Ed Miliband’s promise to cut bills by £300 has been branded a “fantasy” with households facing an electricity bill increase of at least £50 a year because of delays to the government’s clean power strategy.

Two vital projects designed to adapt the electricity grid to green energy will not be ready for the 2030 deadline, ministers have been told. That is a blow to Energy Secretary Ed Miliband’s pledge to cut energy bills by up to £300 by the next election.

It has been suggested that the government could be forced to pay the developers of new offshore wind farms not to produce electricity because the grid would be unable to transport it to people’s homes

Shadow Energy Secretary Claire Coutinho said: “Ed Miliband’s promise to cut bills by £300 was always a fantasy. This is what happens when you set yourself impossible climate targets and ignore the costs. In 2030 you’ll be paying around £100 in your energy bill to pay wind farms literally to switch off when it’s too windy.”

The National Energy System Operator (Neso) calculated that the total cost of the network delays would add £4.2 billion to consumer energy bills in 2030 — at least £50 a year per household.

Meanwhile ministers have abandoned plans to try to lower average energy costs by charging different prices for electricity in different parts of the country.

Jack Richardson, the head of policy at Octopus Energy, said: “Several critical projects needed to upgrade the grid by 2030 are already behind schedule, and who knows how many more will crop up before then. UK billpayers have already paid £700 million so far this year to turn off wind farms because the grid can’t take their power. Billions of pounds of wasted wind will be added if we don’t fix the system, with billpayers fronting the cost through higher bills.”

A spokeswoman for National Grid said: “We welcome Neso’s continued focus on the urgent need for transmission infrastructure to keep pace with the UK’s clean power ambitions. The Norwich to Tilbury and Sea Link projects are critical to reinforcing the network to deliver cleaner and more affordable power.

“It’s always our priority to deliver these projects as early as possible, and we’re working closely with government, Neso, Ofgem and wider stakeholders to ensure they are in place to support a cleaner, more secure and affordable electricity system for the future.”