Petaluma-based Enphase Energy, Inc. (NASDAQ:ENPH), a supplier of solar microinverters, announced today that Paul Nahi is stepping down as president and CEO.

Nahi’s final day with Enphase is today, the company’s announcement stated.

“However, he will continue to assist Enphase as it transitions to a new leader. The company’s board of directors has begun a search for a replacement that includes both internal and external candidates, with the intention to name a successor by August 31, 2017," the company said.

Management of the energy-device manufacturer based in Petaluma will be in the hands of an Office of the CEO, consisting of Bert Garcia, CFO, and Badri Kothandaraman, COO, the company said.

The company makes micro-inverters that attach to individual solar panels to change direct current into alternating current — the kind used in households and businesses for lighting, computing and manufacturing.

“As the Company’s first and only CEO, Paul has led Enphase from pioneering the world’s first micro-inverter to becoming a leading global provider of energy management solutions. We appreciate his many contributions and wish him continued success,” said Steve Gomo, lead independent director of Enphase Energy’s board of directors.

Enphase Energy's stock price has seen a precipitous fall since September 2014, when it soared to $16.85 a share. On Tuesday, the day of Nahi's resignation, the stock was at 89 cents.

The North Bay Business Journal published an in-depth story about Enphase Energy in February 2017. At that time the stock traded near $1.85. Since then the stock price has dropped by half. In January 2017, Enphase cut its workforce by about 18 percent, roughly 75 jobs. "These actions are necessary to create a near-term path to sustained profitability," Nahi said then.

In January 2017, Enphase Energy received an investment boost of $10 million from John Doerr, chairman of Kleiner Perkins Caulfield & Byers, and T.J. Rodgers, founder and former CEO of Cypress Semiconductor. Rodgers was added to Enphase's board of directors. At the time, the $10 million in shares were purchased for about $1 a share.

To diversify beyond micro-inverters, Enphase Energy started producing battery storage units. The new storage units, touted as “AC batteries,” rely on lithium-iron-phosphate cells.

Like other batteries, they store electricity as direct current. The 1.2 kilowatt-hour batteries sell for about $2,200, weigh 53 pounds and can be discharged up to about 7,300 cycles and down to zero percent of capacity without damage. Modular design allows expansion to optimize energy use in relation to system cost.

Four batteries might allow a typical household to store energy needed for one day of rain or clouds with minimal solar power. To get through a rainy week of no sun without grid electricity would require roughly 20 batteries — prohibitively expensive. Under typical use, the batteries last about 10 years.

Raghu Belur, engineer and co-founder of Enphase, worked at Cerent, an optical-equipment telecommunications company in Petaluma acquired by Cisco Systems in 1999 for $7.2 billion. Years ago an efficient inverter in off-grid systems would lose 10 percent of the power as it changed from DC to AC, Belur said. Enphase boosted that inverter efficiency to nearly 97 percent.

Having each panel with its own inverter and going immediately to AC increases fire safety and allows for failure of one panel to not have a big effect on the whole system.

“You get huge advantages by keeping them separate,” Belur said, and for inverting to AC.

Each panel and battery functions as an Internet-of-things appliance, communicating power-generation and temperature statistics to the cloud. “It participates in the resiliency of the grid.” The system can optimize use, drawing from the battery during periods when grid electricity is expensive, drawing from the grid when that source is cheaper.

Because solar panels last about 25 years, the inverters need a comparable lifespan. Attached to the back of panels, the microinverters encounter severe temperature, humidity and wind stresses.

Labs send new versions of the micro-inverter through extremes of temperature as well as high-humidity and ultraviolet radiation. The inverters sit an inch or so beneath the back surface of solar panels on the roof. The inverter casing is filled with a substance that conducts heat away from sensitive electronics and keeps moisture out. The outside of the casing has angled surfaces to reduce ice buildup.

“The failure rate has to be very low. We put these devices under 15 feet of water,” simulated with a pressure cooker, he said. The units are operated at minus 60 degrees Celsius (minus 76 Fahrenheit) to 100 Celsius (212 Fahrenheit), with antifreeze added to the liquid. The testing goes for 21 days while cycling full power.

“You can’t test them for 25 years. You’re accelerating” stresses to simulate the long term, Belur said.

An app on his phone shows him how solar panels on his roof meet or fall short of energy demands of his home. If there’s a net energy deficit, he or his wife might decide not to run the washing machine until later or they might reduce the winter thermostat setting by a couple of degrees.

Traditional inverters are called “string inverters.” Photovoltaic modules are connected in series then electricity is sent through the inverter to power a home or business. A malfunctioning module — covered by dirt or leaves — can diminish performance of the whole string and the entire system suffers.

In resigning from his post as CEO, Nahi said in a statement: “It has been an enormous privilege to lead Enphase since inception and through its growth to become a leading global energy technology company. Our invention of the microinverter and the introduction of module-level data monitoring has transformed the global solar energy landscape. Having managed Enphase from a concept through global leadership, I feel the time is right for a new CEO to continue its growth, while Enphase increases market share, expands into new geographies and explores new opportunities," Nahi said.

“As Enphase continues into its second decade, it is poised for a future of sustained profitability, having successfully embraced the challenges of managing operating expenses while accelerating its investments in next-generation technologies. I remain passionate about Enphase’s bright future, and I am confident its best days are yet to come.”

In May, the company reported that first-quarter revenue was expected between $60 million and $65 million, but it brought in only $54.8 million. The company attributed the weak performance to rain.

James Dunn covers technology, biotech, law, the food industry, and banking and finance. Reach him at: james.dunn@busjrnl.com or 707-521-4257