Understanding your inverter's efficiency loss
Why two efficiency numbers exist on every datasheet
Peak efficiency is the best number an inverter ever hits, usually measured near full rated load under controlled lab conditions. Real solar arrays rarely run at full output. Cloud cover, morning and evening sun angles, and seasonal variation mean an inverter spends most of its operating hours somewhere below peak load. The California Energy Commission developed a weighted testing method, now used industry-wide as the CEC weighted efficiency rating, specifically to reflect that real mix of load levels instead of just the best-case number. The Department of Energy's overview of inverter and grid services covers this conversion behavior in more general terms.
What a percentage point of efficiency actually costs
The gap between a 98.5% peak rating and a 97% real-world CEC rating looks small on paper, but it adds up. On an array producing 11,500 kWh of DC energy a year, that 1.5 percentage point gap works out to about 172 kWh a year, enough to run a typical refrigerator for close to two months. Over a 25-year system life, that gap compounds into a meaningful amount of energy that never shows up on your utility bill credit.
How to compare inverters fairly
When shopping between inverter brands, always compare CEC weighted efficiency numbers against each other, not one brand's peak spec against another's CEC spec. Some manufacturers advertise peak efficiency prominently because it's the larger number, while CEC weighted efficiency sits deeper in the datasheet. The inverter efficiency curve comparison tool lines up multiple models on the same basis, and NREL's PVWatts calculator can model the resulting production difference in more technical detail.
Efficiency loss versus clipping loss
Efficiency loss and clipping loss are two separate things that both reduce your system's real output. Efficiency loss is the inverter's own conversion overhead, present at every load level. Clipping loss happens only when DC input exceeds what the inverter can output, tied to your DC to AC ratio choice. The solar inverter size calculator and clipping loss estimator handle that second factor.
Temperature and efficiency
Inverter efficiency also drops somewhat as internal component temperature rises, which matters most in hot climates or poorly ventilated mounting locations like an uninsulated garage in Phoenix or a direct-sun wall in Las Vegas. The inverter noise and heat derate calculator covers that thermal effect separately from the baseline efficiency loss calculated here. Cooler climates tend to shift the number in the opposite direction, which the cold climate efficiency gain calculator covers on its own.