The power purchase agreement (PPA) with Dutch utility Eneco will begin on 1 January, 2022 and last 15 years.
Power sold through the agreement will power tech giant Microsoft’s datacentres in the Netherlands.
Eneco’s chief customer officer, Hans Peters, said: "Tech companies are facing the challenge to reduce their carbon footprint, mostly due to the vast energy consumption of their datacentres."
Microsoft has now signed two PPAs for Dutch wind farms, buying output from a combined 270MW. Worldwide, it has signed 14 renewable energy PPAs for more than 1.5GW.
Eneco (10% owner of Borssele III & IV) is part of the Blauwwind Consortium that is developing the 731.5MW project. The consortium is completed by Swiss investment manager Partners Group (45%), Shell (20%), Mitsubishi’s Diamond Generating Europe (15%) and Van Oord (10%).
Eneco and Shell both have offtake agreements for 50% of the project’s output, and can secure other power purchasers for their allocated capacity.
Construction of Borssele III & IV is set to begin in the fourth quarter of 2019, with installation of 77 MHI Vestas V164-9.5MW turbines due to be completed by 2021.
Dutch health, nutrition and materials multinational Koninklijke DSM, airport operator the Royal Schiphol Group and Dutch grid operator Stedin have all also signed PPAs for output from Borssele III & IV.