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The electricity sector is the largest source of greenhouse gas emissions (GHGs) in the U.S. Many states have passed and Congress has considered Renewable Portfolio Standards (RPS), mandates that specific percentages of electricity be generated from renewable resources. We perform a technical and economic assessment and estimate the economic costs and net GHG reductions from a national 25 percent RPS by 2025 relative to coal-based electricity. This policy would reduce GHG emissions by about 670 million metric tons per year, 11 percent of 2008 U.S. emissions. The first 100 million metric tons could be abated for less than $36/metric ton. However, marginal costs climb to $50 for 300 million metric tons and to as much as $70/metric ton to fulfill the RPS. The total economic costs of such a policy are about $35 billion annually. We also examine the cost sensitivity to favorable and unfavorable technology development assumptions. We find that a 25 percent RPS would likely be an economically efficient method for utilities to substantially reduce GHG emissions only under the favorable scenario. These estimates can be compared with other approaches, including increased R&D funding for renewables or deployment of efficiency and/or other low-carbon generation technologies.

The largest source of greenhouse gas (GHG) emissions in the U.S. is electric power generation, accounting for about 35 percent of total GHGs in 2008 (Environmental Protection Agency (EPA), 2010a). If U.S. GHGs are to fall substantially, emissions from generating electricity will need to considerably decline. One policy option for reducing GHG emissions is to impose a mandate, such as a Renewable Portfolio Standard (RPS), on electric power providers to generate a specified share of their electricity from renewable sources such as wind and biomass.1 A number of scholars have analyzed the economic costs of using an RPS to reduce GHG emissions and have arrived at differing conclusions. Wiser et al. (2007) concluded that RPSs yield “mixed-results.” Nogee et al. (2007) found RPSs generated “important economic and environmental benefits.” Michaels (2008) concluded they induce “inefficient” and in some cases “pernicious” outcomes. Some studies argue that using a diverse portfolio of low-carbon sources of energy could achieve targeted reductions in GHG emissions at lower cost than the more rigid technological mandates incorporated into RPSs (Dobesova et al., 2005 and Apt et al., 2008). Chen et al. (2009) estimated the anticipated effects of an RPS on retail electricity rates, finding expected increases would be small. In this paper we estimate the net benefits from imposing a national RPS mandate in terms of reductions of GHGs and the corresponding economic costs relative to coal-based electricity.2 Following on an earlier study by Toman et al. (2008), we set a target of generating 25 percent of U.S. electricity from renewable sources by 2025.3 We identify which sources of renewable energy are most economical for achieving such a mandate. We also assess the economic costs and net GHG benefits of the 20 percent Renewable Electricity Standard (RES) included in the American Clean Energy and Security Act (ACES) (HR 2454; Waxman-Markey) passed by the House of Representatives in 2009 (ACES, 2009). The paper is organized as follows. In Section 2, we outline the methodology we use to estimate the costs and quantities of electricity generated by prospective renewable energy sources and associated reductions in GHG emissions. We also introduce scenarios to consider uncertainty in technological developments and costs. Section 3 presents details on the costs and emissions associated with each of the renewable technologies we analyze: hydropower, wind, biomass, solar thermal, and geothermal. We present our results in Section 4. In Section 5, we discuss the implications of our analysis for recent proposals for legislation to reduce GHGs.

5.1. Additional costs of electricity generated by renewable sources of energy As noted above, the EIA projects that the U.S. is likely to consume 4348 TWh of electricity in 2025. Under a 25 percent mandate, renewables would have to generate 1087 TWh. Fig. 1 shows supply curves for new renewable energy to meet this target for all three scenarios. In all cases, the first 392 TWh of electricity (36 percent) would be generated by existing sources of renewable energy, beginning with hydropower (297 TWh), followed by existing biomass, wind, and geothermal sites. Accordingly, Fig. 1 begins at 392 TWh. Additional electricity is then provided by a continuously varying mixture of new wind, geothermal, and biomass sources, with each resource deployed in order of lowest cost. As additional sources of renewables are added to the mix, costs rise, as shown by the upward sloping supply curve. We assume the most expensive coal power is displaced first, as shown by the downward sloping supply curve.Under the Reference Scenario, the cost per kWh of using new sources of renewables rises from 4.6 cents/kWh for the lowest cost biomass to 9.2 cents/kWh for more expensive biomass, with an average cost of 7.8 cents/kWh. We first assume that electricity from these sources of renewable energy replaces coal-fired electricity that ranges in cost from 2.3 to 5.4 cents/kWh, averaging 2.8 cents/kWh in 2009. Electricity generated by renewable energy would cost 5.0 cents/kWh more, nearly three times the cost of the coal-fired power it would replace. The total net economic cost of substituting renewable energy for this coal-fired capacity would be $34.8 billion per year. If we assume that renewables substitute for 131 TWh of prospective new coal-fired generating capacity, the net annual cost would be $32.2 billion, as new coal-fired power is estimated to cost 4.4 cents rather than the 2.4 cents cost of the current coal-fired power at this end of the dispatch curve. Under the Favorable Technology Scenario, costs are much lower. The average cost for renewables is 4.6 cents/kWh. Assuming that electricity generated by renewable energy would substitute for current coal-fired power, the cost of the mandate would be $13.0 billion per year, 63 percent more than the cost of the coal-fired power the renewables would replace. If one assumes that 131 TWh of this electricity would substitute for new coal-fired capacity, the net annual cost falls to $9.8 billion. The Unfavorable Technology Scenario is appreciably more expensive. The additional cost of renewables, relative to coal, is $45.2 billion. In this scenario, renewables average 9.4 cents/kWh, 3.4 times the average price of the coal-fired power it would replace. Utilizing the assumption that renewables would substitute for new coal-fired plants brings this estimate down to $33.9 billion per year. Fig. 2 shows the contributions to meeting the 25 percent goal by each of the renewable sources under the Reference Scenario. Together, co-fired and dedicated biomass would contribute 37 percent of electricity under a 25 percent RPS. Onshore wind provides 28 percent of renewable power and hydropower almost the same.5.2. Reductions in GHG emissions Fig. 3 shows the reductions in GHG emissions that would result from a renewable mandate for electric power and the marginal cost per metric ton of reducing those emissions under all scenarios. The emissions reduction graphed in Fig. 3 corresponds to the output of electricity from additional renewables graphed in Fig. 1. Electricity generated from existing sources of renewables does not contribute to additional reductions in GHG emissions. Because the lowest cost biomass does not always have the lowest associated GHGs, Fig. 3 is not monotonic. Fully deployed, the renewable mandate would reduce GHG emissions by about 670 million metric tons per year in the Reference Scenario. This is equivalent to about 11 percent of U.S. GHG emissions in 2008.In the Reference Scenario, the first 100 million metric tons of GHG emissions could be saved at a cost of less than $36/metric ton. However, marginal costs climb up to about $50 for 300 million metric ton and then to as much as $70 for the remainder. In the Unfavorable Technology Scenario costs are higher: $90 for the last metric ton of carbon dioxide avoided. In contrast, in the Favorable Technology Scenario the costs are appreciably lower, just $23 for the last metric ton of GHG emissions avoided. One alternative way to induce power companies to use renewables would be to impose a tax or charge on emissions of carbon dioxide. In light of these costs of reducing GHG emissions, in the Reference Scenario, power companies would have to face a tax of $70/metric ton CO2e, to induce them to replace coal with renewables for the last increment needed to meet the 25 percent mandate. 5.3. Other estimates Kydes (2007) modeled the effect of a 20 percent RPS requirement by 2020 using similar renewable technologies.7 That analysis used the EIA's NEMS and a relatively more expansive modeling approach, which incorporates general equilibrium relationships between energy markets and the domestic economy. Where Kydes estimates the economy-wide impacts of an RPS, our analysis considers in greater detail the likely availability (and cost) of each renewable energy source and its potential for reducing GHG emissions. Like Kydes, we find that wind and biomass-generated power are the two largest sources of new renewable generation and account for the vast majority of non-hydro renewable power. However, Kydes finds that renewables cause a sharp reduction in natural gas use, while we assume that renewables will replace coal (as detailed in Section 2.1). Moreover, Kydes finds that the cost to the energy industry of a 20 percent RPS would be between $35 and $60 billion per year; our estimate of economic costs for a 25 percent mandate lies at the lower end of that range. Broadly, our results are consistent with Kydes, though we find slightly lower costs and higher emissions reductions. 5.4. Meeting proposed mandates in recent legislation In 2009, the House of Representatives passed the American Clean Energy and Security Act (H.R. 2454; “Waxman-Markey”), which focuses on reducing U.S. GHG emissions. H.R. 2454 contains a Renewable Electricity Standard (RES) similar to the renewable energy mandate analyzed in this paper. The bill would require 20 percent of the U.S. electricity to be produced from renewable sources by 2020, with up to five percentage points of the target eligible to be met by improvements in energy efficiency and excluding existing sources of hydropower.8 Using our models and assumptions of costs in 2025 and excluding existing hydropower, under our Reference Scenario the total cost of meeting the H.R. 2454 renewables requirement would be about $40 billion per year without efficiency improvements and about $26 billion assuming cost-neutral efficiency improvements. The corresponding cost of the last metric ton of GHGs eliminated would be $72 and $62 CO2e, respectively. H.R. 2454 also proposes an economy-wide cap and trade program to reduce GHGs. The program would cap total allowable emissions and allow firms to trade CO2e permits at a cost determined by the market. The EPA estimates the permit price under HR 2454 would be approximately $25/metric ton CO2e in 2025 (EPA, 2010b). At a permit price of $25/metric ton CO2e – but with no renewable energy mandate – our analysis suggests that power companies would only substitute renewable sources of power for an additional 64 TWh of electricity above and beyond the 392 TWh currently generated from existing sources of renewable energy, primarily hydropower. The renewables share in total U.S. consumption of electricity would rise from 9.0 to 10.6 percent of total U.S. consumption in 2025, 1.6 percentage points. The additional power produced through renewable energy would save approximately 64 million metric tons of CO2e annually. These results are substantially lower than the results of the 25 percent mandate considered in this report. 5.5. Policy implications Our analysis indicates that using renewable energy to generate electricity under an RPS would have net economic costs of $13–$45 billion per year. Total annual GHG reductions would be about 650–700 million metric tons. These costs and GHG savings estimates could potentially be used to design strategies for R&D, demonstration, and deployment to accelerate renewable technological development as part of a multi-criteria decision analysis. While costs are substantially reduced with further technology advances, other potential factors for such an analysis include externality values, timing of renewables deployment and associated GHG reductions, the cost of alternative technical approaches to reducing GHGs, and local economic benefits and costs. As noted above, the EPA estimates the cost of a GHG allowance under the H.R. 2454 cap and trade program would be around $25/metric ton. Using that estimate, only in the Favorable Technology Scenario would an RPS by an economically efficient means of substantially reducing GHG emissions. This highlights the value of continued R&D and strategies to accelerate renewable technological development. In the other two scenarios, electric power generators would likely find it more cost-effective to invest in low-cost efficiency options or offsets if available, as well as to purchase GHG allowance permits and continue to operate coal-fired power plants.