The following discussion should be read in conjunction with our financial statements and accompanying notes included in Item 8. Financial Statements and Supplementary Data, of this Form 10-K.

Overview

We focus on solutions that reduce carbon emissions and increase resilience to climate change by providing capital and specialized expertise to the leading companies in the energy efficiency, renewable energy and other sustainable infrastructure markets. Our goal is to generate attractive returns for our shareholders by investing in a diversified portfolio of assets and projects that generate long-term, recurring and predictable cash flows or cost savings from proven commercial technologies.

We believe we were one of the first U.S. public companies exclusively focused on financing solutions to climate change. Our investments, which typically benefit from contractually committed high credit quality obligors, have taken a number of forms including equity, joint ventures, land ownership, lending or other financing transactions. We also generate ongoing fees through gain-on-sale securitization transactions, services and asset management.

We are internally managed, and our management team has extensive relevant industry knowledge and experience, dating back more than 30 years. We have long-standing relationships with the leading energy service companies ("ESCOs"), manufacturers, project developers, utilities, owners and operators. Our origination strategy is to use these relationships to generate recurring, programmatic investment and fee generating opportunities. Additionally, we have relationships with leading banks, investment banks, and institutional investors from which we are referred additional investment and fee generating opportunities.

We completed approximately $1.2 billion of transactions during 2018 , compared to approximately $1.0 billion during 2017 . As of December 31, 2018 , we held approximately $2.0 billion  of transactions on our balance sheet, which we refer to as our “Portfolio.” For those transactions that we choose not to hold on our balance sheet, we transfer all or a portion of the economics of the transaction, typically using securitization trusts, to institutional investors in exchange for a gain on the transfer and in some cases, ongoing fees. As of December 31, 2018 , we managed approximately $3.3 billion  in these trusts or vehicles that are not consolidated on our balance sheet. When combined with our Portfolio, as of December 31, 2018 , we manage approximately $5.3 billion of assets, which we refer to as our managed assets.

We use borrowings as part of our strategy to increase potential returns to our stockholders and have available to us a broad range of financing sources including non-recourse or recourse debt, equity and off-balance sheet securitization structures.

See Item 1. Business for a further discussion of our business, investing strategy, and financing strategy.

Market Conditions

We believe that the sustainable infrastructure markets in which we invest, and overall investment in climate change solutions, will continue to develop given the increasing awareness of the impact of climate change. According to the 2018 U.S Fourth National Climate Assessment Report, a report from the U.S. Global Change Research Program, the earth's climate is now changing faster than at any point in the history of modern civilization, primarily the result of human activities. The report concludes that the impacts of climate change are intensifying across the country but that the severity of future impacts will depend on efforts to reduce carbon emissions and how fast adaptations occur. Without changes to reduce the historic level of carbon emissions growth, the report estimates that annual losses in some economic sectors are projected to reach hundreds of billions of dollars by the end of the century-more than the current gross domestic product (“GDP”) of many U.S. states. Similarly, on a global basis, the World Economic Forum’s 2019 Global Risk Report identified extreme weather and failure to mitigate climate change as the two largest risks facing the global economy according to a survey of business, government, civil society and thought leaders.

At the present time, it is estimated that overall energy efficiency and renewable energy spending is over $500 billion a year globally and over $100 billion a year in the United States. For example, BloombergNEF (“BNEF”) in a January 2019 report estimated that worldwide renewable energy investment was approximately $300 billion with U.S. investing approximately $64 billion. A study by the International Energy Agency (“IEA”) titled Energy Efficiency 2018 estimated global spending on energy efficiency was approximately $235 million with U.S. investing approximately $42 billion. As we focus on certain submarkets, our presently addressable market is less than these overall industry estimates as especially in energy efficiency, projects are often self-financed. However, we believe these estimates provide an indication of market trends.

Positive industry trends and the need to reduce carbon are expected to further increase the investment opportunity. The IEA in the same report estimates that energy efficiency spending pays back on average by a factor of 3 over the life of the improvement as a result of lower energy expenditure. The U.S. Energy Information Administration (“EIA”) in its January 2019 report titled Annual Energy Outlook 2019 estimated that energy efficiency will keep U.S. energy consumption largely flat even as the U.S. economy grows. BNEF in its New Energy Outlook 2018 projects the cost from building and operating a new solar photo-voltaic plant is forecast to decline 71% by 2050, with project costs for onshore wind declining 58%.

The need to reduce carbon emissions is expected to drive further growth. IEA estimated that in order to achieve necessary carbon reductions, global energy efficiency annual spending would need to more than double by 2025 with a further doubling between by 2040. The EIA also estimates that the use of renewables will increase from 18% of U.S. generation to 31% by

2050. To quantify the future opportunity, a joint study was prepared by the International Renewable Energy Agency (“IRENA”) and the International Energy Agency (“IEA”) titled Perspectives for the Energy Transition: Investment Needs for a Low-Carbon Energy Transition , estimated that 76% of total worldwide estimated carbon emission reduction by 2050 would be achieved from energy efficiency and renewable energy investments. The study estimated that over $100 trillion dollars could be spent globally on energy efficiency and renewable energy over the next 35 years while increasing overall GDP growth.

In addition to a focus on reducing the amount of carbon emissions, we are seeing an increasing need to focus on adaptation to weather events or resiliency, including those as a result of climate change. According to the U.S. National Oceanic and Atmospheric Administration, there were 14 disaster events in the United States in 2018 with an estimated individual cost of greater than $1 billion, the 4 ^th highest year ever, with an aggregate cost of approximately $90 billion dollars. The average annual number of such events over the last five years has doubled as compared to the average annual number of events since 1980. Over the last five years, the estimated cost of these events is approximately $500 billion.

Despite these trends, there have been certain federal governmental actions that could have downward pressure on the renewable energy market, such as the potential repeal of the clean power plan (“CPP”), a proposed rule introduced by the EPA under the prior presidential administration and intended to establish carbon emissions guidelines for existing U.S. power plants, the tariff on solar imports in the Section 201 solar trade case (“201”), the withdrawal of the U.S. from the Paris Climate Accords (the “ Paris Accords”) and the 2017 Tax Cut and Jobs Act (“TCJA”). The results of CPP, 201, withdrawal from the Paris Accords, and the TCJA may have caused a short-term reduction in growth in the U.S. renewable energy markets, however, we believe that the growth will continue given the strength in the other factors that are increasing demand.

There were also changes to certain provisions of the tax code in the 2017 TCJA that impacted the renewable energy market included the reduction in the corporate tax rate as well as a minimum tax on organizations with foreign operations referred to as the base erosion anti-abuse tax (“BEAT”) which potentially limits the value of renewable energy tax credits. It does not appear that there was a material impact to the market resulting from the BEAT other than a temporary slowdown in investment taken to understand the totality of the impacts of the provision. On a more positive note, there have been various legislative proposals to increase investment in renewable energy or to place a tax on carbon emissions.

The federal government actions have been largely offset by the continued interest by state and local governments to invest in, and implement policies to encourage, private sector funded investment in, sustainable infrastructure focused on climate change. For example, in the renewable energy markets, much of the U.S. energy policy is implemented at the state level where we continue to see support for the use of renewable energy in many states to address concerns on the long-term environmental impact to our society, to develop a sustainable environment and, in many cases, to encourage jobs creation and economic growth. Along with these policies, there has been an increase in corporate commitments to utilizing renewable energy and other forms of sustainable infrastructure. We believe that the development of projects in our markets help to achieve these goals and thus may benefit from these policies.

Both state and local governmental agencies and other organizations are responding to the ability of renewable energy and energy efficiency to address climate change. State governmental agencies are responding to the potential risks of climate change through the implementation of renewable portfolio standards (“RPS”) as well as energy reduction targets such as energy efficiency resource standards (“EERS”). According to the National Conference of State Legislatures, as of July 2018, there were twenty-nine states, Washington D.C. and three territories that have adopted RPS policies while eight states and one territory have set renewable energy goals with California with the highest goal of 100% renewable energy by 2050. Also, according to the EIA, as of July 2017, 24 states have EERS policies with many states recently increasing their targets in both of these programs. Corporate organizations are also addressing the concern of climate change through their own corporate policies, including increased commitments to the use of energy from renewable sources.

The Federal Energy Savings Performance Contracts ("ESPCs") are an example of a public private partnership that eliminate the need for a federal agency to find appropriated funds to replace, operate, and maintain energy-using equipment while providing multiple benefits, including saving taxpayer dollars from energy savings, improving conditions for federal workers and service men and women and creating private sector jobs. In total, according to the DOE, the federal government has identified, as of February 2019, approximately $8.5 billion of energy conservation measures that could be implemented at existing U.S. federal buildings. In addition, federal agencies, including the U.S. Department of Defense are focused on increased adoption of energy efficiency improvements and on-site renewable energy generation to improve energy resiliency. The recent John S. McCain National Defense Authorization Act for 2019 requires the military to address climate and energy resiliency in their planning with Congress encouraging the use of ESPCs to address these issues.

While we believe that the long-term growth prospects for our business remain positive, there is the potential for financial market and commodity price volatility and interest rate movements that may impact the markets we serve. The current interest

rate environment and increasing investor acceptance of our markets has increased the level of competition we experience. In addition, in 2018, the Federal Reserve Board of Governors continued to raise the rate at which banks lend to each other known as the federal funds rate and have implemented four rate increases equal to 1.0% during 2018 and has indicated for 2019, it will be patient in determining future rate increases. See “Item 7A. Quantitative and Qualitative Disclosures about Market Risk-Interest Rate and Borrowing Risks” for an analysis of the impact of rates on our business.

According to DOE data, the average annual Henry Hub natural gas prices decreased by approximately 30% from 2014 to 2018 with its 2019 outlook forecasting that prices will stabilize or begin to increase. Wholesale electricity prices are closely tied to wholesale natural gas prices in many parts of the country and thus lower natural gas prices have negatively impacted, and are expected to continue to negatively impact, renewable energy projects that sell wholesale power on a “merchant” basis at spot prices. As described in more detail in “Item 7A. Quantitative and Qualitative Disclosures about Market Risk-Commodity Price Risk” for further information on the impact of commodity prices, we attempt to mitigate our exposure to commodity price volatility by focusing on projects with contracted revenues and by negotiating certain structural protections such as a preferred return as well as our on-going active asset management and portfolio monitoring. We also seek to manage credit risk that might arise from commodity price declines thorough due diligence and underwriting processes, strong structural protections in our transaction agreements with customers and on-going active asset management and portfolio monitoring. In addition, we do not generally lend to individual companies but instead focus on projects or portfolios of assets which are typically held in special purpose entities.

Notwithstanding the near-term concerns that current market conditions have raised for our business, we believe significant opportunities exist for us to grow our business in the face of these conditions. Historically, attractive risk-adjusted returns were available in a higher interest rate environment rather than lower. As a long-term participant committed to providing capital for sustainable infrastructure, we plan to continue to fund projects that meet our underwriting standards and look for opportunities to expand our business.

Factors Impacting our Operating Results

We expect that our results of operations will be affected by a number of factors and will primarily depend on the size of our Portfolio, including the mix of transactions which we hold in our Portfolio, the income we receive from securitizations, syndications and other services, our Portfolio’s credit risk profile, changes in market interest rates, commodity prices, federal, state and/or municipal governmental policies, general market conditions in local, regional and national economies and our ability to qualify as a REIT and maintain our exemption from registration as an investment company under the 1940 Act.

Portfolio Size

The size of our Portfolio will be a key revenue driver. Generally, as the size of our Portfolio on our balance sheet grows the amount of our investment revenue will increase. Our Portfolio may grow at an uneven pace as opportunities to originate new assets may be irregularly timed, and the timing and extent of our success in such originations cannot be predicted. To the extent the size of our Portfolio changes due to equity method investment activity, the income or loss from such investments will not be included in revenue but are reflected on a separate line in our income statement and will vary over time. In addition, we may decide for any particular asset that we should securitize or otherwise sell a portion, or all, of the asset, which would result in gain on sale of receivables and investments or fee income as described below. The level of portfolio activity will fluctuate from period to period based upon the market demand for the capital we provide, our view of economic fundamentals including interest rates, the present mix of our Portfolio, our ability to identify new opportunities that meet our investment criteria, the volume of projects that have advanced to stages where we believe a transaction is appropriate, seasonality in our activities and in the various projects where we may provide debt or equity and our ability to consummate the identified opportunities, including as a result of our available capital. The level of our new origination activity, the percentage of the originations that we choose to retain on our balance sheet and the related income, will directly impact our investment revenue.

Income from Securitization, Syndication and Other Services

We will also earn gain on sale of receivables and investments or fee income by securitizing or selling all or a portion of our transactions and by servicing the securitization financings we arrange. For transactions that we securitize to a non-consolidated trust, we recognize a gain on securitization of the receivables. The gain may be comprised of both cash received and a retained interest in securitization assets. We may also recognize additional income such as servicing fees from these securitization assets over the life of the asset.

In many cases, we arrange the securitization of the loan or other asset prior to originating the transaction and thus have avoided exposure to credit spread and interest rate risks that are typically associated with traditional capital markets conduit transactions. In these cases, we avoid funding risks for these financings or other assets given that our securitization partners contractually agree to fund such assets before the origination transaction is completed.

We also generate fee income for syndications where we arrange financings that are held directly on the balance sheet of other investors or if we sell existing transactions to other investors. In these transactions, unless we decide to hold a portion of the economic interest of the transaction on our balance sheet, we have no exposure to risks related to ownership of those financings. We may charge advisory, retainer or other fees, including through our broker dealer subsidiary.

The gain on sale income and our other sources of fee income will also vary depending on the level of our new origination activity and the portion of our originated assets we decide to transfer to other investors. We view this revenue from such activities as a valuable component of our earnings and an important source of franchise value. The total amount of income from securitizations, syndications, and other services will vary on a quarter to quarter basis depending on various factors, including the level of our originations, the duration, credit quality and types of assets we originate, current and anticipated future interest rates, the impact on our leverage, the potential income from a securitization or syndication, the mix of our Portfolio and our need to tailor our mix of assets in order to allow us to qualify as a REIT for U.S. federal income tax purposes and maintain our exemption from registration under the 1940 Act.

Credit Risks

We source and identify quality opportunities within our broad areas of expertise and apply our rigorous underwriting processes to our transactions, which, we believe, will generally enable us to minimize our credit losses and keep financing costs low. While we do not anticipate facing significant credit risk in our assets related to government energy efficiency projects, we are subject to varying degrees of credit risk in these projects in relation to guarantees provided by ESCOs where payments under energy savings performance contracts are contingent upon achieving pre-determined levels of energy savings. We are also exposed to credit risk in our other projects that do not benefit from governments as the obligor such as on balance sheet financing of projects undertaken by universities, schools and hospitals, as well as privately owned commercial projects. In the case of various renewable energy and other sustainable infrastructure projects, we will also be exposed to the credit risk of the obligor of the project’s PPA or other long-term contractual revenue commitments, as well as to the credit risk of certain suppliers and project operators. Our level of credit risk has increased, and is expected to continue to increase, as our strategy increasingly includes mezzanine debt, real estate and equity investments. We seek to manage credit risk thorough due diligence and underwriting processes, strong structural protections in our transaction agreements with customers and continual, active asset management and portfolio monitoring. Nevertheless, unanticipated credit losses could occur and during periods of economic downturn in the global economy, our exposure to credit risks from obligors increases, and our efforts to monitor and mitigate the associated risks may not be effective in reducing our credit risks. See Item 7A. Quantitative and Qualitative Disclosures about Credit Risks for further information on our credit risks and see Note 6 of our audited financial statements included in this Form 10-K for additional detail of the credit risks surrounding our Portfolio.

Changes in Market Interest Rates and Liquidity

Interest rate risk is highly sensitive to many factors, including governmental monetary and tax policies, domestic and international economic and political considerations and other factors beyond our control. We are subject to interest rate risk in connection with new asset originations and our borrowings, including our credit facilities, and in the future, any new floating rate assets, credit facilities or other borrowings. See Item 7A. Quantitative and Qualitative Disclosures about Market Risk for further information on interest rates risks and liquidity.

Commodity Prices

When we make investments in a project that act as a substitute for an underlying commodity, we may be exposed to volatility in prices for that commodity. For example, the performance of renewable energy projects that produce electricity can be impacted by volatility in the market prices of various forms of energy, including electricity, coal and natural gas. This is especially true for utility scale projects that sell power on a wholesale basis such as many of our wind projects as opposed to distributed renewable projects or energy efficiency projects which compete against the retail or delivered costs of electricity which includes the cost of transmitting and distributing the electricity to the end user. See Item 7A. Quantitative and Qualitative Disclosures about Market Risk for further information on the impact of commodity prices.

Government Policies

We make investments in renewable energy projects that typically depend in part on various federal, state or local governmental policies that support or enhance the project’s economic feasibility. Such policies may include governmental initiatives, laws and regulations designed to reduce energy usage and impact the use of renewable energy or the investment in, and the use of, sustainable infrastructure. Policies and incentives provided by the U.S. federal government may include tax credits (with some of these tax credits that are related to renewable energy scheduled to be reduced in the future), tax deductions, bonus depreciation, federal grants and loan guarantees, and energy market regulations. The value of tax credits, deductions and incentives may be impacted by changes in tax laws rates or regulations, including as a result of the TCJA.

Incentives provided by state and local governments may include a RPS, which specify the portion of the power utilized by local utilities that must be derived from renewable energy sources as well as the state or local government sponsored programs where the financing of energy efficiency or renewable energy projects is repaid through an assessment in the property tax bill in a program commonly referred to as PACE. Additionally, certain states have implemented feed-in or net metering tariffs, pursuant to which electricity generated from renewable energy sources is purchased at a higher rate than prevailing wholesale rates. Other incentives include tariffs, tax incentives and other cash and non-cash payments.

Governmental agencies, commercial entities and developers of sustainable infrastructure projects frequently depend on these policies and incentives to help defray the costs associated with, and to finance, various projects. Government regulations also impact the terms of third party financing provided to support these projects. If any of these government policies, incentives or regulations are adversely amended, delayed, eliminated, reduced, retroactively changed or not extended beyond their current expiration dates or there is a negative impact from the recent federal law changes or proposals, the operating results of the projects we finance and the demand for, and the returns available from our investments may decline, which could harm our business.

U.S. Federal Income Tax Legislation

The TCJA, which was signed into law on December 22, 2017, made significant changes to the U.S. federal income tax laws applicable to businesses and their owners, including REITs and their stockholders. Certain key provisions of the TCJA could impact us and our stockholders. See Note 10 of our audited financial statements in this Form 10-K for further information on the TCJA. Prospective investors are urged to consult with their tax advisors regarding the effects of the TCJA or other legislative, regulatory or administrative developments on an investment in our common stock.

Impacts of climate change on our future operations     

As our business is focused on reducing carbon emissions and increasing resiliency to climate change, we are impacted by the effects of climate change and various related regulatory responses. To stem the effects of climate change within the 21 ^st Century, nearly 200 countries agreed in December 2015 to the Accords, which aims to limit the increase in world-wide temperatures to 2 degrees Celsius above pre-industrial levels. Although the United States announced in 2017 its intention to withdraw from the Paris Agreement, numerous governmental, NGO, university, social media and private sector initiatives continue to support the goals outlined in the Accords, push initiatives beyond the Accords and encourage the transition from fossil fuel energy and related physical assets to increased deployment of clean energy and energy efficiency solutions. We understand that the transition to a lower-carbon economy will require sustained creative problem solving and substantial investment of capital over the next several decades across the entirety of the climate change front. We have built our business around providing capital to assets or projects and services aimed at making concrete progress in both mitigating the physical effects of climate change and in supporting the transition to a lower carbon economy.

Because our business focuses on supporting climate change solutions, our results of operations and the growth of our business will be impacted by the risks and opportunities associated with climate change. For example, our business will be impacted by the degree to which governmental initiatives encourage or fail to encourage climate change solutions. In particular, the value of and the revenue we generate from our assets will be positively impacted by governmental action that (1) increases taxes or fees on carbon emitting assets, or (2) increases the value or scope of renewable energy credits. In addition, governmental action that supports additional investment in infrastructure related to renewable energy, storage and energy efficiency assets, is likely to increase the volume of the assets available for us to invest.

If world-wide temperatures continue to rise, it would be reasonable to expect that flooding, storm surges and challenges with electric grid stability would occur more frequently. In this circumstance, we believe that we are likely to continue to see attractive investment opportunities in renewable energy, resiliency, and energy efficiency assets. Renewable energy assets are

becoming increasingly cost competitive when compared to fossil fueled energy assets, for example, and are also an integral component of a distributed generation platform that may be in more demand as the effects of climate change increase. In the case of energy efficiency assets, we expect there will be continued demand for the reduction in operating costs offered by such assets.

Further, all of our assets remain vulnerable to the potential impact of climate-related events, including flooding, increased wildfires, water scarcity, and changes in wind patterns. Each of these events could impact the operation of our assets through physical damage, higher operating expenses, and/or degraded performance. When considering the structure of our investments and environmental risks that may impact our results of operations, we typically negotiate contractual protections to insulate our financial returns from such downside risks outlined above and/or obtain insurance policies intended to mitigate our exposure to physical damage that may occur.

Critical Accounting Policies and Use of Estimates

Our financial statements are prepared in accordance with GAAP, which requires the use of estimates and assumptions that involve the exercise of judgment and use of assumptions as to future uncertainties. The following discussion addresses the accounting policies that we use including areas that involve the use of significant estimates. Our most critical accounting policies involve decisions and assessments that could affect our reported assets and liabilities, as well as our reported revenues and expenses. We believe that all of the decisions and assessments upon which our financial statements are based are reasonable at the time made and based upon information available to us at that time. Our critical accounting policies and accounting estimates may be expanded over time. Those material accounting policies and estimates that we expect to be most critical to an investor’s understanding of our financial results and condition and require complex management judgment are discussed below. See Note 2 of the audited financial statements in this Form 10-K for further details on our accounting policies.

We evaluate our critical accounting estimates and judgments on an ongoing basis and update them, as necessary, based on changing conditions. Additionally, there were certain newly issued accounting pronouncements that may be relevant to our business. See Note 2 of the audited financial statements in this Form 10-K for further details on these newly issued accounting pronouncements.

We have identified the following accounting policies as critical because they require significant judgments and assumptions about highly complex and inherently uncertain matters and the use of reasonably different estimates and assumptions could have a material impact on our reported results of operations or financial condition. These critical accounting policies govern:

Consolidation and Equity Method Investments

We account for our investment in entities that are considered voting or variable interest entities under ASC 810, Consolidation . We perform an ongoing assessment and make judgments to determine the primary beneficiary of each entity as required by ASC 810, which includes an assessment of the type of control we have over the entity. If we would conclude that certain of these entities should be consolidated, we would include the entities assets, liabilities and related activity in our financial statements. Refer to discussion below relating to consolidation considerations for the securitization of receivables. We further discuss our process for evaluating these judgments in Note 2 of the audited financial statements of this Form 10-K.

For those transactions not consolidated, we generally determine our income allocations under the equity method of accounting based on the change in our claim on net assets of the investee entity using a method commonly referred to as the hypothetical liquidation at book value method or (“HLBV”). This method uses a hypothetical liquidation scenario that may require judgment in its application and could have a material impact on our reported financial results. Any changes in this method of application or in certain assumptions could either increase or decrease our net income. We further discuss our process for applying this method of income allocations in Note 2 of the audited financial statements of this Form 10-K.

Impairment of our Portfolio

We evaluate the various assets in our Portfolio on at least a quarterly basis, and more frequently when economic or other conditions warrant such an evaluation, for potential delinquencies or other events that may indicate a potential impairment of the such asset. If an asset is determined to be impaired, any impairment charges would be recorded in the income statement and reduce our net income. We further discuss our process for evaluating these judgments in Note 2 of the audited financial statements in this Form 10-K.

Securitization of Receivables

We have established various special purpose entities or securitization trusts for the purpose of securitizing certain receivables or other debt investments. We make judgments, based in part, on supporting legal opinions, on whether these entities should be consolidated as a variable interest entity, as defined in ASC 810, Consolidation, and whether the transfers to these entities are accounted for as a sale of a financial asset or a secured borrowing under ASC 860, Transfers and Servicing . If we would conclude that certain of these special purpose entities or securitization trusts should be consolidated, we would include the assets and liabilities of the entity and their related activity in our financial statements. If sale accounting is not met in these transactions it would be treated as a secured borrowing rather than a sale in our financial statements. We further discuss our process for evaluating these judgments in Note 2 of the audited financial statements of this Form 10-K.

Results of Operations

We focus on reducing the impact of, or increasing resiliency to, climate change by providing capital and our specialist expertise to the energy efficiency, renewable energy and other sustainable infrastructure markets. Our goal is to generate attractive returns for our stockholders by investing capital in assets or projects that generate long-term, recurring and predictable cash flows or cost savings from proven technologies. We believe we were one of the first U.S. public companies focused on building a diversified portfolio that addresses climate change. Our investments take various forms including equity, joint ventures, lending or other financing transactions, as well as land ownership and typically benefit from contractually committed high credit quality obligors. We also generate on-going fees through gain-on-sale securitization transactions, advisory services and asset management. We manage our business as a single portfolio and report all of our activities as one business segment.

We completed approximately $1.2 billion  of transactions during 2018 , compared to approximately $1.0 billion during 2017 . Our strategy includes holding a large portion of these transactions on our balance sheet. We refer to the transactions we hold on our balance sheet as of a given date as our “Portfolio.” Our Portfolio was approximately $2.0 billion as of December 31, 2018 and December 31, 2017.

Portfolio

Our Portfolio totaled approximately $2.0  billion as of December 31, 2018 , and included approximately $1.0  billion of BTM assets, approximately $0.9  billion of GC assets and approximately $0.1  billion of other sustainable infrastructure investments. Approximately 23% of our Portfolio consisted of unconsolidated equity investments in renewable energy related projects and approximately 20% of our Portfolio was real estate used in renewable energy projects. The remainder consisted of fixed-rate government and commercial receivables and debt securities which we generally refer to as debt investments. Our Portfolio consisted of over 185 transactions with an average size of $10  million and the weighted average remaining life of our Portfolio (excluding match-funded transactions) of approximately 14 years as of December 31, 2018 .

Our Portfolio included the following as of December 31, 2018 :

The table below provides details on the interest rate and maturity of our debt investments as of December 31, 2018 :

The table below presents, for each major category of our Portfolio and the related interest-bearing liabilities, the average outstanding balances, income earned, the interest expense incurred, and average yield or cost. Our earnings from our equity method investments are not included in total revenue and thus we have excluded the income and related interest expense for our equity method investments from these calculations. Our net investment margin represents the difference between the interest and rental income generated by our Portfolio and the interest expense, divided by our average Portfolio balance.

The following table provides a summary of our anticipated principal repayments for our receivables and investments as of December 31, 2018 :

See Note 6 of our audited financial statements in this Form 10-K for information on:

For information on our residual assets relating to our securitization trusts, see Note 5 of our audited financial statements in this Form 10-K. The residual assets do not have a contractual maturity date and the underlying securitized assets have contractual maturity dates until 2055 .

Comparison of the Year Ended December 31, 2018 to the Year Ended December 31, 2017

NM—Percentage change is not meaningful.

Portfolio as compared to 2017. Gain on sale of receivables and investments and fee income grew by $15 million due primarily to an increase in securitization activity and related fees.

Comparison of the Year Ended December 31, 2017 to the Year Ended December 31, 2016