Kent Sheldon, Power-One: Page 2 of 3

Advancing Inverter Technologies

SP: Beyond ongoing questions of reliability due to a short track record in the field, what major concerns did you encounter from the installer community?
KS: Price. Microinverters cost more than string inverters, and installers are focused on up-front cost. Micros can take longer to install, which adds to the cost. Now they have 30 on a rooftop. If one fails, it costs more to replace. The likelihood of repeat failures is higher due to more inverters per system, meaning more truck rolls. The rebate-driven incentive programs don’t reward for energy harvest, so installation cost is critical to installers who see their margins continuously eroded.

Commercial inverters are becoming so inexpensive that micros have little chance of competing anytime soon in the commercial market. The value proposition of higher energy production starts falling apart as the system size grows. You just don’t need module-level MPPT on a large flat array with no shading issues. I think it will be many years before micros are feasible for utility and large commercial projects. Or the micro will need to go through a radical redesign, making it a lot less like a micro.

SP: To what do you attribute the success micros have enjoyed?
KS: I see microinverters performing well in the residential and small commercial markets. They really make sense in places with heavy shading, mixed orientations, small systems and constrained spaces. The large influx of electricians who are entering the PV game can easily understand them. A lot of new blood doesn’t want to worry about shade, layout, orientation, 600 Vdc wiring and all the hard math of string sizing. They understand the ac domain and don’t want to bother with dc.

SP: Where do you see microinverters fitting into the big picture of the US PV industry?
KS: This is the key question and one of the main reasons I joined Power-One. All forecasts show the US market growing much faster in the utility and commercial sector than in residential. Residential will continue to grow at a good rate, but the magnitude is dwarfed compared to the other sectors. Microinverters run into a wall at the large-commercial–project size. The thought of hundreds or thousands of inverters under the array is difficult to swallow when considering a 20-year system warranty and life expectancy. What will be available for replacement if there are major failures in year 10? “Plug-n-play” doesn’t mean much if the plugs are incompatible in the future.

Utility engineers become very animated when asked how they feel about 50,000 micros on a 10 MW PV farm. They don’t want to think about controlling all these things. How will they interact with each other or respond to utility faults? How will their harmonics compound? Utility engineers can get their heads around a small number of central generators with an on/off switch. They need a major shift in their thinking before they will accept micros in large plants.

SP: What’s your current role at Power-One?
KS: Vice president of sales for renewable energy in North America (it hardly fits on my business card). I started my career working on large inverters and progressed to smaller and smaller ones. Now I’ve come back full circle to MW-size inverters.

SP: Now that you have returned to a central inverter manufacturer, do you think there is significant competition from microinverter and module optimizer manufacturers?
KS: There will certainly be some competition in the smaller system sizes, but I don’t see it happening across the board. A large number of systems just don’t need the advantages microinverters offer. The larger the system gets, the more complicated micro installations become. Communication becomes a real challenge. Module-level data is just way too much information to manage in large commercial or utility plants. Optimizers are interesting, but I could never really see the advantage versus the risk. It’s the worst of both worlds in my mind: all the potential issues of microinverters and central inverters combined in one system.

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