More AC Power from Solar Panels

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The concept of small inverters has been around for more than a decade, but there have been technical challenges to making practical devices. "One of the biggest stumbling blocks to micro-inverter technologies in the past has been conversion efficiency," says Marv Dargatz, Enphase's senior director for systems. Enphase converted many analog parts in the circuits to digital to make the inverter smaller without sacrificing efficiency. The conversion efficiency of an individual micro-inverter is 95.5 percent, on par with efficiencies of traditional large inverters, which range from 94 to 96 percent.

Daniel Kammen, a professor of public policy specializing in energy at the University of California, Berkeley, says that the solar industry has held on to the convention of connecting solar panels in a string since the 1960s, when inverters were expensive. "It's sort of crazy that we still hook solar panels together in series," Kammen says. "You take what's now the most expensive part of the system, the solar panels, and just by the way you string them together you cut down their output."

Micro-inverters maximize the power output, but they also make the system very flexible, Kammen says. You can simply plug in more panels to your array if you need more power--"You can't do that with a traditional system," he says. "If you add more panels than the inverter can take, you'd have to go replace the second most expensive part of the system: the inverter."

Semiconductor manufacturer National Semiconductor is taking a different approach to managing the power from a PV system. The company has made a power optimizer device for individual solar panels. The device only has the logic circuit for optimizing current and voltage levels--it doesn't do the DC-to-AC power conversion. Ralf Muenster, vice president of renewable energy at National Semiconductor, says that the company was also considering making micro-inverters. Instead, it chose to make the power optimizer because it can work with a wider range of voltages than inverters can.

But the micro-inverters might have one added advantage. Because they get AC power out of a solar panel, they essentially turn each panel into a separate power source. Enphase's micro-inverters also send data over the Internet to the company's servers. Users can monitor their installation online, look at how much power each makes, and control where the power from each solar panel is consumed. "You might keep some of the panels for selling power and others to power your freezer," Kammen says.