Both firms provide funded lighting solutions, though eEnergy also provides broader energy efficiency and management under an energy-as-a-service banner.
The all-share deal will see eEnergy pay up to £2.2m for Suffolk-based RSL, which last year made a loss of £86,000 on sales of £1.6m.
Focussed on the state-school sector, RSL last year delivered 32 projects with an average contract value of £52,000. It has also recently secured deals with to multi academy trusts to install lighting at 20 schools.
eEnergy has focused more on the independent school sector. So far this year, it has installed lighting at 33 schools, including Marlborough College and Wycliffe College.
The firm estimates that more than 80 per cent of some 25,000 UK schools have yet to install energy efficient lighting, equating to an addressable market in excess of £1bn.
Led by Harvey Sinclair and aided by former Eon business sales chief, Martyn Sheridan, eEnergy yesterday posted an operating loss of £0.6m for the year ending 31 December. It is targeting break even in the second half of 2020.