Chapter 04-03 - Reading Inverter Specifications and Efficiency

A hybrid inverter datasheet contains many fields, but only a specific group determines string design decisions and quality assessment. This document explains the critical fields, how to correctly interpret conversion efficiency, and several parameters that are commonly misread, such as THDi and the DC/AC ratio.

For: technicians reading datasheets and designing systems, sales staff comparing inverters, newcomers who need to understand specification fields.

Quick summary

On the DC side, read the number of MPPT trackers and the MPPT voltage range, maximum DC voltage, maximum input current and maximum short-circuit current per MPPT, and maximum PV power. On the AC side, read the rated and maximum output power, grid voltage, power factor, THDi, and efficiency.

Modern inverter conversion efficiency reaches approximately 97 to over 98% at its best operating point, while the weighted efficiency (which reflects real-world conditions) is a few percentage points lower. It is important to distinguish between the efficiency of the inverter stage alone and the end-to-end efficiency of the full string from PV panels to AC output.

Two figures that are frequently misunderstood are THDi and the DC/AC ratio. A THDi below 3% means the inverter exceeds the standard rather than merely meeting a mandatory threshold, because the international standard IEC 61727 requires no more than 5%. A DC/AC ratio of 1.2 to 1.5 is common industry practice, not a fixed technical standard.

DC-side specification fields

These fields govern string design decisions. The number of MPPT trackers indicates how many independent PV string groups the inverter can manage; more MPPT trackers allow strings facing different orientations or tilt angles to be handled separately. The MPPT voltage range is the band within which the inverter tracks maximum power effectively: the string’s operating voltage must remain within this range. The maximum DC voltage is an absolute limit; the string’s open-circuit voltage must never exceed it, even in cold weather.

The maximum input current and maximum short-circuit current per MPPT limit the number of parallel strings and the current rating of the panels. The maximum PV power limits the total watt-peak of panels that can be connected to the inverter. Four string design rules follow from these fields: the string operating voltage must fall within the MPPT range; the string open-circuit voltage must be below the maximum DC voltage; the string current must be below the maximum input current; and total PV power must not exceed the maximum PV power rating.

Conversion efficiency: understanding it correctly

Efficiency is the ratio of AC power output to DC power input. For example, if 6,800 W DC is fed in and 6,600 W AC comes out, efficiency is approximately 97%. The losses appear as heat in power semiconductors, inductors, capacitors, cooling fans, and control circuitry.

Modern hybrid inverters such as those from Sungrow, Deye, Luxpower, and Chisage achieve conversion efficiency of around 97 to 98% at their peak operating point. This means that for every 100 kWh of DC energy from the panels, approximately 97 to 98 kWh of usable AC energy is produced.

Two efficiency figures must be distinguished to avoid confusion. Peak efficiency is the highest value measured at one optimal load level, typically around half load. Weighted efficiency (referred to as European efficiency or CEC efficiency) is a weighted average across multiple load levels, making it much closer to real-world performance and a few percentage points below the peak. When comparing inverters, weighted efficiency is the figure that accurately reflects annual energy yield.

Two parameters that are frequently misread

THDi (total harmonic distortion of current) measures how clean the current waveform is; lower values mean cleaner power with less interference to other equipment. Datasheets typically specify THDi below 3%. The correct interpretation is that the international standard IEC 61727 requires THDi no greater than 5%, so a value below 3% means the inverter exceeds the standard: it is not a mandatory minimum threshold. Stating that an inverter must achieve below 3% to meet the standard is incorrect.

The DC/AC ratio (the ratio of installed PV power to inverter rated power) involves installing more panel capacity than the inverter’s rated output in order to capture more energy during the early morning and late afternoon. For example, pairing 8 kW of panels with a 6 kW inverter gives a ratio of approximately 1.3. The range of 1.2 to 1.5 is common industry practice, not a fixed technical standard; the optimal value depends on location, equipment, and local weather patterns.