Solar inverter efficiency: peak, European and CEC-weighted explained

Inverters are efficient machines: Fraunhofer ISE puts the efficiency of new PV inverters at around 98% ¹. Yet datasheets quote several different efficiency figures that cannot be compared directly. This guide explains what each one measures and which to use for a UK system.

Three figures, three questions

How the weighting works

An inverter rarely runs at full power. In the morning, evening and under cloud it works at part load, where efficiency is lower. Weighted figures therefore add up the efficiency at several load points, each multiplied by a weight reflecting how often that load occurs ²:

European efficiency puts almost half its weight on 50% load and also counts the 5% and 10% points. CEC efficiency puts over half on 75% load ². The CEC protocol also measures at minimum, nominal and maximum DC voltage and averages all three ³. With the UK’s cloud cover and long part-load hours, the European figure is the better guide.

When comparing: only put like against like. Never compare one model’s European efficiency with another’s CEC or peak figure.

What our database shows

For all 86 inverters in our database we hold both peak and European efficiency from manufacturer datasheets:

• Peak efficiency: 97.0% to 98.6%, median 98.2%.
• European efficiency: 96.4% to 98.2%, median 97.35%.
• Gap between the two: median 0.6 percentage points, but 1.7 points on one model. Judge on peak efficiency alone and you can draw the wrong conclusion.
• 40 models (47%) reach 97.5% European efficiency or more; 15 (17%) reach 98% or more.

Larger inverters score better on average:

The datasheets we use for these models do not state CEC efficiency; European datasheets usually give only the European figure.

What it means in kWh

One percentage point of efficiency is roughly one percent of annual output. On a system producing 3,500 kWh a year, the difference between 96.5% and 97.5% European efficiency is about 35 kWh a year. That adds up over the inverter’s life, but other factors usually matter more:

• Sizing: an inverter that spends most of its time at very low load, or clips a lot (see sizing a solar inverter);
• Shade and string layout (see string vs micro vs hybrid);
• Downtime nobody notices (see inverter lifespan, warranty and monitoring).

MPPT efficiency: the figure often missing

Conversion efficiency tells you how well the inverter turns DC into AC. MPPT efficiency tells you how well it finds and follows the panels’ optimum operating point, including when sunlight changes quickly. The European standard EN 50530 describes measurements of static and dynamic MPPT behaviour and combines them with conversion efficiency into an overall figure ². Many datasheets give no MPPT efficiency, or only a single rounded number.

Hybrid inverters and batteries

In a hybrid inverter, energy also flows from the panels into the battery and from the battery into the home. The European efficiency on the datasheet only describes the path from panels to grid. Some makers quote separate figures for the battery path; compare those only with the same figure for other models.

In short

1. Use European efficiency to compare models.
2. A few tenths of a percent is small; sizing, shade and monitoring matter more.
3. Compare European efficiency, MPPTs and power side by side in our inverter comparison.