Output from wind energy plants recently assisted the electricity system to cope with an amber alert when supply tightened significantly, it has emerged.

Several large electricity plants are currently completing summer maintenance programmes which has left the system with less capacity than usual.

Consequently the presence of wind energy helped to plug a critical gap in supply.

Airtricity last night said the wind energy sector was happy to help with the stability of the system.

The company's trading and regulation manager Dermot O'Kane said, contrary to the often repeated charge that wind was unreliable, the sector had come to the aid of the electricity grid earlier this week at certain times.

An amber alert is when the system has only aproximately 400 megawatts of power in reserve, the equivalent of one large gas-fired station. A red alert is even more dangerous and means the system has no spare capacity at all.

The number of amber alerts has been decreasing in the Republic in recent years, but with the recent cold weather, supply has tightened once again. The giant ESB power plant at Moneypoint in Co Clare also recently suffered a forced "outage", or breakdown, exacerbating the problem.

A spokesman from ESB National Grid, which manages the system, acknowledged yesterday that, from time to time, margins on the power system tightened due to faults and essential maintenance.

"These situations are of course managed very carefully by the transmission system operator (TSO). In managing the situation, the TSO utilises all of the generation resources available including the output of wind farms."

However, wind energy companies yesterday said earlier this week evening peak demand reached 3,760 megawatts with maximum availability of about 4,002 megawatts.

This provided a reserve of just 242 megawatts, meaning its supplies were critical to supporting the system.

The ESB National Grid said the reserve was larger than this and it denied there was any crisis with the system last Monday.

"As is usual, wind power, along with all of the other generation on the system - oil fired, gas fired, coal fired, peat powered and hydro - did play its part in contributing to meeting that demand for electricity," a spokesman for the grid company commented.