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First, check whether your system is actually generating less than it should. Compare its measured production with its own expected production over the same dates, accounting for local sunlight, temperature, system age and operating conditions. A cloudy week, seasonal change or stale monitoring data can make output look low without proving a panel fault. If the gap persists—or the inverter reports a fault, equipment is visibly damaged, or wiring is suspect—contact the installer, manufacturer or a qualified solar operations and maintenance professional.
How to tell whether the shortfall is real
A solar system’s production varies with the sunlight reaching it, temperature, shading and whether the equipment is operating. One low day, a comparison with a neighbor’s array, or a utility bill by itself cannot establish that your panels are underperforming. Start with the system’s production data and a comparison for the same period.
Use PV production data, not just the utility bill
Look at the inverter or production meter’s reported generation, then check the monitoring dashboard’s latest reporting time. A dashboard that stopped receiving data is not necessarily evidence that the array stopped producing. Compare daily, monthly and historical readings, and check whether the inverter or meter shows an alert or an operating-status indicator. The U.S. Department of Energy’s Federal Energy Management Program (DOE FEMP) recommends retaining production reports and checking operational indicators as part of system monitoring.
A utility bill combines more than panel generation. Household consumption, export rules, billing-period dates and battery behavior can all affect what the bill shows. Use the PV production measurement when diagnosing how much electricity the array generated; use the bill to understand how generation, consumption and utility accounting relate.
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Compare equivalent periods and assumptions
Compare actual production with the system’s expected production over the same dates. A sound estimate accounts for local solar resource and temperature, as well as the system’s characteristics, age and known losses. DOE FEMP describes performance ratio as actual production divided by modeled production for a selected period; the modeled value should use the same measured solar-resource and temperature data. The ratio is useful only when its inputs and assumptions are understood.
Raw production figures are not directly comparable across different locations or systems. In a 2022 assessment of 75 federal PV systems, DOE FEMP used system characteristics and coincident weather data in the System Advisor Model before comparing modeled and measured generation. That approach provides a more meaningful basis for diagnosis than comparing a household’s output with a generic panel rating or another array.
When reviewing a monitoring report or installer assessment, check the dates, weather and solar-resource assumptions, whether temperature is included, whether the system was available to produce, and how age and shading or soiling were treated. A model that omits a known loss may make healthy production look deficient; one that assumes ideal operating conditions may set an unrealistic expectation.
Check these possible causes in order
1. Weather and seasonal sunlight
Cloud cover and seasonal changes in available sunlight can reduce production without indicating a hardware problem. A cloudy week should be compared with an estimate that reflects that same week’s local conditions, not with a sunny week from another month or a system’s peak rating. Temperature also matters to modeled output, so it belongs in a like-for-like assessment.
2. New or changing shade
Consider whether trees have grown, nearby construction has changed, or new roof equipment or seasonal shadows now block sunlight at different times. Partial shading can reduce energy yield. Note what is shaded and when, but treat the observation as a clue rather than proof of a specific fault; a qualified assessment can determine whether the shade explains the measured gap.
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3. Soiling, snow or a recent weather event
Visible debris or snow on accessible panel surfaces may affect production. DOE’s Solar Energy Technologies Office identifies soiling as an energy-yield factor, but the cited guidance does not prescribe a particular cleaning tool or establish that cleaning will resolve a given shortfall. Clean only if the manufacturer permits it and the surface can be reached safely. Do not climb onto a roof or use a cleaning method that could damage the equipment. If a severe weather event occurred, look for visible damage from a safe location and arrange a professional inspection when warranted.
4. Downtime, inverter alerts or electrical problems
Review inverter status, alert messages, periods when reporting or production stopped, and any recent maintenance. DOE FEMP identifies inverter issues as a major source of system downtime and maintenance, and lists module damage and wiring faults among possible failure modes. Low output alone does not identify which, if any, of these causes applies.
Do not open an inverter, disconnect wiring, bypass protective devices or repeatedly reset a fault without professional guidance. Record the alert exactly as displayed and follow the manufacturer’s instructions while arranging service if needed.
5. Gradual aging
Some performance decline is expected over time, but fleet-level figures are not a precise forecast for a particular home. DOE-published estimates describe different measures and populations:
| Figure | What it describes | How to interpret it |
|---|---|---|
| About 0.5% per year | DOE FEMP’s guidance on typical module-efficiency degradation; page accessed in 2026. | A general module-efficiency figure, not a guaranteed annual loss for every system. |
| Around 0.75% per year median performance loss | DOE’s Solar Energy Technologies Office summary of NREL PV Fleet findings; page accessed in 2026. The analysis covered nearly 19,000 inverters. | A fleet-level median, not an expected rate for every household array or a count of residential systems. |
These figures are not interchangeable: one concerns module efficiency and the other reports median performance loss across an analyzed fleet. Technology, location, installation, weather and measurement method affect the observed result. Neither figure establishes what your particular array should lose in a given year.
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A practical troubleshooting sequence
- Write down the period and production reading. Choose a date range long enough to compare meaningfully—such as a month rather than a single moment—and capture daily and monthly production along with the historical view. Note the monitoring system’s last report time.
- Check equipment status. Look at the inverter and production meter’s displayed operating status and any alert. Do not assume that a missing dashboard update means generation stopped; cross-check the meter or inverter if available.
- Compare with a weather-aware expectation. Use the system’s expected-production estimate or a suitable model for the same dates. Confirm whether its inputs reflect local solar resource, temperature, system age and known losses.
- Look for changes you can observe safely. Note new shade, snow or debris visible from a safe place, recent construction, roof changes or severe weather. Do not treat an observation as a diagnosis or attempt unsafe roof access.
- Review history and arrange service if indicated. Check for outage periods, previous alerts and maintenance records. Persistent underperformance, inverter faults, visible damage, suspected wiring problems or warranty questions are reasons to contact the original installer, equipment manufacturer or a qualified PV O&M professional.
When to call a professional—and what to prepare
Arrange qualified service if a production gap continues after accounting for weather and data issues, or if there is an equipment alert, suspected wiring fault or visible damage. Consult the warranty and manufacturer instructions before authorizing work; the appropriate service provider depends on the equipment, installation and applicable warranty terms. DOE FEMP recommends trained operations and maintenance personnel or qualified contracted services, along with records of maintenance and response procedures.
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- Dates and production readings for the affected period, plus a comparable historical view.
- Monitoring screenshots and the most recent reporting time.
- Inverter or meter status, with alert text copied exactly.
- Notes on observed shade, snow, debris, weather damage or changes near the array.
- Installation documents, warranty information, previous service records and maintenance dates.
Ask the provider to explain the evidence behind a diagnosis, how expected production was calculated, whether the system was producing during the period in question, and what work is proposed. For a repair proposal, understand the scope, qualifications, likely response timing and any warranty implications before approving work. A credible assessment should distinguish a measured production problem from a monitoring or modeling issue.
How to judge a monitoring report or repair proposal
Use the same questions whether you are reviewing your own report, an installer’s assessment or a proposed repair. A percentage or low-output label without dates and assumptions is not enough to identify the cause.
- Are the dates aligned? Actual and expected generation should cover the same interval.
- Does the expectation fit local conditions? Check the solar-resource and temperature assumptions, system characteristics and age.
- Was the system available? Separate equipment downtime from a monitoring platform that stopped reporting.
- Are known losses represented? Consider whether shading, soiling, snow or other measurable operating losses were reflected in the baseline.
- Is the proposed work supported by evidence? Ask what observation, alert or measurement points to the repair, what the work includes and how it may affect warranty coverage.
DOE FEMP’s performance-ratio approach can help frame the discussion: it compares actual generation with modeled generation for the selected period, using matching solar-resource and temperature data. The calculation does not itself identify a failed component; its value depends on the quality of the measurements and assumptions used.
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