Just one year ago US developer Mayflower Wind was so confident over the economics of its first 804MW offshore wind project that it felt able to promise regulators in Massachusetts a 10% rebate on federal tax credits that would save state consumers $500m over 20 years.

That was then. Now, with a power deal for its second 400MW development under scrutiny by state officials, Mayflower – a joint venture of Shell and Ocean Winds – has joined fellow Massachusetts project Commonwealth Wind in calling for a pause of contract negotiations for Round Three power purchase agreements (PPAs) signed earlier this year, amid forecasts of diminishing returns.