Joulet develops power generation and high-voltage infrastructure for so-called "large load customers" in the US, helping them meet the boom in demand for artificial intelligence tools.
National Grid said the investment was in line with its strategy to go after "select critical infrastructure growth opportunities emerging from the transforming energy landscape".
It also expands the company's exposure to the rapidly growing US large load power market and diversifies its regional presence in the country.
National Grid said the deal will be funded through existing balance sheet headroom and will still allow it to maintain a strong investment grade credit rating.
Joulent's flagship project, Project Kilby, is a 2.67 GW co-located power facility in West Texas, powering a Microsoft-operated data centre campus.
It is being developed in partnership with American energy major Chevron and should deliver first power in 2028.
"Our investment in Joulent is a disciplined, partner-led investment in contracted critical infrastructure for the AI-driven large load economy," said National Grid boss Zoë Yujnovich.
"Through National Grid Ventures, we are gaining exposure to a major source of electricity demand growth that diversifies our regional US exposure and is supported by strong partners."
National Grid shares were down 2.2% at 1,220p by 1429 BST.
See the latest RNS on Investegate.