Electricity grid owner Transpower is being called on to face the music for a power cut estimated to be worth up to $4 million that affected much of the Waikato and Coromandel Peninsula last year.

Electricity regulator the Electricity Authority is to lay a formal complaint with the independent rulings panel against Transpower for allegedly causing a "severe impact" power outage to Hamilton, Cambridge, wider Waikato and Coromandel Peninsula on January 25.

The authority said it took nearly 1.5 hours to fully restore power. The value of the lost load of 185MWh is estimated between $3m and $4m.

The authority