Depreciable lives of Long-lived Assets
We have significant investments in renewable energy facility assets. These assets are generally depreciated on a straight-line basis over their estimated useful lives which range from 23 to 30 years for our solar generation facilities. The major components of our wind plants are depreciated on a straight-line basis over their weighted average estimated remaining useful life of 21 and 23 years as of December 31, 2018 and 2017, respectively.
The estimation of asset useful lives requires significant judgment. Changes in our estimated useful lives of renewable energy facilities could have a significant impact on our future results of operations. See Note 2. Summary of Significant Accounting Policies to our consolidated financial statements regarding depreciation and estimated service lives of our renewable energy facilities.
Recently Issued Accounting Standards
See Note 2. Summary of Significant Accounting Policies to our consolidated financial statements included in this Annual Report on Form 10-K for disclosures concerning recently issued accounting standards. These disclosures are incorporated herein by reference.
Item 7A. Quantitative and Qualitative Disclosures About Market Risk.
We are exposed to several market risks in our normal business activities. Market risk is the potential loss that may result from market changes associated with our business or with an existing or forecasted financial or commodity transaction. The types of market risks we are exposed to are interest rate risk, foreign currency risk and commodity risk. We do not use derivative financial instruments for speculative purposes.
Interest Rate Risk
As of December 31, 2018, the estimated fair value of our debt was $5,789.7 million and the carrying value of our debt was $5,761.8 million. We estimate that a hypothetical 100 bps, or 1%, increase or decrease in market interest rates would have decreased or increased the fair value of our long-term debt by $88.1 million and $96.6 million, respectively.
As of December 31, 2018, our corporate-level debt consisted of the Senior Notes due 2023 (fixed rate), the Senior Notes due 2025 (fixed rate), the Senior Notes due 2028 (fixed rate), the Revolver (variable rate) and the Term Loan (variable rate). Additionally, during the year ended December 31, 2018, we drew and repaid $86.0 million under the Sponsor Line (variable rate). On March 8, 2019, we entered into interest rate swap agreements with counterparties to hedge the variable Eurodollar base rate associated with the interest payments on the entire principal of our Term Loan, paying an average fixed rate of 2.54%. In return, the counterparties agreed to pay us the variable Eurodollar base rate payments due to the lenders until maturity. A hypothetical increase or decrease in interest rates by 1% would have increased or decreased interest expense related to our Revolver, Term Loan and Sponsor Line by $6.2 million for the year ended December 31, 2018.
As of December 31, 2018, our non-recourse permanent financing debt was at both fixed and variable rates. 39% of the $3,496.4 million balance had a fixed interest rate and the remaining 61% of the balance had a variable interest rate. We have entered into interest rate derivatives to swap a majority of our variable rate non-recourse debt to a fixed rate. Although we intend to use hedging strategies to mitigate our exposure to interest rate fluctuations, we may not hedge all of our interest rate risk and, to the extent we enter into interest rate hedges, our hedges may not necessarily have the same duration as the associated indebtedness. Our exposure to interest rate fluctuations will depend on the amount of indebtedness that bears interest at variable rates, the time at which the interest rate is adjusted, the amount of the adjustment, our ability to prepay or refinance variable rate indebtedness when fixed rate debt matures and needs to be refinanced and hedging strategies we may use to reduce the impact of any increases in rates. We estimate that a hypothetical 100 bps, or 1%, increase or decrease in our variable interest rates pertaining to interest rate swaps not designated as hedges would have increased or decreased our earnings by $56.8 million for the year ended December 31, 2018.