More AC Power from Solar Panels

Startup Enphase Energy is shrinking DC-to-AC inverters to extract more juice from solar panels.

There's more to solar power than blue glassy panels shimmering on rooftops. Just as important are the inverters that convert DC power created by the solar panels into grid-ready AC power. Typically, all the panels in a rooftop PV system are connected to one large inverter mounted on the side of a house.

Behind the scene: Small micro-inverters, such as the one shown above, can be attached behind each solar panel to convert DC power into AC and improve the system’s overall efficiency. Credit: Enphase Energy

Startup Enphase Energy of Petaluma, CA, is now making the first micro-inverters. These smaller inverters can be bolted to the racking under each solar panel, to convert DC power into AC for each panel individually. The company claims that the devices will increase a PV system's efficiency by 5 to 25 percent and decrease the cost of solar power.

Enphase has raised more than $20 million in its latest round of funding. The company has teamed up with various distributors and partners, including solar-module manufacturer Suntech Power Holdings and installer Akeena Solar, to bring its device to customers. The micro-inverters could be used on residential, commercial, or even utility-scale PV systems, says Todd Wilson, a general partner at RockPort Capital Partners, one of the leading investors in Enphase's technology.

In addition to DC-to-AC conversion, inverters are in charge of getting the most power from solar modules. They have a logic circuit that constantly searches for the best voltage and current levels at which the panels can operate. (Power is the product of voltage and current.)

Story continues below

In a conventional PV system, solar panels are wired together in series, and their combined high-voltage DC power is fed to an inverter. Therefore, the inverter's logic circuit optimizes the total current and voltage levels. But if one panel's current drops, it limits the overall output of the system. "Something as simple as a leaf blowing over a module, or dust or debris or shade on one module, will affect the entire array of all those modules that are connected in series," says Leesa Lee, director of marketing at Enphase.

Micro-inverters optimize the voltage-current levels at each panel individually. This squeezes the most power from each panel and then adds it together, increasing the system's efficiency. "Any impact on a module is limited to that module alone," Lee says. In addition, the equipment cost for micro-inverters is about 15 percent less than the cost for a traditional system, she says, because expensive DC components, such as signal combiners and disconnects, can be replaced with off-the-shelf AC parts.