While you eat turkey and open presents on Tuesday, dedicated power-station workers will be making sure that your fairy lights remain bright and the holiday roast browns nicely in the oven. There will be few surprises for them, however, as we are creatures of habit when it comes to electricity demand.
"People are predictable," says Michael Power, manager of the ESB's national-grid control centre, which is based in the company's headquarters, on Lower Fitzwilliam Street in Dublin.
Most weekdays at about 5.45 p.m., for example, electricity demand rockets as people arrive home to switch on heat and cook dinner. If there is a big match on, such as the Republic's recent clash with Iran, power demand drops as viewers stop to watch play.
"The whole key to this is the load forecast," Power says. The ESB, which is in the process of separating this transmission activity and vesting it in a stand-alone subsidiary, Eirgrid, has load forecasts going back decades.
The people in the control centre use the information to predict daily electricity loads, factoring in details such as weather conditions, the day of the week, major events and even what's on television. They are usually accurate to within 20 or 30 megawatts over a wintertime daily peak that can reach 4,000 megawatts, he says.
Household activities on Christmas day are no exception. "It is the same pattern for the previous four or five years. It is a very odd day."
A typical winter's day has two demand peaks: lunchtime and at about 6 p.m. On Christmas Day, the heavy load starts early, at about 11 a.m., as 1.5 million ovens go into action to cook the holiday roast, pushing sustained loads to about 2,500 megawatts. "This ends about 1.30 p.m. The load completely drops off by about 4 p.m. The evening peak never appears," says Power.
Things get back to normal by St Stephen's Day, however, with the noon and evening peaks returning to challenge the grid controllers. Christmas-tree lights have virtually no impact on this figure. If every home has one set of lights using 25 watts, then this demand accounts for just 0.1 per cent of total demand.
Knowing what the system is doing is important, because too little power will cause "brown-outs" - or partial blackouts - and too much can damage systems. "In the quantities we use power, it cannot be stored," says Power. Battery back-up is not an option, as batteries supply DC, or direct-current, power, whereas domestic, commercial and industrial systems use AC, or alternating-current, electricity.
Even though the forecasts help and we fall into a predictable pattern of usage, the grid controllers, who issue requests to "spin up" or slow down the generators, have to keep a close watch for unexpected events. They do this by tracking the grid's frequency: how quickly the current alternates. If load and demand are matched, then the frequency holds a steady 50 cycles - or 50 hertz - per second. When demand rises, the frequency drops; and vice versa. If the change is large, the controllers and generating staff have to take action. "If the frequency begins to increase, you have to tell the generators to back off," says Power.
There are automatic systems in place to replace power from a generator if it suddenly drops out of the grid, and small local demand changes of up to five megawatts are also automatic. The grid controllers usually deal directly with any change exceeding 10 megawatts.
They can add power quickly by adjusting the "spinning reserve". The Tarbert generator can produce about 256 megawatts, for example, but it usually runs at no more than 235-240 megawatts. This excess can be called into play if necessary, then dropped back when demand slackens.
Energy demand has experienced unprecedented growth over the past few years, rising by about 5 per cent every 12 months, according to the ESB. Electricity supply has been partially deregulated, so competitors - such as the plant at Edenderry and two plants under construction in Ringsend and north Finglas - can feed into the grid. Transmission is also set for deregulation.
The total national availability, including the ESB's 4,340 megawatts from its 19 stations, plus Edenderry, renewable schemes and the Northern Ireland interconnector, means up to 4,700 megawatts of power are available. The record for electricity demand - 4,003 megawatts, reached in January this year - is well below this.
Looking back, the highest demand was 3,080 megawatts in 1996, 2,100 megawatts in 1986 and just 308 megawatts in 1954.