers. This increased power consumption has driven us to build out our new IBX data centers to support power and cooling needs twice that of previous IBX data centers. We could face power limitations in our IBX data centers, even though we may have additional physical cabinet capacity available within a specific IBX data center. This could have a negative impact on the available utilization capacity of a given IBX data center, which could have a negative impact on our ability to grow revenues, affecting our financial performance, operating results and cash flows.
Strategically, we will continue to look at attractive opportunities to grow our market share and selectively improve our footprint and offerings. As was the case with our recent expansions and acquisitions, our expansion criteria will be dependent on a number of factors, including but not limited to demand from new and existing customers, quality of the design, power capacity, access to networks, capacity availability in the current market location, amount of incremental investment required by us in the targeted property, lead-time to break even on a free cash flow basis and in-place customers. Like our recent expansions and acquisitions, the right combination of these factors may be attractive to us. Depending on the circumstances, these transactions may require additional capital expenditures funded by upfront cash payments or through long-term financing arrangements in order to bring these properties up to Equinix standards. Property expansion may be in the form of purchases of real property, long-term leasing arrangements or acquisitions. Future purchases, construction or acquisitions may be completed by us or with partners or potential customers to minimize the outlay of cash, which can be significant.
Our business is based on a recurring revenue model comprised of colocation and related interconnection and managed infrastructure offerings. We consider these offerings recurring because our customers are generally billed on a fixed and recurring basis each month for the duration of their contract, which is generally one to three years in length. Our recurring revenues have comprised more than 90% of our total revenues during the past three years. In addition, during any given quarter of the past three years, more than half of our monthly recurring revenue bookings came from existing customers, contributing to our revenue growth. During both the three months ended March 31, 2019 and 2018, our largest customer accounted for approximately 3% of our recurring revenues. Our 50 largest customers accounted for approximately 39% and 38%, respectively, of our recurring revenues for the three months ended March 31, 2019 and 2018.
Our non-recurring revenues are primarily comprised of installation services related to a customer's initial deployment and professional services we perform. These services are considered to be non-recurring because they are billed typically once, upon completion of the installation or the professional services work performed. The majority of these non-recurring revenues are typically billed on the first invoice distributed to the customer in connection with their initial installation. However, revenues from installation services are deferred and recognized ratably over the period of the contract term. Additionally, revenue from contract settlements, when a customer wishes to terminate their contract early, is generally treated as a contract modification and recognized ratably over the remaining term of the contract, if any. As a percentage of total revenues, we expect non-recurring revenues to represent less than 10% of total revenues for the foreseeable future.
The largest components of our cost of revenues are depreciation, rental payments related to our leased IBX data centers, utility costs, including electricity, bandwidth access, IBX data center employees' salaries and benefits, including stock-based compensation, repairs and maintenance, supplies and equipment and security services. A majority of our cost of revenues is fixed in nature and should not vary significantly from period to period, unless we expand our existing IBX data centers or open or acquire new IBX data centers. However, there are certain costs that are considered more variable in nature, including utilities and supplies that are directly related to growth in our existing and new customer base. We expect the cost of our utilities, specifically electricity, will generally increase in the future on a per-unit or fixed basis, in addition to the variable increase related to the growth in consumption by our customers. In addition, the cost of electricity is generally higher in the summer months, as compared to other times of the year. To the extent we incur increased utility costs, such increased costs could materially impact our financial condition, results of operations and cash flows. Furthermore, to the extent we incur increased electricity costs as a result of either climate change policies or the physical effects of climate change, such increased costs could materially impact our financial condition, results of operations and cash flows.