EVERETT — The Snohomish County PUD’s switch to monthly billing late last year has caused an uproar with many ratepayers who say their bills have spiked since the switch.

PUD officials say customers are — ultimately — only paying for electricity they use, and that they are working with people to ensure no one is being overcharged.

Until late last year, most PUD residential customers received a bill every other month. The amount charged was based on a district employee actually reading that house’s electricity meter.

Following a substantial software upgrade last fall, the district began billing customers every month. However, a meter reader still only comes every other month. So, the PUD estimates the first monthly bill based on how much power that location used during the same period 12 months prior. The following month’s bill is based on an actual meter reading, and includes any difference between the prior estimate and actual usage.

“It’s raising a lot of questions” from ratepayers, said Jim West, the PUD’s assistant general manager for customer and energy services.

District customer service workers can go over a customer’s bill if there are questions, he said.

Oftentimes, there is a simple explanation for a sudden jump to a customer’s bill, including a change in the home, such as caring for a new baby, or simply colder weather, West said.

The PUD’s rates also are slightly higher now than they were a year ago. The district passed along to customers a 4.5 percent rate increase from the Bonneville Power Administration, which provides about 80 percent of the PUD’s energy. The district also increased its residential rate about 1.7 percent in April 2015.

The PUD has been switching many internal software programs to a unified system provided by SAP, a German software vendor. The change has allowed the PUD to scrap many stand-alone programs that did not talk to each other. It is a multi-year process covering dozens of functions, such as work schedules, customer billing, and budget management, among others.

In 2014 and 2015, the district spent more than $34.3 million for those changes, and it plans on spending $13.2 million this year.

It would cost even more to customize the SAP billing software to continue billing residential customers every other month, West said.

The PUD sends monthly bills to businesses and residential customers on a payment plan. The billing system is not set up to bill some customers monthly and others every other month, he said.

Similarly, adding more people to read meters would be prohibitively expensive, he said.

The district is working on a fix for one glitch it has found. If an estimated bill is much higher than the amount of electricity actually used, then the next bill — based on an actual meter reading — could be so low that a customer is charged a minimum-use fee. The $0.51 a day charge is meant to cover some of the fixed costs of providing power to a location that is hardly using any electricity.

“We’ve got a fix that will correct” bills with erroneous minimum-use fees “and will prevent those from happening,” West said.

The software is being tested now, and should be in effect by mid-March, he said.

The PUD plans to switch customers to smart meters, which don’t require a person to physically read the meter, starting, possibly, as soon as late 2017, district spokesman Neil Neroutsos said.

The switch will take up to five years. Once it happens, there should not be any need to estimate bills.

For example, Puget Sound Energy’s automated meters allow data to be easily collected for each monthly bill for nearly all of its 1.1 million electric customers, PSE spokeswoman Akiko Oda said. “We estimate bills only for specific circumstances such as for a stopped and reset meter, and interference with the automated reading of the meter.”

Dan Catchpole: 425-339-3454; dcatchpole@heraldnet.com; Twitter: @dcatchpole.