Solar System Generating Less Than Promised? Order the Causes Before Calling Anyone
Decision details
Fits if: Owners of existing systems seeing monthly kWh fall short of promised numbers, or unsure whether their readings are normal.
Main drawback: No measured nationwide Malaysian yield distribution is held in the registry; this page anchors checks to the official example and manufacturer warranties, not market averages. Professional cleaning at RM150–RM300 per visit and inspections at RM200–RM400 are installer estimates [source] [source].
Compare: kWh/kWp per day across 3 months; shape of the sunny-day curve (short bell versus clipped); new shade (trees/buildings); soiling; per-string/per-panel deltas in the app; commissioning date versus warranty degradation.
In this guide
The check sequence: cheapest first
The most expensive mistake owners make is buying new hardware for a problem visible for free in the monitoring app. Follow this order:
- Normalize expectations. Compute kWh per day per kWp; compare the same month last year. Official starting point: TNB’s example uses 3.5 effective hours/day [source]. A 6 kWp system at 3.5 hours ≈ 21 kWh/day design average; monsoon months sit below it with nothing broken.
- Shape of the sunny-day curve. A full but short bell = reduced light/soiling. A curve clipped at certain times = moving shade; clipped flat at a fixed value = inverter clipping (a DC/AC design choice — discuss with the installer, not a fault).
- Compare panels/strings in the app. One string or a few panels persistently low on clear days: local shade, local soiling (bird nests, leaves), or a failing panel/string. Note from the NREL trial: microinverters tolerate residential shading better than strings on identical arrays [source] — string systems are more sensitive to one shaded panel.
- Compare cleaning scope, then observe. One installer publishes RM150–RM300 per visit as a professional-cleaning estimate and 1–2 visits per year as a site-dependent frequency [source]; this is not a market price or required schedule. Before accepting a quote, ask whether it covers water-only cleaning or chemical treatment, what roof access and safety are included, and whether testing or inspection is a separate line item. Compare written scopes rather than totals alone. Record comparable readings before and after, with weather and other changes; a rise after service is useful evidence to investigate, not proof that cleaning alone caused it.
- System inspection. One installer publishes RM200–RM400 as an estimate for visual, electrical and mounting checks every 1–2 years, with an inverter check included [source]. This is that installer’s budget estimate, not a required interval or market price. Ask what tests, access, written findings and follow-up work the actual quote includes. Dedicated inverter service (health check, firmware, fan cleaning) is offered ‘from RM200’ by one vendor [source] — a vendor offer, not a fixed market price.
- Only now think about replacing hardware. Panels failing below the warranty curve = a manufacturer power-warranty claim (example curve: 1% year one, 0.4%/year after, one Tier 1 model) [source]. Early inverter failure or a misconfigured string is an installation issue — go back to the workmanship agreement.
| Symptom in app/bill | Likely cause | Check | Cost |
|---|---|---|---|
| All months low since day one | Over-optimistic design assumptions; shaded/hazy site | Compute kWh/kWp/day against the 3.5-hour basis [source]; ask for the site simulation | Free |
| Seasonal dip, recovers | Monsoon/cloud | Same month across years | Free |
| Curve clipped by time of day | Moving shade (tree, building, water tank) | Match clip times to sun path; staged photos | Free |
| One string/panel persistently low | Local shade, soiling, panel failure; string sensitivity [source] | Per-panel/per-string readings; clean and compare | Cleaning RM150–RM300/visit [source] |
| Curve flat-topped at a fixed value | Inverter clipping (DC/AC ratio) | Ask the installer for the design ratio; not a fault | Free (question) |
| Sharp drop across seasons | Equipment failure; not normal degradation (0.4%/yr after year one, one Tier 1 model [source]) | System inspection RM200–RM400 [source]; inverter error codes | Moderate |
Bird droppings on the panel, or nesting under the array?
These are different jobs. Persistent material on the panel face is a cleaning question; nesting beneath the array is an inspection and possibly a bird-exclusion question. A low app reading alone does not show which one is present. Note visible conditions from the ground and compare like-period readings before paying for work; if neither a surface build-up nor signs beneath the array are apparent, there is no automatic reason to buy a guard.
If surface soiling persists, ask for a cleaning-only scope and the proposed method for your exact module. Manuals differ: Trina’s February 2025 manual for its 210 Vertex modules says failure to follow its handling and maintenance requirements can invalidate that product’s limited warranty; it is not guidance for other module models. Give the provider your panel make/model and ask the original installer or manufacturer to confirm the applicable instructions and terms.
If there is evidence of nesting below the array, ask the installer or a qualified PV provider to assess it; do not treat a surface wash as nest removal or assume a guard fits. Keep any exclusion proposal separate from cleaning and inspection. Before accepting it, get written scope and price, including access/travel, exact panel-frame and rail compatibility, attachment method, airflow/drainage and cable access, future maintenance or removal, and who handles existing nesting material. Ask the original installer and panel maker whether that exact method affects named workmanship or product cover; third-party work does not automatically void every warranty.
Three records that make your complaint strong
Before contacting the installer (or a tribunal), prepare: (1) screenshots of daily/monthly readings for three consecutive months, per-panel if available; (2) TNB bills for the same months showing import/credit; (3) roof photos and shade lines. These records make it easier to ask for a written explanation; they do not identify the cause by themselves.
When it becomes a seller-promise problem
If the numbers promoted at sale were never reached even after steps 1–5, your complaint is contractual, not technical: ask in writing for the original formula behind the promise (assumed kWh, sun-hours, self-consumption share) and set it against actual readings. The verify installer claims page owns how to test savings claims; the bill-rise page owns the bill side. Here the evidence is narrow: promised figure versus metered figure — record both.
Frequently asked questions
How many kWh per month should my system generate?
No number is guaranteed for your site, but there is an official starting point: TNB's worked example for 5 kWp domestic estimates generation = kWp × 3.5 hours × 30 days (≈ 525 kWh/month at 5 kWp), with 75% assumed self-consumed [source]. If your readings sit far below that arithmetic for months in normal season, enter the cause-check sequence.
Output dips — weather or fault?
Cloudy/monsoon months cut effective sun-hours for every system — compare the same month across years, not this month against last dry season. If the drop persists across seasons OR some panels/strings read far below others in the app, investigate shade, soiling or a fault; those readings point to checks, but do not identify the cause by themselves.
Can a little shade really cut that much?
Yes, depending on design. NREL's side-by-side trial on identical 8 kW arrays found microinverters held relative yield better than a string inverter under simulated residential shading [source]. In a string system, one shaded panel can pull down that whole string. Check for new shade (tree growth, new buildings) and whether the string layout groups shaded panels together.
Old panels just get weak over time, right?
Slowly, not sharply. Example manufacturer power-warranty curve: 1.00% degradation in year one and 0.40% per year after that over 30 years for one Tier 1 model [source]. A five-year-old system down tens of percent is not normal wear — that is a fixable issue (shade, soiling, stringing, inverter), and if panels themselves fail below the warranty curve, the manufacturer power-warranty claim is the route, model permitting.
How should I compare cleaning or inspection quotes?
Ask each provider to separate the work being quoted: panel-cleaning method and any chemicals, visual checks, electrical tests, mounting checks, roof access, travel, exclusions and the record you will receive. Compare written scopes line by line; a higher quote may include different work, but that alone does not establish better quality. Keep comparable app readings and note weather and other changes before and after service. A later output change does not by itself prove the service caused it or guarantee a recovery.