Fusion energy technology startup Helion Energy announced Friday that it has raised $500 million to build a net positive electrical generator that generates more electricity than it consumes.

The latest round gives the start-up a valuation of $3 billion. Helion said that $1.7 billion in follow-up investments will be pledged if it can prove that its technology works. This would be a crucial step towards making fusion technology a practical solution for generating carbon-free electricity.

Fusion is the process that fuels the sun, as the nuclei of two atoms fuse under extreme heat, generating enormous amounts of energy.

Founder and CEO David Kirtley said Helion’s latest prototype, called Polaris, will feature renewable energy technology, similar to fusion technology that has already been developed to generate electricity.

The start-up broke ground in Everett, Washington, in July to build a generator the size of two shipping containers and, Kirtley said, generate enough electricity to power 40,000 homes. If successful, the $1.7 billion in follow-up investment would be used to develop a commercial system, he added.

Sam Altman, a prominent Silicon Valley investor and artificial intelligence researcher, led the panel, saying that Helion’s long-term goal is not only to produce clean energy, but also to keep the price of electricity at just one cent per kilowatt-hour.