A Guide to Green Energy Adoption for Transit Agencies Chapter 1: Green Energy Sources for Transit

  • Date: February 16, 2022

Transportation and electricity production are the two largest sources of greenhouse gas emissions in the United States, according to the US Environmental Protection Agency (EPA). Transit agencies deploying zero- emission vehicles lower transportation emissions, but these deployments often result in increased electricity usage from charging infrastructure or auxiliary services necessary to power the vehicles. The increasing prevalence of zero-emission vehicles and increased electricity usage presents an opportunity for transit agencies to further contribute to environmental and community benefits not only through the deployment of clean vehicle technology but also through their choice of electricity resources. Technological innovations in the twenty-first century have led to decreased costs of green energy sources, making green energy options more affordable for more organizations. This chapter defines and describes green energy resource options and their relative applications for transit agencies.

Total US greenhouse gas emissions by economic section in 2018
Figure 1. Total US greenhouse gas emissions by economic section in 2018 (“Sources of Greenhouse,” n.d.)

For transit agencies looking to make their operations greener, there are a variety of avenues that allow agencies to produce or purchase energy from green energy resources. According to the EPA, green power is the subset of renewable energy sources that contains energy sources which are the most beneficial to the environment (EPA, 2018). The energy resources that the EPA classifies as green are shown in Figure 2. As agencies transition to zero-emission vehicles for their fleets, it will be important to ensure that the source of the replacement fuels—electricity and hydrogen—also support the zero-emission goals.

Depiction of conventional, renewable, and green energy sources in relation to their benefit to the environment
Figure 2. Depiction of conventional, renewable, and green energy sources in relation to their benefit to the environment. Source: EPA (2018) Guide to Purchasing Green Power

Green Power Resources

The following sections provide brief overviews of common green power resources and technologies. The specific green power resources that a transit agency may use will be dependent on local availability of the resource, operational needs and financial resources of the agency, and particular environmental goals. There are some generalities that can be stated regarding green power resources: solar power is by far the most prevalent resource and technology in use for on-site power generation. It is likely the cheapest to install on-site, as well, although particular circumstances could result in different resource cost calculations. Further information on each resource’s relevant infrastructure can be found in Chapter 2: Green Energy Infrastructure. Information on green power procurement can be found in Chapter 4: Procurement Options for Green Energy.

Table 1: Average cost assumptions for green power energy systems 1

Average Installed Cost (2019 $/kW) Fixed Operation and Maintenance Cost (2019 $/kW/yr)
Solar PV, commercial $1,539 $16
Wind, commercial $1,548 $230
Biomass $4,104 $125.2
Small hydropower $600 – $4,500 $36 – $270
Geothermal $2,000 – $5,000 $115

1 Solar, wind, and biomass costs are from the Energy Information Administration. Hydropower and geothermal project costs are highly project and site specific. These approximate cost data are from a 2019 report by the International Renewable Energy Agency.


Solar Power

Solar power is one of the most common forms of green energy production, and the technologies are already in use by many transit agencies. The most common technology for solar power production is solar photovoltaic (PV), in which solar cells are arranged into panels and capture energy from the sun to convert it into electricity. Although power production may dip slightly on cloudy days, solar PV panels can still produce electricity with indirect sunlight. These systems are also versatile in that they can be installed on the ground, on building roofs, or as canopies over parking lots. Solar power is an easily scalable energy option, as the modular nature of the panels allows agencies to customize the size and orientation of the system.

Alameda-Contra Costa Transit District (AC Transit) in California has four different solar arrays across their facilities, with the first installed in 2007. Three of the four arrays are roof- mounted on different facilities, and one is a trellis array. The trellis array is positioned in one of AC Transit’s parking lots which provides shaded parking underneath for the agency’s buses and is more accessible for maintenance purposes. Prior to receiving five battery-electric transit buses in 2019, AC Transit’s solar arrays and on-site solid oxide fuel cell produced more energy than AC Transit consumed.

Bus's at station
Figure 3. AC Transit’s solar trellis array with buses parked underneath (Source: AC Transit).

Solar PV technology has become popular for both residential and commercial applications. Costs for solar PV systems have fallen dramatically over the past decade, and the industry has matured to the point where there are many solar manufacturers, installers, and financing options to choose from when purchasing a solar power system. Agencies can size systems to power their entire operations or can offset a portion of utility-provided power from the grid for financial or environmental benefits. Solar power is often paired with an energy storage system (see Energy Storage section below) to provide resiliency or store power for operations at night.

Wind Power

Wind power is another quickly growing green power option. Wind energy is converted to electricity through the use of wind turbines. While wind itself is as ubiquitous as the sunshine that powers solar energy generation, wind turbines require certain wind speeds, known as the wind resource, to operate. The U.S. Department of Energy (DOE) recommends that the area where the wind turbines will be placed receives at least 10 miles per hour average annual wind speeds (DOE, 2021). In areas with adequate wind resources, wind power can be a very effective green power option and is often used for utility-scale power generation. Large, utility- scale wind turbines can stand over 260 feet tall and generate several megawatts of power each. Fortunately, smaller wind turbines exist for more local applications. Small wind turbines typically generate less than 100 kW and can require roughly one acre of land to install. According to the American Wind Energy Association, small or “distributed” wind turbines can be found at farms, rural electric cooperatives, and schools throughout the country. Wind power could be an appealing options for many rural transit agencies with access to adequate wind resources. The WINDExchange Wind Resources tool, maintained by Department of Energy, has wind energy maps by state that can provide a good starting point for agencies to explore the feasibility of on- site wind at their facilities.


Biomass is organic material that comes from plants and animals such as wood pellets, agricultural residues, and animal manure, which can be converted into electric power typically by combustion. Biomass systems may be an option for on-site electricity production at transit agencies that have access to a constant and sufficient source of local biomass fuel to maintain electricity production. On-site biomass systems may require more staff attention than sources such as wind or solar, as the fuel must be ordered and received on-site. In particular geographic areas, transit agencies may have access to local, third-party biomass generating plants as a green power option. More information on procuring green power through such third-party producers can be found in Chapter 4: Procurement Options for Green Energy.

Low-Impact Hydropower

Hydropower systems convert energy from flowing water to electricity through the use of turbine. Most hydropower, while considered renewable power, is not considered green power because of environmental impacts that hydropower dams have on local ecosystems and the greenhouse gas emissions required to produce the concrete to build the dams. Even small hydropower systems, categorized by the US Department of Energy as those generating 10 megawatts or less, produce much more power than any one transit agency will likely require. Low-impact hydropower projects are those that “involve little change to water flow and use and are unlikely to affect threatened and endangered species” (FERC, 2020). Hydropower’s role as a green energy source for transit agencies is likely limited and will likely be considered only in very specific, location-dependent circumstances. More often, hydropower will be part of a utility’s power generation mix and may provide power to a transit agency through the grid.


systems harness the heat from the earth in order to do two main things: produce electricity and heat or cool buildings. To produce electricity, geothermal plant operators drill deep wells, up to two miles, into the earth and pipe steam or hot water to the surface to power a turbine and create electricity (U.S. Energy Information Administration, 2020). Geothermal power plants must be in an area that has sufficient geothermal reservoirs, which are found mostly in the western United States, making this a very location-dependent resource. Geothermal energy is often used to heat or cool buildings using a geothermal heat pump, which utilizes the near- constant temperature of the earth directly below the surface to moderate heating and cooling systems. This application is not location-dependent like geothermal power production as shallow earth temperatures are mostly constant throughout the United States. Transit agencies interested in offsetting facility heating and cooling loads with green power could explore geothermal heat pumps as an efficient and environmentally-friendly option to offset some electricity or gas consumption in facility heating/cooling loads.

Energy Storage

While not necessarily an energy resource itself, energy storage is a crucial component of many green power systems and on-site green energy installations. For transit agencies using hydrogen-powered fuel cell electric vehicles, energy is stored as liquid or gaseous hydrogen and used as vehicle fuel. The fuel cell then converts this hydrogen in to electric power onboard the vehicle. Historically, storing electricity has been difficult. In recent years, however, improvements in battery technology and reductions in costs for battery components have made lithium-ion batteries the go-to technology for energy storage in the electricity industry. Many green power systems now use lithium-ion battery storage to store energy.

Given that it is not consistently sunny or windy, green power systems relying on these resources produce power intermittently. Energy storage allows an agency to store energy that is generated, say, when it is sunny out for use when the solar PV system is less productive—at night or on cloudy days. This capability is especially important for transit agencies that may want to charge buses overnight in a depot but also find that solar power is generally their best green power option. Energy storage systems also increase the resiliency and reliability of an agency’s power supply, whether that is on-site green power or supplied via the local grid. Even for agencies relying on grid power, energy storage can provide a backup power source for charging buses and maintaining operations in the event of a grid outage.

The decision to install a complementary energy storage system is going to depend on how an agency intends to procure green power. For agencies planning to power their operations entirely via on-site green power generation, energy storage will likely be a necessity and should be factored into the planning and cost analyses. For agencies interested in procuring green power through utility green power programs, energy storage may be less of necessity but could still add value in terms of resiliency and energy management options. The value proposition of energy storage should be researched in the context of a particular agency’s green power needs and constraints

Assessing Green Energy Resource Options

The green energy resources available to a given transit agency will depend on location and agency-specific factors. While solar and wind resources are ubiquitous as on-site generation and as utility resources, other resources like low-impact hydropower, geothermal, and biomass may only make sense to investigate as on-site options for agencies located in geographies with these resources nearby. More likely, these green power resources will be available through third-party generators or as utility resources that supply power to the electrical grid. Some agencies may, after consulting with their local electricity provider, realize that their provider already produces green energy from one of these energy resources and may have the opportunity to participate in a green power program or tariff. More information on green power procurement options can be found in Chapter 4: Procurement Options for Green Energy.

Resources for Transit Agencies

Once an agency decides to incorporate green energy into their operations, there are still quite a few more decisions to be made, as there are a variety of avenues to acquire green power. The EPA produced a Green Power Supply Options Screening Tool that could be a beneficial tool for agencies that are early in their decision-making process. An agency fills in some organizational details on the spreadsheet, and the tool provides green power supply options that might be best for the agency based on the details provides.

Additional Resources