There are various efficiencies that go into modeling the overall production success of a solar PV system. One of the most important to understand is the inverter’s, as the muscle that makes DC-to-AC magic happen.
Power from the solar array travels through the inverter, converting DC power from the PV array into usable AC power. During this conversion, some power is lost as heat. Inverter efficiency represents the amount of power that passes through the inverter and is not lost.
Having the inverter efficiency as close as possible to 100% is obviously ideal because then all generated solar power is usable and profitable.
“You don’t want your precious solar energy turned into heat,” said Raymond Hudson, global solar segment director for DNV GL. “You want it to go to the grid or the loads.”
Peak vs. weighted efficiency
When looking at inverter efficiency, there are two important types to consider. The peak efficiency rating often comes from internal manufacturer testing, although it can come from third-party testing. It’s the highest efficiency that is captured during testing and the number manufacturers commonly advertise. Realistically, an inverter may only reach its peak efficiency for an hour each day in an actual installation.
Weighted efficiency is a blended average that is obtained by measuring inverter efficiency at different power range points and combining them together. It’s a more realistic measure of how the inverter would operate in a project under real-world conditions.
“Part of the time the inverter operates at lower powers and part of the time at higher power, and weighted efficiency weights those appropriately,” Hudson said.
Today, weighted efficiency is tested in a standardized way, but that hasn’t always been the case.
The history of inverter efficiency testing
Hudson has studied and designed inverters in renewable energy since 1990, so he remembers when inverter efficiency wasn’t standardized.
“If you go back to the ’90s and early 2000s, solar inverter efficiency was kind of the Wild West,” he said. “The manufacturers made some measurements and reported them. There really wasn’t a standard way of doing those measurements. It was hard to interpret the data.”
After California launched its solar incentives program in 2004, a formal method of measuring grid-tied inverter efficiency was finally put in place. Sandia National Labs wrote a protocol for the California Energy Commission (CEC) that included standard weightings typical for California or a sunny location. The Sandia protocol required tests be witnessed by a nationally recognized testing party, such as UL, CSA and ETL. There were also requirements for testing over the inverter’s operating power levels and at different DC voltages—both can affect efficiency—and all losses had to be measured.
“There was a little table you had to fill out, which really leveled the playing field and provided a lot of information that was good for accurately comparing products, to see how much the system would produce,” Hudson said. “That was a pretty big deal.”
Hudson also said that once the protocol was announced, inverter manufacturer started optimizing their designs to achieve a good number through the calculation. “This benefited the solar industry to have more efficient inverters. Having your efficiency visible for comparison on the internet was quite motivational,” he said.
Today CEC efficiency is listed on many inverter data sheets. The CEC also posts test results on its website, making it easy to compare data. Another weighted efficiency that is commonly used is “European” efficiency.
“It gives buyers a way to look at efficiency in an apples-to-apples comparison right on the internet,” Hudson said. “There are many, many inverters. Some are no longer produced, but the data is still there.”
How efficiency has changed
Hudson has been able to study how inverter efficiency has increased through the last two decades. In his opinion, no single type of inverter category (micro, string, central, etc.) has particularly stood out over the long term in terms of efficiency. He’s seen models in each category with high efficiency and some a bit lower. But he said efficiency has improved in each type overall.
“It’s definitely gone up,” Hudson said. “It’s risen from the low- to mid-90s, and now most are in the high-90s, even reaching 99%. Through different topologies, the industry has done a good job of increasing inverter efficiency over time.”
For example, SolarEdge’s HD-Wave inverter has a 99% CEC-weighted efficiency. The company said the inverter creates a sine wave that is very close to the final sine wave required for AC, so less energy is lost, resulting in a higher efficiency.
Hudson said many inverters are reaching their practical efficiency limits—inverters can’t be more than 100% efficient. Meanwhile, efficiency is a major focus for panel R&D—panel efficiency hovers around 20%.
Hudson’s impressed by the continued innovation. Even small increases matter.
“A 1% higher efficiency means you’re going to get close to 1% more energy delivered to the grid than you would with a lower efficiency inverter,” he said. “Even 1% is a number certainly worth considering in modeling the efficiency of a solar system and its energy output. You’re looking at the return on investment, and 1% can help quite a bit to get the returns the solar plant is looking for.”
Lior Handelsman, company founder and vice president of marketing and product strategy for SolarEdge, also stressed that inverter efficiency is directly related to the bottom line of a system.
“If a homeowner puts up a 5-kW system but the inverter is only 95% efficient, then in terms of energy production, it is only a 4.75-kW system,” he said. “Over the system’s lifetime, this lowered production will lower revenue and lengthen the payback period.”
Another advantage Handelsman noted is that as efficiency improves, less heat is lost so there’s less of a need for cooling components. This allows the inverter to be lighter, easier to install and less expensive.
When it comes to considerations for choosing an inverter, Hudson said efficiency is definitely near the top of the list—in addition to reliability, price and manufacturer capability.
“A lot of inverter efficiencies are close together in the high 90s, so efficiency can be weighted in one of the tradeoffs,” he said. “If you can get an inverter with higher efficiency for the same price from a manufacturer with similar confidence, you would. You don’t want to be burning up good solar-generated electricity.”