Yield losses: how to distinguish power limiting from a technical fault

On a sunny day, the generation curve may reach a certain level and maintain an almost constant power output for several hours. This plateau does not in itself indicate a fault. It may be caused by the inverter’s AC power limit, plant control settings or the grid connection’s export limit. It is important for the owner to establish whether the limit is consistent with the design and grid connection conditions.

A distinction must also be made between energy generated and energy exported to the grid. At a site with self-consumption, lower exports do not mean lower generation. Before assessing losses, check the measurement points and whether the accounting includes self-consumption and, where present, the operation of the energy storage system.

Inverter clipping occurs when the available DC power from the panels exceeds the inverter’s conversion capability under the prevailing conditions. This may result from a deliberate design choice rather than inadequate maintenance. Export limiting, by contrast, is implemented by a control system that adjusts generation to the permitted grid export power and the site’s consumption.

Power reductions caused by grid voltage conditions or elevated inverter temperatures must be assessed separately. In these cases, event logs, voltage and temperature data, and active control signals are essential. Generation curves with similar shapes can have different causes, so a visual review of the graph alone is not sufficient.

For the selected period, compile inverter power output, grid connection meter data, site consumption, power setpoints and fault messages. Where available, include plane-of-array irradiance measurements and module temperatures. Data must have synchronised timestamps and sufficient resolution, as hourly averages can conceal short-term limiting.

Compare actual operation with the system design, inverter parameters and permitted control settings. IEC 62446-1 inspections and handover documentation can provide an initial baseline for assessing the system’s electrical condition, but they alone do not determine the energy lost through power limiting. Electrical testing and changes to settings must be entrusted to qualified specialists; protection functions must not be disabled without authorisation.

Lost energy is estimated as the difference between a soundly modelled generation baseline without the specific limit and actual generation over the relevant intervals. The model must account for weather conditions, system configuration and data quality. Present the result as an estimate with stated assumptions, rather than as a directly measured loss.

Where several limits apply simultaneously, their individual estimates must not simply be added together. For example, removing panel soiling during an interval when the system has already reached its export limit may not increase the energy exported to the grid. The report should distinguish between limits built into the design, external control limits and technically remediable faults.

Preventive maintenance helps keep cooling systems, connections and other system components in working order, but it does not remove the grid connection’s power limit. If inspections identify soiling, choose a cleaning method that complies with the manufacturer’s requirements; filtered water for panel cleaning is part of the cleaning solution, not a means of overcoming control limits. Inappropriate settings, meanwhile, should be reviewed with the responsible specialists and, where necessary, the grid operator.

pvcare.eu can help assess operational data and plan maintenance tailored to the cause in accordance with LVS EN IEC 62446-2:2020. The company provides robotic panel cleaning, site mowing, thermal drone inspections with reports available on dronepv.eu, as well as monitoring and O&M contracts.