Why Does My Solar System Generate Less Than the Installer Estimated?
Six causes account for most Singapore solar system underperformance, in order of likelihood: new shading, soiled panels, inverter clipping, monitoring app misreading, micro-cracking from poor installation, and irradiance variance from the estimate year. Most are diagnosable in under 30 minutes.
Key Takeaways
- 1
New shading from tree growth or a neighbour's extension is the most common cause of unexpected underperformance in Singapore — not panel degradation
- 2
Soiled panels reduce output by 5 to 20% in Singapore's dusty urban environment — cleaning twice a year restores full performance within minutes
- 3
A reading discrepancy (monitoring app showing DC generation instead of AC export) is the most overlooked cause of the 'underperformance' that turns out to be normal performance
Your installer said the 10kWp system would generate around 11,000 kWh per year. After six months, you are tracking around 9,200 kWh on an annualised basis. That is a 16% gap. Is this normal? Is something wrong? Before you call the installer, these six causes account for almost every case of Singapore solar underperformance, in order of how frequently they appear.
Cause 1: New Shading (Most Common)
The site survey your installer performed captured the shading profile of your roof on the day they visited. That profile changes over time. A young tree planted two years ago may now be large enough to cast a shadow across one string of panels in the early morning. A neighbour's new extension may block an afternoon sun path that was unobstructed at survey time. Even a telecommunications cable box installation on a nearby pole can create a new shade line.
Shade on even one panel in a string can reduce the entire string's output significantly with a standard string inverter (shading effect on one panel pulls down the whole string). Check your roof from a position where you can see the panels at 9am, 12pm, and 4pm on a sunny day, any shadow on a panel tells you the cause immediately. The fix depends on whether the shading source is yours to address (trim the tree) or external (discuss with neighbour or check if a power optimiser or microinverter retrofit would isolate the affected panel).
Cause 2: Soiled Panels
Singapore's urban environment deposits road dust, construction particulate, and bird droppings on panel surfaces. Each contaminant reduces the light reaching the silicon cells. Moderate soiling reduces output by 5 to 10%. Heavy soiling (construction dust, significant bird activity near nesting sites, proximity to the Pan-Island Expressway) can reduce output by 15 to 20%.
Singapore's rainfall does a reasonable job of rinsing flat surfaces, but panels tilted at the shallow Singapore angles (5 to 15 degrees) accumulate deposits at the lower rail edge that rain does not clear. A physical clean, hosing with clean water and a soft brush, never abrasive, takes 30 minutes for a 10kWp roof and typically recovers the full soiling loss immediately. If your panels have not been cleaned in over three months, do this before concluding that something is broken.
Cause 3: Inverter Clipping
When the panels can produce more power than the inverter's rated AC output, the inverter clips, it caps generation at its rated limit rather than passing through the full panel output. This is intentional (inverters are sized to their rated capacity) but should not affect the annualised yield significantly. Where clipping becomes a problem is when a system was sold with more panel capacity than the inverter can handle, so clipping occurs regularly during peak sun hours in the morning.
Check your inverter monitoring logs for the maximum power output recorded. If it consistently caps at exactly the inverter's rated kW output rather than ever exceeding it, clipping is occurring. Your installer should have sized the system to avoid material clipping. If it is significant, the fix is either a larger inverter or reducing panel capacity in the underperforming string.
Cause 4: Monitoring App Showing DC Instead of AC Generation
This is the most overlooked cause of apparent underperformance that turns out to be accurate performance read incorrectly. Some inverter monitoring apps display the panel DC generation figure rather than the AC output delivered to your home and grid. DC generation is always higher than AC output because the inverter's conversion has losses (typically 4 to 6%). If your monitoring app shows 11,500 kWh but your SP Group meter shows 10,800 kWh exported plus consumed, the system is performing correctly, you were comparing the wrong figure.
Verify this by cross-referencing your inverter monitoring figure against your SP Group bi-directional meter reading over the same period. The meter is ground truth.
Causes 5 and 6: Micro-Cracking and Irradiance Variance
Micro-cracking occurs when panels are mishandled during transport or installation, internal cell cracks that are invisible to the eye but reduce current output. This typically manifests as one or two panels performing noticeably below the others when measured individually. An installer can identify micro-cracking with a thermal camera or EL (electroluminescence) imaging. If found, the panel should be replaced under the manufacturer's product warranty, which typically covers this defect.
Irradiance variance is the simplest explanation: Singapore's actual solar resource varies ±5 to 8% year to year depending on El Niño and La Niña cycles, monsoon intensity, and dust haze from regional forest fires. Your installer's estimate is based on a long-run average. A year with more cloud than average will produce less than projected. This is normal and not a system fault, compare your generation against a 3-year rolling average rather than the first-year estimate.
Before calling your installer about underperformance, clean your panels. In approximately half of Singapore cases, the entire gap closes within 30 minutes and a garden hose.
If you have worked through all six causes and the system is still materially underperforming, request a formal yield assessment from your installer, they should provide one within their workmanship warranty period at no charge. See expected generation for your system size here.
Further reading: EMA performance standards for solar PV systems · NEA particulate matter and haze monitoring.
What is a normal year-one vs projected generation gap for a Singapore solar system?
A gap of ±10% from the installer's projected annual generation is considered normal variance for a Singapore system. Generation in the first six months may also be affected by the seasonality of the installation date, a system commissioned in November (northeast monsoon) will have a lower first-half generation than one commissioned in February (drier period). Compare your annualised generation against the projections on a full 12-month basis before drawing conclusions. If the gap exceeds 15% on a full-year annualised basis, begin the diagnostic sequence starting with shading and soiling.
How do I tell if my inverter monitoring app is showing DC or AC generation?
Cross-reference the cumulative kWh figure on your inverter app against your SP Group bi-directional meter's export reading plus your household consumption over the same period. Your SP Group meter records actual kWh flows, it is the authoritative figure for billing purposes. If your inverter app shows a consistently higher number (5 to 8% more), it is displaying DC generation before inverter losses. If the app and meter align within a small margin, the app is showing AC output. Contact your inverter manufacturer's support or your installer to clarify which figure the app displays if you are unsure.
See your numbers
What does this mean for your home?
Tariffs and technology change the math. The calculator uses current SP figures to show your actual payback and savings.
