Pelham fought off Brunswick and FD for the brief.
Econogy made the PR change after switching analysts from Numis Secuities to Dresdner Kleinwort.
The independent firm started as a carbon trading company but is now moving its business model towards full renewable power production.
Pelham founding director Chelsea Hayes heads the account. Pelham has a history of working with clients in the alternative energy arena, advising the Chinese energy infrastructure investment firm Fortune Oil and the Indian coal exploration company Great Eastern Energy.
Last month Pelham won the account for FTSE 250 firm Gem Diamonds, which listed earlier this year. Gem Diamonds made headlines when it sold the 603-carat Lesotho Promise diamond, the biggest diamond discovered for 13 years, for more than £6.5m.
Separately... in the power-generating arena, Edelman has scooped the corporate account for Mitsubishi Electric Europe in the UK, a subsidiary of Fortune 500 Company Mitsubishi Electric.
The account will be led by Edelman corporate practice MD Pamela Fieldhouse and associate director Nick Barron.