NEW YORK, Aug 27 (Reuters) - An exact tally of North Americans who lost power during the mammoth Aug. 14-15 blackout may never be known, clouded by the way electric utilities count their customers.

For utilities, a "customer" is not an individual, but a meter box linking the power grid to an individual power account that could be feeding anything from a home with a family of 10 to a chicken farm with 1,000 birds.

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According to a Reuters survey of the utilities impacted, at the height of the blackout, about 22 million customers lost their lights in the Canadian province of Ontario and eight states in the U.S. Northeast and Midwest.

Within the first hour of the outage, grid operators started referring to it as the biggest blackout to ever hit North America, surpassing the 1965 blackout that struck an estimated 30 million people in roughly the same geographical area.

But the number of people at the other end of those 22 million meters remains a mystery.

"Utilities recognize customers to be someone who has an account, and how many people are reflected on those accounts is difficult to determine," said Alberto Bianchetti, spokesman for National Grid Transco Plc (London:NGT.L - News) subsidiary Niagara Mohawk Power Corp.

"Some of those may be business accounts, some are residential accounts, some are schools and churches. There is no way we can say how many people those accounts represent," Bianchetti said.

DIFFICULT TO DETERMINE

Niagara Mohawk sends power to about 1.5 million upstate New York customers in a service territory with a population of about 3.5 million people.

The blackout knocked out electric service to 867,000 of Niagara Mohawk's customers, or about 58 percent, representing an unknown number of people, Bianchetti said.

"When we say 'customers,' that is the number of homes and businesses, not the number of people in homes," said Kristen Baird, spokeswoman for FirstEnergy Corp. (NYSE:FE - News), which lost power to about 1.4 million customers during the blackout.

FirstEnergy sends power to about 4.3 million customers in parts of Ohio, Pennsylvania and New Jersey, an irregular patchwork of service territories whose population Baird did not immediately know.

The joint U.S.-Canadian investigation into the origin of the blackout has focused on an area of Ohio controlled by utility FirstEnergy.

Not all FirstEnergy customers lost service since the grid, when functioning properly, automatically shuts down lines with irregular voltage flows and re-routes the power to stable lines, isolating the problem and skirting the outage.

ACCEPTABLE "GUESSTIMATE"

In the media, the two-day blackout was widely reported as having affected 50 million people, a number based on rough population estimates for the areas pitched into the dark.

That estimate may be off by up to 16 million using a crude calculation of three people per customer account. That means the 22 million customers utilities reported had been hit by the blackout translates to about 66 million people.

The 3-to-1 ratio is based on a service area's population and its number of utility customers.

For example, Consolidated Edison Inc. (NYSE:ED - News) subsidiary Consolidated Edison Co. of New York Inc., provides electricity to 3.1 million customers in and near New York City, representing a total population of just over 9 million people.

However, none of the utility officials Reuters contacted were willing to endorse the 3-to-1 calculation, or any straight multiple of their customers, for that matter.

"I don't know of any general formulas that will work," Bianchetti said.