A new report by the consultancy, prepared for TenneT and Energinet.dk, said 180GW of the total capacity would be deployed in the North Sea and another 50GW in the Baltic and Irish seas, and the Atlantic Ocean.
Restricting global warming to well below 2C in line with the Paris climate accord would require electricity generation to become carbon neutral by 2045, Ecofys noted.
To calculate the needed offshore wind capacity, it further assumed a 50% reduction in total energy demand by 2045 as well as a 45% electrification level.
The report identified spatial planning as an international challenge, arguing that the region as a whole has sufficient space available to meet the combined offshore wind target.
It noted that countries such as Denmark and the UK, which have "overabundant space" in their part of the North Seas to meet national offshore wind capacity requirements, can compensate for others with more limited space, including Belgium and France.
Ecofys also recommended the countries develop a 2045 roadmap for flexibility options, such as energy storage, demand response and capacity reserves, to ensure grid stability with a higher share of variable renewables in the system.
It added that increased interconnection capacity is needed to pool these flexibility resources from across the region.
In a separate study for the European Commission, Ecofys and RPS outlined the potential environmental impacts arising from the development of renewables and a grid in the Irish and North Seas up to 2030.
In a high renewables scenario, up to 76.6GW of renewable energy would be deployed in the region — the vast majority of it wind.
"Much of the impact from the offshore elements relate to biodiversity through direct conflict (e.g. collisions, loss of habitat, smothering etc.) or indirect impacts (increased effort required for feeding, avoidance behaviour)," the report said.
"Facilitating the development of international grid infrastructure, including offshore grid connection hubs to support the exploitation of the lowest levelised cost of energy, is key to allowing more [offshore wind] capacity to be installed," a spokesperson for industry association WindEurope said.