Most UK warehouse solar systems installed since 2020 have been well-designed and are performing within 2-5% of PVSyst yield model. But the difference between a system running at 78% Performance Ratio and 84% PR is a 7.7% generation gap — and 7.7% of a 1 MW system's annual output is roughly 71,000 kWh, worth £15,000/yr at 22p/kWh. Getting O&M right matters financially.
What Performance Ratio tells you
Performance Ratio (PR) is the ratio of actual yield to theoretical yield under site-specific irradiance, adjusted for system losses. A healthy UK commercial PV system in 2026: PR 80-85%. Below 78%: investigate (string faults, shading, inverter underperformance, soiling). Above 87%: double-check irradiance data (sometimes an optimistic PR is a measurement artefact). PR is calculated monthly by your monitoring platform and should be reviewed quarterly.
Remote monitoring: what to actually look at
Don't just log in and look at "total generation today." The meaningful checks, weekly: (1) String-level output uniformity — any string generating >10% below the median is a fault candidate. (2) Inverter DC input vs AC output ratio (efficiency) — below 96.5% sustained suggests inverter health issue. (3) Performance Ratio trend — declining PR over 2-3 months signals a systemic issue. (4) Irradiance-adjusted daily yield vs PVSyst forecast — sudden drops vs forecast signal string or inverter fault.
Annual O&M programme: minimum standard
We recommend: Annual electrical inspection (visual + IR thermographic scan of string combiner boxes, DC cables, inverters, AC distribution); Annual module cleaning (if PR shows >2% soiling loss vs expectation, which is common in autumn-winter); Annual mechanical check (fixings, flashings, drainage, wind uplift clips — particularly important after severe weather events); Inverter health check (firmware update, capacitor health, log review). For systems above 500 kW: half-yearly for electrical + quarterly string-level review via monitoring platform.
Thermographic survey: when and why
Annual thermographic survey using FLIR or equivalent IR camera identifies: hot spots on shaded or degraded cells (potential fire risk); string isolation failures; connector corrosion (DC arc risk). Thermographic survey is increasingly required by commercial property insurers for warehouse solar above 250 kW. Our O&M contract includes annual thermographic survey as standard.
String inverter vs central inverter O&M differences
String inverters (10-15 year lifespan): each inverter serves 10-30 strings. Failure is localised — lose 3-5% generation until replaced. Replacement cost: £600-£1,500 per unit. Central inverters (15-20 year lifespan): each inverter serves the full system. Failure is catastrophic — system offline until repaired. Redundant inverter specification is available and recommended for systems above 500 kW. Replacement cost: £8,000-£25,000. SMA, Fronius, and Sungrow all have UK-based field engineering teams for 4-hour emergency response on commercial systems.
O&M SLA: what to specify
A good O&M SLA specifies: Response time (4 hours for inverter fault, 24 hours for string fault, 48 hours for soiling-only); PR guarantee (e.g. 82% annual PR guaranteed, with financial remedy for shortfall); Annual thermographic survey; Monitoring platform access 24/7 with anomaly alerting; Annual report (actual vs PVSyst yield, PR trend, degradation tracking); IWA workmanship cover coordination.
See more
Full O&M guide: /guides/warehouse-solar-om-guide/. Monitoring platforms: /guides/warehouse-solar-monitoring-platforms/. Inverter replacement guide: /guides/warehouse-solar-inverter-replacement/. Contact: /contact/.