The return from investments in PV systems in the main PV markets depends mostly on the electrical energy exported to the grid (kWh). Efficient system operation is generally assured by assessment of performance indicators, such as performance ratios (PR) that are calculated from monitored system performance data. Low site-specific PRs indicate operational problems that might result in a loss of profit or even a warranty case if a PR was previously guaranteed and this paper establishes the boundaries in which sub-optimal performance can be identified. This is achieved by calculating the uncertainties of the required measurements. It is shown that the uncertainty of performance indicators and thus the boundary at which sub-optimal performance can be identified varies with the operating environment and instrumentation set-ups. This study has investigated those boundaries for three measurement systems, three locations and two module types. It is shown that the uncertainties in the annual PR can be as high as 4.5% for Northern Europe, 50% higher than for a Southern European site. It is also shown that the determination of low system performance based on power measurements is extremely difficult and riddled with significant uncertainties, while the evaluation based on energy is much more robust. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.