TenKsolar solar arrays are designed to work in snowy conditions — but that wasn’t the problem with panels that saw their output reduced this summer. (The Minnesota Star Tribune)

TenKsolar, a solar panel manufacturer based in Bloomington, says it will reprogram or replace a failed component on about 100 recently installed solar power arrays mostly at Minnesota homes.

The failure has curtailed solar panels' output in late summer — usually a time of high energy production — and reduces the utility-funded incentive payment to homeowners for buying Minnesota-made panels. The incentive is paid over 10 years based on power production.

As TenKsolar fixes the problem, the state Commerce Department, which administers the Made in Minnesota solar subsidy, is studying whether anything can be done to address homeowners' expected shortfall. Some solar homeowners could lose hundreds of dollars in production payments.

"We noticed a sudden drop in energy production," said Linnea Tani, who with her husband, Doug, had TenKsolar panels installed at their Roseville home in May.

She said that output from their 24 panels was off 40 to 50 percent in August compared with July. After complaining to her installer and others, the couple waited for weeks for answers. On Oct. 18, TenKsolar told solar installers that it would reprogram or replace problem units.

"It's been very disheartening," said Tani, whose system is one slated for reprogramming.

The problem centers on devices called micro-inverters that typically are placed on the back of each panel. One of their main jobs is to convert electricity from direct to alternating current. TenKsolar uses an innovative design that combines multiple micro-inverters so that fewer are needed for each solar project. TenKsolar calls its unit a Redundant Inverter Bus.

APsystems of Seattle custom manufactured the micro-inverters that failed or caused breakers to trip in the TenKsolar bus units. In a statement, APsystems said "the failure resulted from an unforeseen software interaction" between its technology and TenKsolar's. "We have worked closely with TenK to determine the root cause," Jason Higginson, APsystems senior director of marketing, said in an e-mail.

TenKsolar Chief Executive Joel Cannon said the problem was discovered in July, and it took weeks to investigate. He said TenKsolar will pay the original installer or use its own contract installation team to reprogram or replace problem units over the next few weeks.

Most of the affected installations are residential, but a few are commercial, he said. TenKsolar mostly sells solar panels for commercial and institutional projects because the company offers an integrated rack for flat rooftops or ground mounting. Cannon said the company's residential sales are almost entirely in Minnesota thanks to the incentive program.

In its replacement plan notice, TenKsolar said only micro-inverters in two serial number ranges need reprogramming or replacement. Even in the range where failures occurred, APsystems said not all units have failed or are failing. No micro-inverters sold to other solar companies have experienced the problem, APsystems said.

Jon Kramer, chief executive of Sundial Solar of Edina, said four to six of his customers have TenKsolar panels with problems. Installers like him have fielded most of the customer complaints.

"Although there might be frustration over how long it has taken, they have been able to identify the problem and resolve it with the manufacturer of the micro-inverter," Kramer said. "It seems to me they are stepping up to the plate."

Bill Grant, deputy commissioner of energy and telecommunications at the Commerce Department, said officials are studying whether the Made in Minnesota incentives can be adjusted to a new start date after problems are corrected and the systems are recommissioned.

"We obviously would like to help people out if we can and the statute allows it," he said in an interview.

TenKsolar employs more than 90 workers in its Bloomington factory and headquarters. Cannon said the company expects sales of $40 million in 2015 and $100 million next year. About 95 percent of sales are out of state, he added.

David Shaffer • 612-673-7090 Twitter: @ShafferStrib