A 10 kWp Rooftop Loses Fifteen Performance-Ratio Points in Fourteen Months
A 10 kWp grid-tied array on a college workshop roof has twenty-four 415 Wp modules in two strings of twelve. In its first March the site pyranometer recorded 145 kWh/m2 and the meter logged 1,250 kWh. This March the pyranometer records 143 kWh/m2 and the meter logs 1,020 kWh. The roof faces a busy road, and a water tank six metres away is used by roosting pigeons. The last recorded cleaning was four months ago.
Visual inspection shows a dust film on every module and droppings along the top edge of two modules near the tank. One module in string B has a brown patch and a fine crack line across two cells; it sits beside the walkway where installers stepped during a cable repair eight months ago. An infrared scan at 700 W/m2 shows most modules at 44-46 C, the cracked module with one cell at 68 C, and a warm connector at the string B junction box. String currents at the same irradiance are 7.35 A on string A and 6.10 A on string B. No fuses blown, no earth fault, no inverter error log entries.
1. Calculate the performance ratio at both dates, quantify the loss, and allocate it between temperature, soiling and the module/connector fault. Justify the order in which you would investigate.
Read the worked answer
Performance ratio. PR = energy / (kWp x irradiation).
- First March: 1250 / (10 x 145) = 0.862.
- This March: 1020 / (10 x 143) = 0.713.
That is a fall of 14.9 PR points, or about 17% relative, roughly 230 kWh lost in the month.
Allocation.
- Temperature, about 3-4 points. Cell temperature is probably 8-10 K higher this March, and at 0.4%/K on a 0.86 PR that is about 0.86 x 0.04 = 3.4 points. Nothing can be repaired here, but it must not be blamed on the equipment.
- Soiling, about 4-5 points. A roadside site with pigeon activity and four months without cleaning is a classic heavy-soiling case.
- Module and connector, about 7 points. String B runs at 6.10/7.35 = 83% of string A. Since a string of twelve is limited by its worst element, one half-array losing 17% costs the whole array about 8%, i.e. around 7 PR points.
Order of investigation. Clean first, then re-measure. Soiling is free to remove, affects both strings, and contaminates every other measurement; if you test string current through a dust layer you cannot separate the module fault from the soiling, and if you replace a module before cleaning you cannot prove the repair worked. After cleaning, record PR again, then repeat string currents, then the infrared scan, then an IV curve on the suspect module. The warm connector at the string B junction box must be checked before condemning the module, because a high-resistance joint produces exactly the same low-current signature without any module damage.
2. Write the monitoring and maintenance regime that would have caught this decline within one month rather than fourteen.
Read the worked answer
The regime must produce a number every month that can be compared with a fixed baseline, and it must be cheap enough that it actually gets done.
Monthly, by the site caretaker (15 minutes).
- Read the inverter and generation meter on the same date each month and write the kWh in a register kept at the inverter.
- Read the pyranometer or download the nearest irradiation data, and compute PR with the formula energy / (kWp x irradiation).
- Visually check for droppings, dust lines, shade growth, loose conduit and water ingress.
Trigger for action. Any month whose PR falls more than 5 points below the same month last year, or below 0.75, raises a work order. In this case the alarm would have fired within two months instead of fourteen, saving roughly 1,000 kWh.
Cleaning. Every 15 days through the dry season and monthly otherwise, early morning or after sunset, water and a soft brush only, no detergent, no pressure jet, no walking on modules. Record each clean; the cost is trivial against 4-5 PR points.
Quarterly, by the technician.
- String current comparison at a fixed irradiance, logged so trends are visible.
- Torque check and thermal check of MC4 connectors and junction boxes.
- Inverter fault log download and fan filter clean.
Annually. Infrared scan of all modules and all joints under load, IV curve trace of both strings, insulation resistance test, earth continuity test, and a written report with before/after PR.
What made this case invisible. There was no register, no fixed comparison date and no threshold, so a 17% loss sat unnoticed until someone happened to look at an old commissioning report.