# Mainspring Energy
Mainspring Energy, Inc. is a [Menlo Park, California](https://www.edgepedia.org/menlo-park-california) clean-power company, incorporated in 2010 by Stanford engineers [Shannon Miller](https://www.edgepedia.org/shannon-miller), Adam Simpson, and Matt Svrcek, that builds a fuel-flexible generator for onsite electricity production; per its own April 2025 announcement it has secured more than $800 million in total financing.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) • [2](https://spectrum.ieee.org/mainspring-energy) The product, the Mainspring Linear Generator, converts fuel to electricity without a spinning turbine or a conventional engine. The company operated earlier as Affinity Power, Inc. and EtaGen, Inc., with the EtaGen-to-[Mainspring](https://www.edgepedia.org/mainspring) name change recorded in a 2017 Delaware filing.[3](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt)
| Fact | Detail |
| --- | --- |
| Founded | 2010, Menlo Park, CA, by Shannon Miller, Adam Simpson, and Matt Svrcek[2](https://spectrum.ieee.org/mainspring-energy) |
| Legal and former names | Mainspring Energy, Inc.; formerly Affinity Power, Inc. and EtaGen, Inc.[3](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt) |
| Product | Fuel-flexible Linear Generator for onsite power, scalable from 250 kW to 100+ MW arrays[4](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html) |
| Fuels | Natural gas, biogas and landfill gas, hydrogen, syngas, ammonia, propane, alcohols[2](https://spectrum.ieee.org/mainspring-energy) • [5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users) |
| Total financing | More than $800 million as of April 2025 (company statement)[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) |
| Notable investors | Khosla Ventures, Bill Gates/Gates Frontier, NextEra Energy, Lightrock, General Catalyst, Amazon Climate Pledge Fund, Temasek[2](https://spectrum.ieee.org/mainspring-energy) • [1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) |
| Status | Private, operating; commercial shipments began 2020, with hundreds of megawatts in field operations per the company[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) |
## Founding and history
The technology began at Stanford University's Advanced Energy Systems Laboratory, where mechanical engineering professor Christopher Edwards posed a question to his Ph.D. students about how to build a more efficient fuel-to-electricity system. Three of those students, Shannon Miller, Adam Simpson, and Matt Svrcek, incorporated the company in 2010 to turn the research into a commercial product.[2](https://spectrum.ieee.org/mainspring-energy) [TechCrunch](https://www.edgepedia.org/techcrunch) likewise reported that the technology was initially developed in a Stanford thermodynamics lab where the three co-founders worked.[6](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/)
Development ran for roughly a decade before commercial rollout. [Khosla Ventures](https://www.edgepedia.org/khosla-ventures) provided the initial seed money.[2](https://spectrum.ieee.org/mainspring-energy) The first production unit was delivered to a paying customer in June 2020, and the company formally launched the product commercially in March 2021, announcing the launch alongside a $150 million contract with NextEra Energy Resources.[2](https://spectrum.ieee.org/mainspring-energy) • [6](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/)
## Technology: the Linear Generator
A [linear generator](https://www.edgepedia.org/linear-generator) makes electricity by moving magnets directly through copper coils in a straight line, without a crankshaft, flywheel, or rotating shaft. In Mainspring's design, a low-temperature reaction of air and fuel drives the magnet assembly; the company describes the reaction as flameless, with near-zero NOx (nitrogen oxides) emissions on any gaseous fuel.[6](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/) • [4](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html) This places it apart from reciprocating engine-generators, which burn fuel in cylinders at high temperature, and from microturbines and fuel cells, which use rotating machinery and electrochemical conversion respectively.[6](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/)
**Fuel flexibility** is the design's central feature. The machine runs a wide range of fuels, including natural gas, biogas, hydrogen, ammonia, syngas, and alcohols, without compromising performance, because its variable compression can be adjusted to fuels with very different properties; hydrogen needs little compression while ammonia needs a lot.[2](https://spectrum.ieee.org/mainspring-energy) The company says the product can switch from one fuel to another seamlessly without hardware changes, and can interchangeably use methane-based gases such as drilled natural gas, landfill gas, and biogas, or gas derivatives like hydrogen, syngas, ammonia, and propane.[4](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html) • [5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users) Mainspring has run a stock unit on hydrogen and on anhydrous ammonia, and reports a customer project running on renewable landfill biogas.[2](https://spectrum.ieee.org/mainspring-energy)
Individual units in the early fleet produced 230 to 460 kW; the company now describes products scalable from 250 kW up to arrays of 100+ MW.[2](https://spectrum.ieee.org/mainspring-energy) • [4](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html)
## Funding and investors, round by round
A Delaware Form D filed on July 6, 2021 reported a $110 million exempt offering with $108,100,265 sold as of the filing; the first sale under that offering was dated December 16, 2020, and 19 investors participated.[3](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt) The company's round history, as compiled from press reports and its own announcements, is:
- **Series C, January 2018:** $83 million (unverified; reported only in a templated profile page).[7](https://www.teardown.ai/companies/mainspring-energy)
- **Offering of 2020–2021:** $110 million offered, $108.1 million sold per Form D.[3](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt)
- **Series D, May 2021:** $95 million led by Devonshire Investors, with the announcement stating the round brought total capital raised to $228 million (unverified; reported only in a templated profile page).[7](https://www.teardown.ai/companies/mainspring-energy)
- **Series E, closed September 2022:** $290 million led by Lightrock (unverified; reported only in a templated profile page).[7](https://www.teardown.ai/companies/mainspring-energy)
- **Series F, April 14, 2025:** $258 million led by [General Catalyst](https://www.edgepedia.org/general-catalyst), joined by Amazon's Climate Pledge Fund, DCVC, Temasek, Marunouchi Innovation Partners, M&G Investments, and Pictet Group, with participation from earlier investors including Lightrock, LGT Bank, Khosla Ventures, and Gates Frontier. The company said the round brought total financing to more than $800 million.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board)
A note on totals: an IEEE Spectrum article written from the founders' perspective stated the company had raised more than US$500 million from investors including Khosla, American Electric Power, Bill Gates, and [NextEra Energy](https://www.edgepedia.org/nextera-energy),[2](https://spectrum.ieee.org/mainspring-energy) while the company's April 2025 release puts the total above $800 million.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) The figures are compatible as snapshots at different dates, and the later, company-stated figure supersedes the earlier one. With the Series F, Tom Linebarger, former chairman and CEO of Cummins, and Bethany Mayer, former CEO of Ixia, joined the board.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) Shannon Miller signed the 2021 Form D as Chief Executive Officer; the filing lists Shannon Miller, Adam Simpson, Samir Kaul, Jagdeep Singh, Alexander Weiss, and Nael Kassar among its executive officers and directors.[3](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt)
## Customers and deployments
The commercial launch in March 2021 was anchored by the $150 million NextEra Energy Resources contract, and the generators were under contract with a national supermarket chain deploying them at 30 grocery stores; IEEE Spectrum identifies a Kroger store in [Southern California](https://www.edgepedia.org/southern-california) among early installations, alongside a [Lineage Logistics](https://www.edgepedia.org/lineage-logistics) cold-storage facility.[6](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/) • [2](https://spectrum.ieee.org/mainspring-energy)
Later deployments are broader and larger:
- The Utah Municipal Power Agency is using a 48 MW Mainspring system on its distribution network, and a 35 MW project with another electric cooperative was announced as pending.[5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users)
- Mainspring's first data-center project, with [EdgeConneX](https://www.edgepedia.org/edgeconnex), is operational, with more data-center deployments planned; the company cites AI-driven demand for onsite power.[5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users)
- Kroger has deployed linear generators as a behind-the-meter power solution, and Amazon is moving forward with them for last-mile sorting facilities and electric vehicle charging, including a large microgrid.[5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users)
- Per its April 2025 release, the company has hundreds of megawatts in field operations across data centers, EV charging microgrids, hospitals, and wastewater treatment plants.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board)
## Manufacturing and recent developments, 2024–2026
On October 22, 2024, the U.S. Department of Energy's Office of Manufacturing and Energy Supply Chains selected Mainspring for an $87 million award under the Bipartisan Infrastructure Law's Advanced Energy Manufacturing & Recycling Grants Program, for a manufacturing facility in [Allegheny County, Pennsylvania](https://www.edgepedia.org/allegheny-county-pennsylvania) with capacity of up to 1,000 Linear Generators annually, the power equivalent, per the release, of meeting the electricity needs of up to a quarter of a million U.S. homes.[4](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html) The Series F round and board additions followed in April 2025, with the stated purpose of scaling the linear generator power business amid data-center demand.[1](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board) • [5](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users)
## References
1. [Mainspring Secures $258 Million in Financing to Scale Linear Generator Business — company press release, April 14, 2025](https://www.mainspringenergy.com/news/mainspring-secures-258-million-in-financing-to-scale-linear-generator-business-adds-energy-and-tech-leaders-to-board)
2. [This New Breed of Generator Can Run on Almost Any Fuel — IEEE Spectrum](https://spectrum.ieee.org/mainspring-energy)
3. [Mainspring Energy, Inc. — Form D, Notice of Exempt Offering of Securities (filed 2021-07-06) — SEC EDGAR](https://www.sec.gov/Archives/edgar/data/1716836/000171683621000001/0001716836-21-000001.txt)
4. [U.S. Department of Energy Awards $87 Million Grant to Mainspring Energy — PR Newswire, Oct 22, 2024](https://www.prnewswire.com/news-releases/us-department-of-energy-awards-87-million-grant-to-mainspring-energy-to-build-linear-generator-manufacturing-facility-in-allegheny-county-pa-302283226.html)
5. ['Linear' Gas Generation Adds Flexibility for Big US Power Users — E&E News via Mainspring newsroom](https://www.mainspringenergy.com/news/linear-gas-generation-adds-flexibility-for-big-us-power-users)
6. [Mainspring Energy launches its flexible fuel generator with a $150 million NextEra Energy contract — TechCrunch, March 9, 2021](https://techcrunch.com/2021/03/09/mainspring-energy-launches-its-flexible-fuel-generator-with-a-150-million-nextera-energy-contract/)
7. [Mainspring Energy: business model, traction, and outlook — Teardown](https://www.teardown.ai/companies/mainspring-energy)
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*Topic: Encyclopedia › Technology and the built world › Energy technology › Batteries and energy storage*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026; Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license