- Microsoft cofounder and former CEO Bill Gates is the second richest person in the world, with a net worth over $100 billion.
- He's invested a sizeable chunk of cash into clean-energy companies, largely through a billion-dollar fund he spearheads called Breakthrough Energy Ventures.
- Most of the startups he funds address three solutions Gates says are necessary for the clean-energy transition: Long-duration storage, nuclear energy and carbon capture, and power transmission.
- Business Insider compiled a list of the 24 clean-energy companies that Gates has invested in, using data from PitchBook.
- Visit Business Insider's homepage for more stories.
Bill Gates is rich. His net worth is on the order of $100 billion, according to Forbes' real-time billionaires list, making him the second-wealthiest person on the planet.
He's also one of the most generous, as measured by how much money he's given away. Through the Bill and Melinda Gates Foundation, Gates and his wife have dished out about $50 billion since 1994 to support public health, education, and development.
But what Gates receives less attention for is his investments in startups — of which there are dozens. Many of them fall into the category of clean energy.
"What's needed are affordable ways to bring clean energy to nearly a billion people who deserve more access to electricity," Gates said in a video that's part of his personal blog, Gates Notes. "That's why I'm making investments in clean, affordable, and reliable ways to generate electricity, and I'm encouraging others to do the same."
Gates' most public venture vehicle is a $1 billion fund he spearheaded with a coalition of other extremely high-net-worth individuals called Breakthrough Energy Ventures (BEV). Richard Branson, Michael Bloomberg, Ray Dalio, and Vinod Khosla are all investors in BEV, one of the world's most active clean-energy venture funds.
Read more:The top funds making bets on the clean-energy industry
"Our goal is to generate a financial return on our investments, each of which will have the potential of significantly reducing greenhouse gas emissions," the fund's website says. 
Gates has also invested in startups through other funds including Gates Ventures, Cascade Investments, and Village Global, according to the startup finance database PitchBook.
Most startups he backs are working on one of three solutions that Gates says the world needs "for the transition to clean electricity," according to his blog.
The first is long-duration energy storage — essentially, big batteries that can store power for days or months.
"Finding ways to store that energy to use after the sun sets and the wind stops blowing is a big challenge we need to solve," he wrote in a blog post.
The second is a mix of nuclear energy and carbon capture, a seemingly unusual pairing. Citing an MIT study, he said that if the energy transition includes nuclear power and carbon capture and storage technologies — which can reduce emissions from fossil fuel plants — it would "make carbon-free electricity up to 62 percent cheaper than using renewables alone."
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Finally, he says we also need high-voltage, long-distance power lines that can send direct current from renewable energy sources to areas of demand.
"Connecting our renewable energy supply with demand will require us to build transmission lines that can handle large amounts of power over very long distances," he said.
Business Insider compiled a list of clean-energy startups that received investment from Gates or one of his investment funds, according to data from PitchBook and company statements.
The startups below are listed by how much total capital they've raised in increasing order.
Bluefield Technologies — $2 million
Year founded: 2017
What it is: The startup measures methane emissions using imagery gathered by backpack-sized satellites. "Our sensor detects the spectral signature of methane in sunlight that is reflected off the ground. We then use machine vision algorithms to further enhance the optically rich data our sensor captures," the company's website states.
Total raised: Over $2 million, the company said in a statement
Other notable investors: Sequoia Capital and former Microsoft executive Charlie Songhurst, according to the company
Seaborg — $6 million
Year founded: 2014
What it is: Denmark-based Seaborg is developing compact molten salt nuclear reactors designed to limit waste. The startup is planning to have commercial reactors by 2027.
Total raised: About $6 million in equity. That doesn't include an additional $2 million in grants, the company said in a statement.
Other notable investors: PayPal cofounder Peter Thiel, according to PitchBook
Quidnet Energy — $8 million
Year founded: 2015
What it is: A startup that develops a pumped-hydro energy storage technology. Water is pumped into an underground cavern, where it becomes pressurized, and then released to power energy-generating turbines.
Total raised: About $8 million, the company said
Other notable investors: Clean Energy Venture Group and the Will and Jada Smith Family Foundation, according to PitchBook
Heliogen — $9 million
Year founded: 2013
What it is: The startup's system uses a mosaic of mirrors to concentrate sunlight, which generates extreme heat necessary for industrial processes like cement or steel manufacturing. The technology could eventually replace fossil fuels in those processes, as Business Insider previously reported.
Total raised: About $9 million, per PitchBook. A company spokesperson declined to share the total capital raised and mentioned that Heliogen is preparing to announce new funding.
Other notable investors: Energy giant NRG Energy and Brad Feld, cofounding partner of Foundry Group, according to PitchBook
Arnergy — $9 million
Year founded: 2013
What it is: Nigeria-based Arnergy sells distributed energy solutions, such as rooftop solar and storage, in emerging markets.
Total raised: About $9 million, the company said
Other notable investors: Norfund, a private-equity fund founded by the Norwegian government, according to PitchBook
Fervo Energy — $11 million
Year founded: 2017
What it is: California-based Fervo Energy is a geothermal energy startup. Geothermal energy typically refers to using the heat stored in the Earth's crust to power turbines or heat homes.
Total raised: About $11 million, per PitchBook
CarbonCure — $11 million
Year founded: 2007
What it is: CarbonCure sells a technology that enhances concrete using recycled carbon dioxide, or CO2. The company pumps CO2 into wet concrete while it's being mixed, at which point the gas reacts with water and calcium ions in the cement, forming solid limestone. The carbon is stuck in the limestone indefinitely. What's more, is that this mineralization process makes ready-mix concrete slightly stronger than some alternatives, according to the company.
Total raised: About $11 million, per PitchBook. That amount does not include recent, undisclosed funding. The company declined to share the total capital it's raised.
Read more: The 5 buzziest startups working to bring carbon capture and storage technology to market
SparkMeter — $14 million
Year founded: 2013
What it is: SparkMeter designs and sells smart meters and other equipment to utilities in developing markets. The startup's technology "enables utilities operating in remote locations to access a range of features — prepaid billing, customer communications, and remote monitoring and control — that improve their operations and help them achieve financial sustainability," the company's website says.
Total raised: About $14 million, according to the company
Other notable investors: Clean Energy Ventures, Powerhouse Ventures, and energy giant Total's venture arm, Total Carbon Neutrality Ventures, according to PitchBook
Lilac Solutions — $24 million
Year founded: 2016
What it is: Lilac developed a technique to extract the element lithium — used to make lithium-ion batteries, which power electric cars — in a more efficient and cost-effective way, the company says.
Total raised: About $24 million, according to the company
75F — $25 million
Year founded: 2012
What it is: 75F is a smart-building startup that sells various tools to make commercial buildings more energy-efficient. The company says it can cut costs by up to 50%.
Total raised: $25 million, according to the company
Other notable investors: A funding coalition led by executives in the oil and gas industry known as the Oil and Gas Climate Initiative, and the Wells Fargo Innovation Incubator, per PitchBook
Malta — $26 million
Year founded: 2018
What it is: Incubated at Alphabet's secretive X, known formerly as Google X, Malta develops a long-duration energy storage technology. The startup turns surplus electricity into thermal energy — both heat and cold, which are stored separately — which it can then convert back into electricity.
Total raised: $26 million, the company said in a statement
Boston Metal — $30 million
Year founded: 2012
What it is: MIT spin out Boston Metal uses a process called molten oxide electrolysis to turn raw metals into molten products used by various industries, such as steel production. The startup says the process produces far less carbon dioxide emissions.
Total raised: $30 million, according to the company
Other notable investors: Oil and Gas Climate Initiative, per PitchBook
Varentec — $37 million
Year founded: 2009
What it is: The Silicon Valley startup sells hardware and software to optimize the electric grid. The technology can reduce the "voltage volatility" by as much as 72% and prevent watt loss, the company says.
Total raised: About $37 million, per PitchBook
Other notable investors: 3M Ventures and Khosla Ventures, according to a public statement
Natel Energy — $48 million
Year founded: 2009
What it is: California-based Natel Energy is a hydropower startup that is developing fish-friendly turbines, which the company says are cheaper to build and easier to permit. Natel also works with developers to design sustainable hydropower projects.
Total raised: About $48 million, per PitchBook
Other notable investors: Schneider Electric Ventures, according to a public statement
ESS Inc. — $47 million
Year founded: 2011
What it is: ESS manufactures flow batteries. Unlike a traditional cell, flow batteries store an electrical charge in two tanks of liquid electrolyte. In this case, the electrolyte contains a mixture of iron, salt, and water. When the battery is plugged in, charged atoms called ions flow between the two tanks, generating an electric current.
Total raised: About $47 million, according to the company
Other notable investors: Wells Fargo Innovation Incubator, per PitchBook
Form Energy — $51 million
Year founded: 2017
What it is: Form Energy, a storage startup run by a Tesla veteran, is developing a handful of different electrochemical batteries designed for grid-scale storage. According to the company, its batteries will last for hundreds of hours. And at that duration, they'll be far cheaper than Li-ion alternatives, the company said.
Total raised: About $51 million, per PitchBook
Other notable investors: Eni Next, Macquarie Asset Management, and Saudi Aramco Energy Ventures, according to PitchBook
Sierra Energy — $63 million
Year founded: 2004
What it is: Sierra Energy is commercializing a gasification technology that can turn municipal waste into valuable products like diesel and hydrogen gas.
Total raised: About $63 million, including $30 million the startup received through its parent company, Sierra Railroad Company, the company said
Ambri — $80 million
Year founded: 2010
What it is: MIT spin out Ambri is developing a liquid metal battery for long-duration storage.
Total raised: About $80 million, according to the company
Other notable investors: Khosla Ventures and Total Carbon Neutrality Ventures, according to the company
Carbon Engineering — $84
Year founded: 2009
What it is:Technology designed by Carbon Engineering pulls CO2 directly out of the air. A large fan (or array of fans) essentially pulls in enormous quantities of air and passes it through a chemical solution that reacts with CO2, sucking the gas out of the air.
The solution, now loaded with carbon, is then mixed with another chemical called calcium oxide to produce pellets of limestone.
Finally, those pellets are heated to create what's called quicklime, a substance that, if heated to even higher temperatures, will produce a stream of pure CO2 that can be used to make synthetic fuel, among other things.
Total raised: About $84 million, not including government grants, the company said
Other notable investors: Oil and gas giants Chevron, BHP, and Occidental, according to PitchBook
Commonwealth Fusion Systems — $115 million
Year founded: 2017
What it is: MIT spin out Commonwealth Fusion Systems is developing a compact reactor to generate nuclear fusion, a source of clean energy.
Total raised: $115 million, according to the company
Other notable investors: Khosla Ventures and Eni, per PitchBook
Mainspring Energy — $133 million
Year founded: 2010
What it is: Formerly known as EtaGen, Mainspring Energy develops a technology called a linear generator, which uses a reaction between air and fuel to move magnets through copper coils and generate electricity. The company says it's efficient and produces "near-zero" nitrogen oxide emissions.
Total raised: $133 million, according to the company
Other notable investors: Norwegian oil major Equinor and Khosla Ventures, per the company
TerraPower — $178 million
Year founded: 2006
What it is: Founded by Bill Gates himself, TerraPower is developing various nuclear technologies including a modular reactor that uses molten chloride instead of water as a coolant, as Business Insider previously reported. TerraPower believes the design will be safer and more efficient than today's reactors.
Total raised: $178 million, per PitchBook
Other notable investors: Khosla Ventures, according to PitchBook
1366 Technologies — $107 million
Year founded: 2007
What it is: The Boston-based startup makes wafers for solar panels directly from molten silicon.
Total raised: About $107 million, the company said
Other notable investors: Solar energy giant First Solar and General Electric, per PitchBook
QuantumScape — $296 million
Year founded: 2010
What it is: Stanford spin-out QuantumScape is trying to develop solid-state batteries — which, unlike lithium-ion batteries, rely on solid-not-liquid electrolytes — in partnership with Volkswagen. Volkswagen says solid-state batteries would more than double the range of its electric e-Golf care.
Total raised: $296 million, according to PitchBook
Investors: Volkswagen, Khosla, and Kleiner Perkins, per PitchBook
Read more: These are the top 21 clean-tech startups to watch that are set to transform the energy industry