Although the deal is not yet finalized, the move will reportedly see the STA re-launching as a trade body affiliated to the REA.
The two parties have said they will merge their memberships to form an association with around 400 members. The result will see a trade body, which represents both the solar thermal and PV industries. They will share an office in central London.
In a statement, the REA explains: "Some detail of the arrangements are still being worked out, but the plans will result in the re-launching of the Solar Trade Association, as a trade body affiliated to the REA."
The two parties involved believe the merger expected to create the UKs largest solar trade body will offer the industry a "stronger voice and better representation, and the resources to fully engage in standards and certification work as well as far more targeted promotional campaigning."
Howard Johns, chairman of the STA commented: "With the growth in the solar industry in the UK, this move to bring the industry together more fully could not have come at a better time. We really need good representation for solar in the UK and I believe this new revamped STA will provide it."
Gaynor Hartnell, chief executive of the REA added: "Having won the feed in tariffs, which fired the starting gun for PV in the UK, were keen to catch up with our European neighbors. With the solar tariffs now under review, and the continued delay to the heat incentive we need unity more than ever before."
Existing members have reportedly welcomed the move.