Millions of people made their heaters work harder in January than ever before.

Power plants had to worker harder, too. The sweeping national cold spell caused mechanical problems for some generators and difficulties getting natural gas to others.

Calling January "one of its biggest weather challenges" in its nearly 87-year history, regional grid operator PJM Interconnection said it managed to maintain a stressed grid during the arctic cold, but not without some unusual measures.

This included several rare requests last month asking its 61 million customers, including South Jersey residents, to conserve electricity and lower thermostats if they could.

"We had a set of circumstances in January that may not be replicated in the future in terms of availability," PJM spokesman Ray Dotter said. "Cold brings a whole different set of challenges for the machinery compared to the summer."

Dotter said the conservation requests helped reduce demand, but to what extent is difficult to quantify.

"If everyone turned off one incandescent light, it quickly adds up. In our region, it can quickly add to being a couple of power plants," he said.

January brought unusual strains to the power grid as temperatures dipped dangerously low, and winter demand peaked locally and in much of the country. Conservation requests in South Jersey are common during long summer heat waves but are rare in winter.

In Atlantic City Electric's territory, peak winter demand this year was on Jan. 7 with 1,790 megawatts, utility spokesman Frank Tedesco said.

This is more than 9 percent above last winter's peak on Jan. 23, 2013. A megawatt can power about 1,000 homes.

Dotter, whose Valley Forge, Pa.-based company oversees the grid serving 13 states and the District of Columbia, said January provided eight of the top 10 winter power use days in its history.

On many days, the above-normal demand was the equivalent of powering greater Washington, D.C., and Baltimore regions again, PJM said.

As people cranked up their heaters during the cold, wholesale natural gas prices spiked because supply was running short for power plants and customers, the U.S. Energy Information Administration said. Coal and natural gas are the leading fuels for power plants, and natural gas is the most popular heating fuel in the U.S.

In the Mid-Atlantic, some power plants went offline from mechanical problems related to the cold. Even some oil-powered generators used for peak times ran into problems when frozen rivers blocked oil barges, Dotter said.

The grid operator is tasked with managing electricity supply to avoid potential blackouts across a major region that includes New Jersey, Delaware, Pennsylvania, Virginia, Maryland, Indiana, Illinois, Michigan, Kentucky, North Carolina, Ohio, Tennessee, West Virginia and Washington, D.C.

Electricity is used as a primary heating source for 15 percent of households in Atlantic, Cape May, Cumberland and Ocean counties, but it also powers motors on heaters that burn natural gas, propane and other fuels.

And for the most part, large amounts of electricity cannot be stored for a long time, meaning power plants need to produce the energy as people use it.

PJM and Atlantic City Electric issued several calls for public conservation in January, asking people to turn off unnecessary lights, wait until off-peak times to use major appliances and turn down their thermostats if their health permitted.

On Jan. 6, PJM issued a voltage reduction across the grid, about 5 percent, which one official said meant slightly dimmer light bulbs.

Dotter said the temporary voltage reduction is "one of the tools in our toolbox" to manage supply but is something the company tries to avoid.

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