Nowadays, the electronic converter which connects the photovoltaic array (PV) with the utility grid is subject to multiple requirements in terms of energy efficiency, power quality and grid stability. The paper focuses on a comparison among grid-connected Power Conditioning Units (PCUs) with different sizes, technologies and PV system architectures. In particular, the comparison includes the following items: single-phase and three-phase systems; with low-frequency or high-frequency transformers and transformerless version; with MOSFETs and IGBTs transistors. The comparison, based on experimental tests in which the signals of voltage, current and power are sampled at fast rate and with low uncertainty, is performed thanks to the normalisation of the input/output powers with respect to the power ratings. A set of meaningful parameters permits to characterise the behaviour of the PCUs which are studied both in field operation and in laboratory with known and constant test conditions. Regarding the PV systems in field operation, five plants have been analysed with PCUs from 230 kW down to 3 kW. The PCUs under study in laboratory include 1-kW inverter, one string-inverter with multi-MPPT, one per string, and two module integrated inverters. The experimental results demonstrate that the different PCUs mostly have similar performance, although the single-phase PCUs exhibit lower DC-AC efficiency at partial load, whereas a three-phase PCU shows lower MPPT efficiency, when defects in the solar cells generate multiple maximum power points in the current voltage curve of the PV array. (C) 2014 Elsevier Ltd. All rights reserved.