Maine utility regulators decided Tuesday to start a preliminary, fact-finding review of what is behind an apparent spike in electric bills for some Central Maine Power customers.

Staff from the Public Utilities Commission will pull together data that includes electric meter performance, billing system accuracy and CMP’s response to customers, the commissioners decided during deliberations. If the results warrant, the agency will open a formal investigation.

No timeline was set for either process.

The inquiry comes roughly two months after customers began contacting the PUC, the Office of the Public Advocate and media outlets to complain about inexplicably high monthly bills. In some instances, customers were reporting bills four times higher than the same time last year.

As of Feb. 23, the PUC had tallied 274 complaints, most dealing with billing and usage. There were 85 complaints from CMP customers in January and 172 through Feb. 23. By contrast, Emera Maine, which serves northern and eastern Maine, tallied 10 in January and seven through Feb. 23, according to the PUC.

CMP’s customer base is nearly four times larger than Emera’s — 600,000 customers versus 159,000 — but CMP’s complaint numbers are proportionately much higher.

Determining a root cause or causes may be a challenge, though, because every customer’s pattern of electric use is unique and a number of variables could muddle the data on individual bills.

Among them:

• Many complaints involve the period in late December and early January when Maine experienced a prolonged, brutal cold spell. People used more energy to stay warm and may have consumed far more electricity than they realized.

• On Jan. 1, “standard offer,” electric rates — the default price of electricity for people who do not pick their own supplier — rose 18 percent for customers whose electricity is delivered by CMP. That would make bill rise, even if consumption was identical to the same period last winter.

• Some customers who get their power from competitive suppliers, such as Electricity Maine, have seen even larger rate increases than the 18 percent hike in the standard offer.

• Appliances can malfunction and push consumption to high levels. Common culprits are water heaters and well pumps, according to CMP.

• Billing periods can be confusing. CMP has 20 billing cycles each month. Customers on the cycle from Dec. 8, 2017, to Jan. 8, 2018, for instance, were billed during the worst of the arctic blast.

Aside from variables that might affect individual customers, the bill spikes could be linked to a system-wide problem involving software or hardware.

For instance: CMP switched to a new billing software in October. To make the transition, some bills had a 40-day cycle, much longer than the typical period of 27 to 33 days. That would drive up November bills.

It is also possible the new billing system has a technical glitch, something CMP has been studying and the PUC is aware of.

Electric meters also can malfunction. The PUC’s consumer assistance division has checked the accuracy of meters at homes of some residents who complained, but did not find problems.

“We have done several meter tests and have not found an issue,” said Harry Lanphear, the PUC’s spokesman.

CMP’s smart meters, which send power-use data from each customer for billing, also have come under scrutiny. Some residents suspect the meters were damaged by the prolonged power outages from last October’s windstorm.

CMP says smart meters have been installed for several years now and have gone through multiple outages. They undergo random testing each year for accuracy, CMP says, and some have been specifically evaluated following complaints.

“So far, all meters have tested within very strict standards for accuracy,” said Gail Rice, a CMP spokeswoman.

Rice also dismissed conspiracy theories, contained in some emails, that allege CMP is somehow using the meters to recover part of the $69 million cost of restoring power following the October windstorm, which caused the most widespread outages in state history.

“All of our operations are very transparent and subject to PUC regulations and scrutiny,” Rice said.

But some residents say they just cannot find a satisfactory explanation for the astronomical bills they received. Their frustration is reflected in emails sent to the newspaper.

Patricia LeRoyer said she was not surprised by a $250 bill last fall, when renovation work was being done on her Parsonsfield home. The same was true at Christmas time, when holiday lights went up. But as a retired person in a home with oil heat, LeRoyer said nothing prepared her for the most recent bill.

LeRoyer wrote: “With our lowest usage by our calculations, I just opened a bill for just shy of $600!!! Our old house had new services and panels and was updated to breakers from fuses. So there is no reason that OUR equipment is responsible.”

LeRoyer said CMP told her the meter is working correctly.

“We live on a fixed income and cannot afford this atrocity,” she concluded.

Daniel Davis also is struggling to make sense of his $531 bill. His consumption went from 800 to 900 kilowatt hours, he said, to 3,800 in two months. His Sabattus home gets 70 percent of its heat from propane, he said, so the spike in use does not make sense.

Davis wrote: “When I started digging for clarity, every point made was met with conflicting information. The cold snap in December is understandable, but my bill in January was dramatically lower than the one received in February, where the average temps were actually higher.”

Davis said he tested his appliances and also analyzed his use on Energy Manager, CMP’s online tool to let customers track their patterns.

“Huge spikes at 2 a.m., usage when we are not home. Fluctuation when we shut everything off. Going all the way back to October, we have a week of about 17 kilowatt hour per day, when we were without power.”

Hundreds of customers also contacted WGME, according to the Portland station, which has been conducting its own investigation. The PUC said it received 112 of the WGME complaints to review. To compare year-over-year usage for January, the station analyzed bills for customers who had lived in the same home for at least a year. It found average, daily usage rose 111 percent over last year.

Getting to the bottom of these complaints will be a very labor-intensive process, Lanphear said. The PUC must receive a customer’s permission to examine their meter’s performance and go through their bill with them to look for abnormalities. In most instances, Lanphear said, when the PUC staff already has done this, it has been able to uncover a use pattern that was not evident to the customer.

“We look at this on a case-by-case basis,” he said. “In the complaints where we’ve worked through the process, we’ve been able to explain to the customers, ‘Here’s where your use went up.’”

Rice said CMP is continuing to study all possibilities, including the new billing software.

“If any errors are found on the part of CMP,” she said, “we will correct the problems and adjust customers’ bills.”