EngiMetric Arena / Diagnose the Failure
A solar array produces 7.8 kWh instead of 11.5 kWh. Why?
Competency Fault diagnosis primary Evidence interpretation supporting Quantitative reasoning supporting
What you should take away: Compare measured yield with the design yield and use string-level current data to distinguish shading or soiling from inverter or battery limitations.
The situation
What is happening
A 2.7 kWp off-grid array is designed to produce about 11.5 kWh/day at 5 peak sun hours and 15 % derating. On a clear day it produces 7.8 kWh, about 32 % below expectation. The inverter is not clipping, the battery is accepting charge, and one string shows a lower current than the others. What is the most plausible diagnosis?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Array rating | 2.7 | kWp | Six 450 W panels. |
| Design yield | 11.5 | kWh/day | At 5 peak sun hours and 15 % derating. |
| Measured yield | 7.8 | kWh/day | About 32 % below design. |
| Inverter status | No clipping | - | Output is below the inverter limit. |
| String currents | One low | - | The other strings are normal. |
Assumptions this challenge makes
- The irradiance and temperature on the test day are close to the design conditions.
- The battery is accepting charge and is not limiting the array.
- The inverter is operating and is not clipping or derating.
Your call
Your answer
Hints
- Compare the measured 7.8 kWh with the engine's design yield. How large is the shortfall?
- String-level current is the clue. Are all strings low, or only one?
- Rule out the system-level limits first: inverter clipping and battery acceptance are both stated to be normal.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The array engine sizes six 450 W panels at 5 peak sun hours and 15 % derating, giving a design output of 11.475 kWh/day.
- The measured 7.8 kWh is about 32 % below that design value, which is too large to ignore.
- One low string with the others normal points to a localized current limitation: shading, soiling, a damaged module, a bypass diode or a connection fault.
- The inverter is not clipping and the battery is accepting charge, so the shortfall is not a system-level export limit.
- The next step is to measure irradiance and string current together, inspect the affected string, and compare module temperatures. The calculation narrows the cause; it does not prove which component failed.
What this challenge does not establish
- The design yield is a screening estimate; actual irradiance, temperature and spectrum vary.
- The diagnosis assumes the string currents were measured at the same irradiance and temperature.
- The calculation does not test modules, bypass diodes or wiring; those require electrical measurements.
- Shading can be seasonal or time-of-day dependent, so a single measurement may not capture the full loss.