Understanding CEC weighted efficiency
Why one efficiency number can be misleading
Inverter datasheets often lead with a single peak efficiency figure, but that number is usually measured at the load point where the inverter performs best, frequently 50% to 75% of rated capacity. A real system rarely operates at exactly that point; it spends real time at low output in the morning and evening and on overcast days, and at or near full output around midday in summer. Comparing two inverters on peak efficiency alone can miss real differences in how they perform across that full range, and it also leaves out how ambient temperature shifts that curve, which the hot climate efficiency loss calculator covers separately.
Why the CEC method uses those specific weights
The California Energy Commission's weighting formula, now the industry-standard method referenced on most inverter datasheets, assigns 4% weight to the 10% load point, 5% to 20%, 12% to 30%, 21% to 50%, 53% to 75%, and 5% to 100%. Those weights are meant to approximate a typical annual distribution of operating points for a grid-tied solar inverter, with the heaviest weighting at 75% load reflecting how often inverters run in that range during normal daytime operation. The Department of Energy's inverter systems overview covers inverter fundamentals more generally if this weighting method is new to you.
Why small percentage differences still matter
A 1 percentage point difference in weighted efficiency sounds small, but applied against a system producing tens of thousands of kWh over its life, it adds up to a real number, the same kind of small-percentage-compounds-over-time effect seen in the inverter clipping loss estimator. It's worth calculating the actual kWh and dollar impact with the cost per kWh produced calculator rather than dismissing a percentage point gap as immaterial, especially when comparing options at a similar price point. NREL's PVWatts calculator can model that annual kWh figure at a finer level of detail than the estimate here.
What this tool doesn't cover
CEC weighted efficiency is a standardized comparison metric, not a guarantee of real-world performance, since actual operating conditions, temperature, and site-specific irradiance patterns vary. It also doesn't account for other differentiators between inverters, like reliability history, monitoring features, or warranty length, covered separately by the inverter warranty value calculator and central vs string inverter cost tool. Efficiency is one factor among several worth weighing together. The cost per kWh produced calculator turns this result into dollars and payback time.