A solar inverter’s efficiency is the share of the panels’ DC power that comes out the other side as usable AC power, and one home inverter on the market converts 99.2% of it under test conditions, per its maker’s own spec sheet. That headline number isn’t the one that predicts your power bill.
The test protocol California adopted from Sandia National Laboratories produces a second, lower “weighted” number built from how an inverter performs across a full day. How hot the box runs and how the array is sized against it move the real number even further from that headline figure.
What the percentage on a spec sheet measures
Efficiency here means one specific ratio. Inverter efficiency is the ratio of AC output power to DC input power, the way Sandia National Laboratories defines it in the test method the industry uses. Whatever DC power doesn’t come out the other side as AC left the box as heat instead of current. That’s the gap between the sunlight hitting the panels on your roof and the power your meter counts.
Inside the box, transistors switch the incoming DC current’s direction back and forth rapidly, and a filter smooths that switching into the clean 60-hertz wave a refrigerator or a laptop charger expects. There’s no fuel and nothing that spins. That efficiency number sits downstream of how well the panels themselves turn sunlight into DC power.
Peak efficiency and weighted efficiency are different numbers
Manufacturers advertise peak efficiency, one point on the curve. SolarEdge’s SE7600H-US home inverter lists a maximum efficiency of 99.2%, while its own CEC weighted efficiency, the figure built from performance across a full range of loads, comes in at 99%. On a microinverter the gap runs a little wider. Enphase’s IQ8 and IQ8+ models list a 97.7% peak efficiency against a 97% CEC weighted efficiency.
Both figures come from the same California Energy Commission protocol, which makes a CEC weighted rating from one brand directly comparable to a CEC weighted rating from another. If two quotes for your own roof list peak numbers a fraction of a point apart, ask which CEC weighted number backs each one. That’s the figure closer to what gets credited to your account over a year of changing sun.
What CEC weighted efficiency counts that a European rating doesn’t
CEC weighted efficiency and the older “European efficiency” number share a formula and differ in their assumptions about how much time an inverter spends at each power level. Both come from the protocol California adopted from Sandia National Laboratories in 2004, which sums an inverter’s measured efficiency at seven power levels, each multiplied by its own weighting factor.
The two weighting sets pull in different directions because they model different climates. The CEC weighting is built on irradiance and temperature data for the Southwest US, where clear, high-sun days push an inverter toward 75% of its rated output more often. The European weighting assumes cloudier, lower-sun conditions and leans instead on the 50% load point. If your town gets the kind of clear, high-sun afternoons the Southwest does, the CEC weighted number on a spec sheet is the closer match to what your own system will produce.
| Load point | CEC (Southwest US) weight | European weight |
|---|---|---|
| 5% | 0% | 3% |
| 10% | 4% | 6% |
| 20% | 5% | 13% |
| 30% | 12% | 10% |
| 50% | 21% | 48% |
| 75% | 53% | 0% |
| 100% | 5% | 20% |
Source: California Energy Commission inverter test protocol, developed by Sandia National Laboratories, Endecon Engineering and BEW Engineering, 2004.
The same inverter tested under each method can end up with two different official efficiency numbers. Neither one is wrong. They’re measuring the same box under two different assumptions about the sky.
Why arrays get built bigger than the inverter
Installers routinely wire more DC panel capacity into a system than the inverter can output in AC, a practice measured as the DC-to-AC ratio. US solar plants typically report 10% to 30% more DC capacity than AC capacity, the US Energy Information Administration found. The capacity-weighted average for large US plants moved from about 1.17 in 2010 to about 1.26 in 2016, and climbed further to a 1.33 median by 2018, Lawrence Berkeley National Laboratory reported. In that 2018 data, fixed-tilt projects ran a 1.41 median ratio against 1.31 for projects on single-axis trackers.
The tradeoff is clipping, where the inverter caps its output once the array sends more DC power than it can convert. Modeling of a fixed-tilt system in Oak Ridge, Tennessee found a 1.25 ratio cost about 1% of potential annual generation to clipping, a small price on a roof built to hold more panels than the inverter alone would use. Pushing the ratio to 2.0 cost 16% of potential generation on a fixed-tilt array and 22% on a single-axis tracker. That gap between clipped output and full potential runs alongside a plant’s capacity factor, the separate measure of how much of its rated capacity a plant delivers over a year.
Regulators don’t let this run unchecked everywhere. Australia’s Clean Energy Council requires a grid-connected inverter’s AC rating to be at least 75% of the array’s DC rating, which caps the DC-to-AC ratio at about 1.33 for systems the guideline covers.
What heat does to that rating
None of the weighted numbers above get measured at temperature extremes. Efficiency drops once the electronics get hot enough to trigger derating, where an inverter steps down its own output to protect itself. SMA’s own technical documentation puts it plainly: “the power is reduced in steps,” and “in extreme cases, the inverter will shut down completely.”
There’s no single temperature that triggers this across every model or install. It depends on the DC voltage the array runs at and how much bigger the array is than the inverter. In one of SMA’s worked examples, an inverter fed at 800 volts DC with an array 40% larger than its AC rating can deliver full rated power at any ambient temperature up to 31°C before it has to start backing off. That box is often mounted on an outside wall of your house or garage, so the same summer heat radiating off the siding is what it has to shrug off to keep production up.
Operating range is a separate promise from full-power range. SolarEdge rates its Home Hub inverter to operate from -40°C to 60°C, though surviving a temperature and delivering full rated output at that temperature are different claims, and only the second one is what a hot rooftop tests.
String inverters and microinverters trade efficiency for shade tolerance
A string inverter wires many panels into one circuit feeding a single box, and it’s the cheaper, more common setup. Shading or a fault on one panel drags down the output of the whole string, the US Department of Energy notes, because the panels in a string share one electrical path.
A microinverter sits behind each panel and converts that panel’s power on its own. Shading or damage to one panel won’t touch what the others produce, though microinverters cost more per panel than a single string inverter serving the whole array. If a chimney or a tree shades one corner of your roof for part of the day, a string inverter lets that shaded panel drag down the whole array and a microinverter doesn’t. You pay for that protection in cost per panel.
Microinverters post a slightly lower CEC weighted efficiency in the numbers above, 97% for Enphase’s IQ8 line against 99% for a comparable SolarEdge string inverter. Both ratings come from the same CEC protocol, so that two-point gap reflects a real difference between the tested products.
The industry asked for an updated test in 2018 and is still waiting
The test protocol behind both weighted efficiency numbers dates to 2004, built by Sandia National Laboratories before power optimizers and battery inverters were common on US roofs. The Solar Energy Industries Association asked the California Energy Commission in November 2018 to update the protocol with test procedures for battery inverters and power optimizers.
No revised protocol has replaced it. Every spec sheet you compare today, whether it’s a SolarEdge or an Enphase, is still scored against the same seven load points Sandia chose when it wrote the protocol, and whatever inverter ends up on your roof keeps the rating it shipped with for as long as it’s there.
