Transpower has told power firms that there is only a small buffer of surplus power generation available to cover any unexpected outages this morning, as cold weather spreads to the North Island.

The electricity system operator advised that the difference between the amount of generation that had been committed to the market and expected demand was projected to fall below the 200 megawatt buffer that it aims to maintain, between the hours of 7.30am and 9am.

For comparison, electricity demand can peak at just over 7000MW.

Transpower’s notice, which takes account of the expected weather, came shortly after operations manager Stephen Jay says it was closely monitoring the supply situation in September, in the wake of projected possible generation shortfalls then.

Relatively few notices of the nature issued by Transpower on Thursday progress to become full scale “grid emergencies”, and even then it can often avoid power cuts by ordering lines companies to turn off ripple-controlled hot-water heaters.

But the electricity system is facing an elevated risk of a power cut, for example if generating equipment broke down or failed to start as planned on Friday morning.

“While electricity supply is tight tomorrow morning, we don’t expect any impact on consumers. We are of course always susceptible to changing system conditions and faults to equipment,” says Jay.

About 7 per cent of the country’s electricity generation is offline due to repairs and maintenance at the moment, including the largest of the five turbines at Genesis’ Huntly power station, which broke down at the end of June and which Genesis recently discovered might not be fixed until May.

As is normal when the supply buffer drops below 200MW, Transpower called on generators to increase their output if they could.

- Tom Pullar-Strecker/Stuff.