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If you care about the world’s transition to clean power, then you should pay attention to the emerging market for energy storage. A lot of times when “grid-scale” energy storage is discussed, it’s usually meant to describe big battery farms, but it can also mean other forms of energy storage like thermal storage (using heating and cooling), and the next-generation of compressed air storage.

It’s been over a year since California set the country’s first ever mandate that says that the state’s utilities need to buy over a gigawatt of energy storage services by 2020. But you don’t have to live in California, to be interested in this new market because what’s happening in the Golden State will likely be emulated elsewhere down the road.

Wind and solar power plant owners can use energy storage systems to bank electricity and discharge it at any time when they need it. That helps utilities dispatch solar and wind energy to homes and businesses even when their solar power plants and wind farms aren’t able to generate electricity (like solar panels during the night). The ability to do so makes it possible for utilities to forgo building more fossil fuel power plants, which are able to produce electricity around the clock.

Some companies are selling energy storage services to businesses that can use electricity from battery banks — instead of the power grid — when utility rates are high. Homeowners typically can’t save as much money as businesses can because they don’t face the same special charges on their bills from utilities. These types of battery services will become more attractive to customers once battery system prices drop low enough to make them more affordable.

Here is a list of battery technology and energy storage service startups that have completed sizable demonstration projects or started to deploy their energy storage systems more widely:

1). Aquion Energy: This startup just scored a major customer in a private estate off Hawaii’s Kona Coast, where it will install a 1 megawatt-hour battery that will store electricity from a 176KW solar energy system. Aquion has figured out a novel cocktail of materials for creating the chemical reaction needed to charge and discharge energy. The Pennsylvania-based company uses manganese oxide and activate carbon for the electrodes, sodium sulfate for the electrolyte and a synthetic material with a structure similar to cotton.

2). Stem: Software development is key to Stem’s growth. The California company assembles lithium-ion battery cells and power electronics made by other companies. But it creates the software to monitor and predict its customer’s energy demand, and manage the charging and discharging of the battery systems to help customers reduce the use of electricity from the grid when rates are high.

Stem recently added Total Energy Venture and Constellation Technology Ventures as investors and raised a $27 million series B round. Some of the money will be used to develop, license or buy software technology to boost its energy management service, Stem’s CEO, John Carrington, told me. The company has lined up 85 MW contract with Southern California Edison.

3). EnerVault: A flow battery is made up of two tanks containing energy-storing chemicals and liquid electrolyte, which flows through a cell to react with the electrodes inside for generating power. EnerVault celebrated the completion of a 1 megawatt-hour flow battery system, which has iron in one tank and chromium in the other for storing energy, at an almond farm in California’s Central Valley last year. It was an important project for the company to demonstrate its technology. The performance of the system will affect whether EnerVault will line up more customers and financing.

4). Sunverge Energy: Raising a $15 million Series B last year and teaming up with SunPower are propelling Sunvege’s growth as a battery system designer and installer. Sunverge integrates lithium-ion battery cells and power electronics into a cabinet and develops software to run the system.

SunPower and Sunverge started a pilot project with KB Home last year to show how a SunPower’s solar energy system could work well with Sunverge’s battery setup. The two companies are now co-marketing that solar-and-battery combination for a commercial rollout this year. One of the Sunverge’s investors is Total Energy Ventures, the investment arm of French oil and gas giant, Total, which owns the majority stake in SunPower.

5). Ice Energy: Using electricity to make ice at night, when electric rates are lower, and melting it to provide cool air for the building during the day is what Ice Energy offers as an cheaper alternative to running air conditioning in the hot afternoon, when electric rates are high. Ice Energy’s ice-making system, called Ice Bear, is located near a building’s air conditioning system, which is modified to integrate Ice Bear. The company recently won its biggest deal with Southern California Edison, which wants 25.6 MW of service from Ice Energy systems across different locations.

6). Advanced Microgrid Solutions: This company was quiet until it got publicity for being selected by Southern California Edison to provide 50 MW of services. Helmed by Susan Kennedy, a former member of California Public Utilities Commission and chief of staff of former California Gov. Schwarzenegger. Advanced Microgrid Solution appears to be a pure project developer which doesn’t do in-house hardware or software development.

7). Coda Energy: This is not the Coda Energy that wanted to make electric cars, though it was part of that enterprise until a group of investors bought the energy storage business during a bankruptcy sale in 2013. Coda Energy installed several energy storage projects in 2014.

The company designs lithium-ion battery systems and develops the electronics and software to manage the systems. Its largest project, a 510KW system that could deliver 1.05 megawatt hours of energy, is a demonstration project at its headquarters that was co-funded by a state incentive program and the South Coast Air Quality Management District in southern California.

8). Imergy Power Systems: Imergy is a flow battery developer that has developed a process to purify low-grade vanadium, such as those from the waste stream from mines and oil fields, to make the energy storing compound in the liquid electrolyte. The vanadium technology is newer for the company, which sold several dozen iron and chromium flow batteries for off-grid use by telecom firms in India before making the switch a few years back. Imergy has announced several demonstration projects over the past year, including a 100KW system that will deliver 400 kilowatt hours of energy at a microgrid project at a U.S. Navy site in Port Hueneme, Calif.

9). GELI: The acronym stands for Growing Energy Labs, Inc., and this startup has developed an operating system to make sure the battery systems work well with other pieces of equipment, such as solar panels or heating and cooling systems. Developing good software to predict energy use of a building and store and dispatch energy from a battery system efficiently is a major pursuit and challenge of any energy storage developer and service provider. You can expect to see more startups in the battery software space as the market grows.

10). Green Charge Networks: When I caught up with Green Charge’s CEO, Vic Shao, about a year ago, he had lined up $10 million for financing projects that use Green Charge’s software to manage the lithium-ion battery systems designed by the company, using battery cells from suppliers such as Samsung. Like its competitors such a Stem, Green Charge also is going after business and industrial customers, such as 7-Eleven and Walgreens. It has also lined up customers in public agencies, including school and cities.

11). Ambri: The liquid metal battery developer raised a $35 million series C round last year to bring its technology to market. Ambri’s battery, unlike common battery designs, uses liquid anode and cathode, along with a molten salt electrolyte. The company plans to install prototype systems this year, look for a location to build a larger factory and complete the sale of its technology’s commercial version in 2016, according to a Boston Globe article.

12). Solar Grid Storage: The company is part of a growing number of project developers that sell and install a bundle of battery systems with solar panels and electric car charging equipment. When I spoke with Solar Grid’s CEO, Tom Leyden, over a year ago, he talked about the challenge of securing financing separately for the energy storage and solar portion of a project. The company’s projects are mostly on the East Coast.