Solar - Inverters

solar-manufacturers-inverters - done reading, important - done reading, important

As for replacing parts, there are a few components in a solar system that could need replacement. If a panel fails outside of the manufacturer’s warranty, it costs between $150 and $200 to replace. If your system uses a centrally-located inverter to change the DC power produced by the panels into AC power that can be used by the electric grid, it might need to be replaced in 10 or 15 years at a cost of about $2,000, but many inverter manufacturers offer 10- to 12-year warranties; some even extend those warranties to 25 years. And micro inverters, the kind that attach to each individual solar panel, cost much less to replace and are usually warranted for 25 years.

What are the benefits of using micro-inverters?

We’re talking about one inverter PER SOLAR PANEL all linked in parallel, whereas a traditional string inverter configuration links several panels together in SERIES. This allows microinverters to maximize the energy harvest of each panel.

If you shade enough panels in a row, you can bring the voltage of the entire string low enough so that the inverter just stops, creating zero power output. However, if you have say, 5 panels shaded on an 8-panel array, those 3 panels just keep kicking with micro inverters, each one creating maximum power.

Furthermore, if dust, grime, or bird poop are getting on panels disproportionately, they can drag down the power of the whole string, much like stepping only a small portion of a water hose. Remember: the string inverter maximizes power output for the whole string, not each panel, so if some panels are suboptimal, so is the whole string.

If you have a bunch of different panels lying around, or you have a system installed but your buddy gave you a bunch of different type of panels that fell off a truck, you can mix and match them with micro-inverters since each are independent to their own panel. As string inverters require that all panels on a string be the same orientation, they must also be the same type.

No Single Point of Failure.

Because it’s panel by panel, monitoring system sophistication is really powerful. Not only does the software know your system as a whole is underperforming, but which panels are the culprits. The monitoring system is smart enough to know that power depression on all panels is probably a cloud, whereas if one panel is blinking on and off you’ve got an issue. You currently can’t get this type of information with string inverter monitoring. There also is some other cool whizbang stuff like getting text messages if a panel is out. Your installer gets this info too, so as soon as you know there is a problem, so does the installer. The “EMU” is smart as hell and just plugs into the wall, grabbing RF signals that the Enphase modules throw out. No need to connect it directly to the system!

Orientation. Let’s say you have a small roof with a west and south face. You can fit 10 panels on the south face and 5 panels on the west face. String inverters require all panels on a string to be the same orientation and tilt, so a string of 5 and a string of 10 is probably not a possible configuration, and splitting them up 8 and 7 won’t work because of this requirement. Even if you were allowed to perform these two string configurations, different string orientations need to be carefully designed and can lead to disastrous situations if not carefully looked at.

In California, inverters are all warranted for 10 years anyway, and the lifespan of a string inverter isn’t much longer than that. I could see being concerned in say, year 5, if tons of them started failing all at once and the company goes belly up. This is a legit concern.

Currently Enphase is $200 MSRP last I checked. Let’s call a 3000 Watt string inverter $1600 for simplicity’s sake. So once you go over 8 panels you’re looking are more money on the product side, but you’re weighing that against some other things like, well, the advantages listed above. The extra power output is worth money, and so is the decreased labor costs, so you have to do that calculation. On a tiny system, micro-inverters are a slam dunk in my mind.

There are other micro inverters that will be manufactured, as well as some string inverter technology that attempts DC based max power point tracking like National Semiconductor’s Solar Magic technology, which as far as I’ve heard, isn’t in the field yet. There’s something called “solar bridge” but there’s virtually nothing on the internet about it and I haven’t seen it in the field yet.

In some large, perfectly unshaded, same orientation, residential applications you could still make a case for string inverters.

How does heat impact performance of the inverters?

You can see that light hitting panels changes their max current, and heat changes their max voltage. The Max Power Point moves accordingly (Imp and Vmp). See

With a string inverter configuration, the inverter is getting several of these panels linked in series. In series, their voltages of the panels add together but the current does not. It’s the inverter’s job to try and track the maximum power point, but it can only “see” a string 0f panels, it doesn’t know which panels are doing what.

The big benefits of micro-inverters come from the fact that they can maximize this power point for EACH PANEL. So that if one panel is say dusty, faulty, aligned differently, a different type of panel, etc., it does not drag down the whole string. Additionally, conventional string inverters are limited in the configurations they can accept, and disregarding those limitations can sometimes result in zero system output at times. micro-inverters, on the other hand, are free of most of those limitations.

Due to this individualized “panel harvest,” Enphase claims you will see anywhere from a 5%-25% gain in power output when compared to a string inverter configuration. The low end of that range would assume a well engineered string configuration with panels on the same orientation and tilt, without shading.

What is MPPT?

“The hardware components and software by which the system adjusts voltage to find the PV system’s maximum power point is known as the maximum power point tracker (MPPT),” GTM Research explained in its last report on the inverter market. “Traditional PV system topologies limit maximum power point tracking to the inverter level.”

Microinverters and embedded power optimizer topologies relocate the MPPT to the level of the module. That allows monitoring of individual modules for electrical safety issues, theft, shading, and soiling. It increases system performance but adds cost. “Cost and increased performance must be balanced,” GTM Research explained.

What is a DC optimizer?

The DC optimizer is attached to the central inverter.

The DC optimizer approach beats microinverters on both cost and performance, SolarEdge North America General Manager John Berdner said. “It is extremely difficult to attach a microinverter to the laminate,” he explained. “You end up heating the cells.”

SolarEdge’s topology “is very high efficiency — 98.7 percent average efficiency and 99.5 percent peak efficiency. Whatever is not efficiency is heat — in our case, 1.3 percent average heat, and less than that under peak conditions. Microinverter solutions are in the range of 93 to 95.5, maybe 96, percent average efficiency best-case, so 4 percent of the module’s output needs to be dissipated as heat.”

Enphase PR Manager Kady Cooper rejected Berdner’s claim. “Enphase believes there is no technical reason to not have a frame-attached microinverter on a module.”

SolarBridge Marketing VP Craig Lawrence agreed. “It depends on how you mount it and how the inverter is designed,” he explained. “We mount with an air gap between our microinverter and the back sheet,” he said. “And we have designed the inverter so it directs heat away from the module.”

Lawrence said thermal chamber and rooftop data back his claim. “The impact of the microinverter on the temperature of the cells,” he said, “is not zero, but it is not a significant impact on the module’s performance.”

Losses in transmission to central inverters, Berdner said, are also “typically lower in a DC-based topology because the DC voltage is higher than the corresponding AC voltage.”

But, Lawrence pointed out, “a DC optimizer only does a part of the inverter’s function. It is not actually converting the DC to AC, which is where you lose all of your efficiency. The inefficiency still lives in the central inverter that you still need to have.”

Without a microinverter, Lawrence said, “you still have high-voltage DC on the roof. And you still have a single point of failure at the central inverter. With a microinverter, if you do have a module or inverter failure, it only impacts that particular module — it doesn’t take down the whole system.”

For the best microinverters, Berdner said, “overall wiring and conversion losses are about a half percent to 1 percent less efficient than a DC optimizer with a separate inverter.”

On cost, Berdner acknowledged, microinverters beat SolarEdge’s topology in small systems. “When you get to 50 kilowatts, we are 35 percent lower cost. For large-scale systems, we are less than half the cost of a microinverter-based solution.”

“Our solution is geared toward distributed generation, not utility-scale,” Lawrence admitted. “Central inverters are so cheap, when you are at the one-megawatt scale, the benefits you get from the microinverter aren’t enough to make up for that cost differential.”

That was Berdner’s point. “If you buy one microinverter, the cost is X. If you buy 1,000, the cost is 1,000X. Fundamentally, the costs are linear.” With the SolarEdge topology, central inverters do more cost-effective service in larger systems. Also, “you eliminate the added cost for mounting an external device and you eliminate the cost of redundancies in the system like cables and connectors.”

Enphase Energy Commercial Product Manager Gene Choi disagreed. “If we compare on a strict inverter-to-inverter pricing scenario, this seems valid. However, if we look at the total cost of ownership of the PV system, microinverters come out on top for larger systems. We see installs in the hundreds of kilowatts because of the value microinverters bring to the total cost of ownership.”

“I think you will see DC optimizers getting used in very large systems,” Lawrence said, but “costs of microinverters are dropping, and the power rating is going up. Right now you do get a scale cost advantage on very large systems. Microinverters have only been around for about three years and they now account for over 30 percent of all U.S. residential installations. It is a better solution for residential rooftop systems.”

Leading inverter makers and traditional PV system topologies continue to dominate the marketplace, according to GTM Research, because established companies like SMA and Power-One (ETR:S92) (NASDAQ:PWER) are seen as more bankable due to their reliability records and existing distribution channels.

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