Performance and energy
- Performance ratio (PR)
- Measured AC energy divided by the reference energy implied by plane-of-array irradiation and installed DC capacity. The main weather-normalised indicator of solar plant performance. Read the guide →
- Temperature-corrected PR
- PR adjusted to a reference module temperature, so hot and cool periods can be compared fairly. Read the guide →
- Capacity utilisation factor (CUF)
- Energy produced divided by the energy the plant would produce running at rated capacity all the time. Stated on an AC or DC basis.
- Specific yield
- Energy produced per kWp of installed DC capacity (kWh/kWp) over a period.
- Plane-of-array (POA) irradiance
- Sunlight power per square metre falling on the tilted surface of the modules, in W/m². Summed over time it becomes POA irradiation (kWh/m²).
- Global horizontal irradiance (GHI)
- Sunlight power on a horizontal surface. Not the right input for PR, which needs POA.
- DC/AC ratio
- Installed DC module capacity divided by inverter AC capacity. Higher ratios increase clipping at midday.
- Clipping
- Energy lost when DC power exceeds the inverter's AC rating, so output is capped. Read the guide →
- Soiling loss
- Energy lost because dust or dirt on modules blocks sunlight. Read the guide →
- Loss waterfall
- A chart walking from target to actual generation one loss category at a time. Read the guide →
- P50 / P90
- Annual energy estimates that will be exceeded with 50% and 90% probability. P50 is the usual budget.
- Curtailment
- Output deliberately reduced on instruction from the grid operator or because of an export limit.
Availability and reliability
- Plant availability (PA)
- Share of time (often capacity-weighted, during generating hours) that the plant's own equipment was available. Read the guide →
- Grid availability (GA)
- Share of time the grid was available to accept the plant's output. Read the guide →
- Time-based availability
- Available time divided by total time, with agreed exclusions. Common in wind contracts. Read the guide →
- Energy-based availability
- Actual energy divided by actual plus lost energy. Reflects when downtime happened. Read the guide →
- Mean time to repair (MTTR)
- Total repair time divided by the number of repairs. Read the guide →
- Mean time between failures (MTBF)
- Total operating time divided by the number of failures. Read the guide →
- Response time
- Time from an alert being raised to someone acknowledging it. Read the guide →
- Resolution time
- Time from an alert being raised to it being closed. Read the guide →
Equipment and systems
- Inverter
- Converts DC power from modules (or batteries) into AC power for the grid.
- String
- A series of modules connected together, feeding an SCB or a string inverter.
- String combiner box (SCB)
- Combines several strings, with fuses and monitoring, before the central inverter.
- IGBT
- The power semiconductor switches inside an inverter. Their temperature is a useful health signal.
- Multi-function meter (MFM)
- Meter measuring energy, power, voltage, current and power factor at a point in the plant.
- Data logger
- Device that collects readings from inverters, meters and sensors and sends them to SCADA or the cloud. Read the guide →
- SCADA
- Supervisory control and data acquisition: the real-time supervisory and control system at a site. Read the guide →
- RMS
- Remote monitoring system: software for viewing plant data remotely, often provided by an OEM. Read the guide →
- Power curve
- A wind turbine's expected output at each wind speed. Read the guide →
- Cut-in / rated / cut-out speed
- The wind speeds at which a turbine starts generating, reaches rated power and shuts down for safety. Read the guide →
Software and maintenance
- CMS
- Centralised monitoring system: portfolio-level monitoring and performance analysis across sites and OEMs. Read the guide →
- CMMS
- Computerised maintenance management system: plans, assigns and records maintenance work. Read the guide →
- Daily generation report (DGR)
- The daily summary of generation, irradiation, performance, availability, losses and issues for each plant. Read the guide →
- Preventive maintenance (PM)
- Scheduled maintenance done to prevent failures. Read the guide →
- Corrective maintenance (CM)
- Maintenance done to fix a failure after it occurs. Read the guide →
- PM compliance (conversion rate)
- PM tasks completed divided by PM tasks scheduled in a period. Read the guide →
- Work order
- An assigned piece of maintenance work with an owner, steps, due date and record of what was done. Read the guide →
- Ticket
- A tracked issue, raised automatically from an alert or manually, followed through to closure. Read the guide →
Battery storage
- State of charge (SoC)
- How full a battery is, as a share of usable capacity. Read the guide →
- State of health (SoH)
- Remaining capacity compared with rated capacity at beginning of life. Read the guide →
- Round-trip efficiency (RTE)
- Energy discharged divided by energy charged, at a stated measurement boundary. Read the guide →
- Equivalent full cycles (EFC)
- Total energy discharged divided by usable capacity. Read the guide →
- Depth of discharge (DoD)
- How much of the usable capacity is used in a cycle. Read the guide →
- Augmentation
- Adding capacity to a storage system to offset degradation and keep meeting contracted output. Read the guide →
- BMS
- Battery management system: monitors cells and protects the battery. Read the guide →
- PCS
- Power conversion system: the inverter between the battery and the grid. Read the guide →
- EMS
- Energy management system: decides when the battery charges and discharges. Read the guide →