Just follow the money.

That motto -- a favorite of investigative reporters and law enforcement -- also happens to be the best way to understand the rapid transformation of solar. 

As the costs to build and generate solar electricity have plummeted, the amount of installed PV has skyrocketed globally. For many years, following the money trail (i.e., cost reductions) was mostly a matter of tracking falling module prices. But now that panel prices have come down so dramatically, the ongoing and escalating influx of solar also depends on finding cost reductions elsewhere. 

“Initially, our goal was to introduce solar and demonstrate its effectiveness in grid-tied systems,” said Allan Gregg, a multi-decade solar industry veteran and director of application engineering at inverter manufacturer Sungrow. “Then it was to reach grid parity, and now it’s to go below grid parity so that solar stands on its own without subsidies or tax incentives. And that is all about money; the only way to reduce costs is to increase performance and reduce installed costs and [operations and maintenance].”

Inverters will play a key role in driving necessary cost reductions and also ensuring a well-functioning distributed grid. This requires inverter manufacturers like Sungrow to innovate in ways that simultaneously reduce costs while also improving controllability and intelligence and maximizing performance.

“All of those things are becoming more and more important because of the high volumes of installations,” said Gregg. “The evolution of inverter design follows the need. Innovation has been tied to cost reductions.”

The emergence of string inverters

Allan Gregg knows all about the sort of inverter design innovations that yield cost reductions. In the early 2000s, he designed the very first 100-kilowatt central inverter to house all of its components in a single outdoor cabinet.

“If you have the inverter, the recombiner, transformer, switch gear and everything in one cabinet that was factory-tested and delivered to the work site, all you need to do is connect the combiner box to inputs and the AC to the output and you’re done,” he said. “By doing that, you reduce your costs and increase your reliability, because you control everything and don’t have all of the components installed in the field by a contract electrician.”

In the early days of his career, Gregg worked exclusively on designing and developing central inverters. He was more than a little skeptical when the first string inverters came along.

The fact that the initial string inverters had capacities of only 5 kilowatts or so and were able to handle only one or two strings of modules made them extremely expensive and impractical, he said. But string inverters have come a very long way -- thanks in large part to the work Gregg has done at Sungrow. Today, string inverters have a capacity of 60 kilowatts and are able to connect to 16 strings of modules.

And compared to central inverters, said Gregg, string inverters still have a lot of upside. 

“The evolution of these small compact inverters to become three-phase and larger capacity has led them to capture at least 50 percent of the market today. Their costs have gone down and their efficiencies have gone up, but the main thing is that installed costs and O&M costs are continuing to drop down,” he said.

“With central inverters, they’re not. They’re flat and there is much more room for innovation to reduce costs with string inverters.”

Cost-saving features

At Sungrow, those cost-reducing innovations are already apparent -- and they are advancing quickly.

To start with, Sungrow’s string inverters are both high-power and lightweight, which helps reduce installation expenses. “Other similar capacity inverters weigh 250 or 300 pounds, as opposed to Sungrow’s 150-pound inverter,” said Gregg.

This is meaningful because it eliminates the need for expensive, heavy equipment to install, replace or service the inverter, all of which helps reduce O&M costs. 

In addition, Gregg notes that it’s simple to remove just the combiner box section of Sungrow inverters, which provides flexibility and ease of service and replacement -- another area of cost saving. Furthermore, Sungrow string inverters have AC and DC disconnect switches built in.

“That means you don’t have to install separate DC or AC disconnects, and that’s a cost and labor savings,” said Gregg.

Sungrow has also incorporated a host of features into its string inverters that were once only found in central inverters, including dynamic power factor control, low-voltage and frequency ride-through and power curtailment.

“Sungrow took the lead in putting all those features in even the 30-kilowatt string inverters. So now you can put large groups of string inverters together in parallel and have higher output voltage and lower losses and lower costs,” said Gregg. “There’s no differential as far as functions between string and central inverters.”

A benefit to the grid

Importantly, many of these features can also provide a significant benefit to the grid.

One example is the role string inverters can play when the operation of a lot of pump and electric motors near a substation causes power quality to lag. If the substation is fed by a significant amount of solar power through inverters, grid operators can adjust the power factor using the inverters in order to improve the overall power quality. This can save utilities a lot of money.

“Before they learned to manipulate inverters, they would have to spend a lot of money to put huge banks of capacitors to correct the power factor,” said Gregg. “This can save them money in the long run because they can make those inverters function like a big capacitor.”

Sungrow inverters provide a benefit to the entire grid when power quality is not at its optimal unity power factor. But they also offer bonus generation at times when unity power factor is achieved. That’s because Sungrow inverters are designed with 10 percent extra capacity than their AC output rating would indicate. 

This “overload” capability is what ensures that full rated kilowatt output will be achieved when power factors are above or below unity. It’s even better news in the unity power factor sweet spot.

“If you operate at unity power factor and don’t vary from it, that means a 60-kilowatt inverter can deliver 66 kilowatts if you have enough solar array capacity to derive it,” said Gregg. “You are getting that 10 percent surplus for free.” 

System design still matters

String inverter improvements are still being made. Already on the drawing board at Sungrow is an increase in the maximum DC voltage from 1,000 to 1,500 volts. With the size and weight of the inverter remaining the same, this increased maximum voltage translates into more rated kilowatts coming out of basically the same inverter.

“If we take a 60-kilowatt inverter and increase the voltage from 1,000 to 1,500, we reduce the current and can probably get 90 kilowatts out of that same inverter,” said Gregg. “That reduces costs because you have essentially the same equipment but you’re getting more power out of it.” 

Gregg said Sungrow has an ambitious cost reduction goal. “In the future, we will be down in the 4- or 5-cent per watt range for a string inverter, which is unheard of,” he explained.

Though inverter design and innovation are essential to lowering costs and improving solar performance, capturing all of those benefits also depends on good system design and installation. To help ensure the best performance, Sungrow provides guidance in a white paper, which can be found here