If your solar production seems lower than you expected, the first step is to understand what normal variation looks like. Solar output naturally rises and falls with the time of day, season, weather, and even the temperature of the panels. A drop in production does not always mean something is broken. This guide explains the difference between routine fluctuations and signs of a real equipment issue, so you can decide when to call a professional. If you are new to diagnosing solar performance, start with our Home Solar Troubleshooting guide for an overview of common alerts and checks.

Short version: Most low-production days are caused by clouds, shorter winter days, or seasonal shading. Check your monitoring data and inverter lights first. If you see a persistent drop that does not match weather patterns, or if the inverter shows a fault light, contact your installer.

Normal Variation in Solar Production

Time of Day, Season and Latitude

According to the U.S. Energy Information Administration, the intensity of solar radiation at any location is greatest when the sun is at its highest apparent position in the sky, at solar noon, on clear, cloudless days. Production naturally ramps up in the morning, peaks around midday, and tapers off in the afternoon. Latitude, climate, and weather patterns are major factors that affect insolation, the amount of solar radiation received on a given surface area during a specific amount of time. The EIA notes that locations in lower latitudes and in arid climates generally receive higher amounts of insolation than other locations. Seasonal variations in solar resources increase with increasing distance from the earth’s equator, so systems farther north see a larger swing between summer and winter output. This means a lower winter bill credit is normal and not a sign of trouble.

Weather and Cloud Cover

The EIA also points out that clouds, dust, volcanic ash, and pollution in the atmosphere affect insolation levels at the surface. A run of overcast or rainy days will depress daily production figures. Comparing your system’s output against local weather records can quickly show whether a dip is weather-related. If production rebounds when the sun returns, the system is likely working as designed.

Temperature Effects

The U.S. Department of Energy explains that solar cells generally perform better at low temperatures. Higher temperatures cause the semiconductor properties to shift, resulting in a slight increase in current but a much larger decrease in voltage. Extreme increases in temperature can also damage the cell and other module materials, leading to shorter operating lifetimes. This means a rooftop array on a blistering summer afternoon may produce less power than on a bright but cool spring day, even though the sunlight is intense. The effect is temporary and part of normal operation.

Shading Changes

Shading is not always static. The EIA notes that buildings, trees, and mountains may shade a location during different times of the day in different months of the year. A tree that was small when the system was installed can grow enough to cast a shadow across several panels during part of the day. Seasonal changes in the sun’s path can also bring new shading from chimneys or neighboring structures. If your system uses microinverters or power optimizers, the U.S. Department of Energy’s design guidance states that a microinverter allows for independent operation of each panel, which is useful if some modules might be shaded. With a string inverter, however, shading on one panel can reduce the output of the entire string. Understanding your system’s design helps you interpret production patterns. For more on how roof conditions affect output, see our Roof Suitability page.

Soiling and Debris

Dust, pollen, bird droppings, and tree sap can accumulate on panels and block sunlight. The Queensland Government’s solar maintenance guidance, which is Australian but reflects universal safety principles, states that dust and debris build-up in vents can reduce a panel’s exposure to sunlight. Overhanging trees, dust and debris, and storm damage can all take their toll on solar panels. The guidance emphasizes that you should visually inspect your system from the ground, looking for excessive build-up of hard-to-remove debris like tree sap and bird droppings. It also warns that electricity can be dangerous: never attempt to repair electrical issues yourself and conduct visual inspections from the ground. Only use an appropriately qualified professional, such as a licensed electrician or an accredited installer, to perform close-quarters inspections or any work that requires going onto your roof. If you notice heavy soiling, have a professional clean the array; do not climb onto the roof yourself. Follow the maintenance schedule provided by your installer. For safe homeowner checks, visit our Maintenance page.

Gradual Degradation Over Time

All solar panels lose a small fraction of their output each year. A review by Jordan and Kurtz at the National Renewable Energy Laboratory (NREL), published through OSTI, analyzed nearly 2,000 degradation rates from field studies and found a median value of 0.5% per year. As simple arithmetic with round numbers, a 0.5% yearly decline adds up to roughly 5% over ten years, all else being equal. A slow, steady decline of this magnitude is normal. If your annual production is dropping much faster, it may point to an equipment issue.

When to Suspect a Real Equipment Problem

Check Your Monitoring Data and Inverter Indicators

The Queensland Government’s maintenance guidance recommends keeping an eye on your system’s performance so you can spot issues quickly. This includes checking the generation levels shown on your electricity bills, looking for drastic changes to your energy usage or solar feed-in tariff, checking monitoring data for decreased performance, and checking the data screens on your inverter or its online interface for error messages. Each system is different, so refer to your owner’s manual for specific instructions. Many modern systems come with a monitoring portal or app; a sudden, unexplained drop in daily or monthly production that does not recover is worth investigating. For more on reading monitoring data, see our Monitoring Systems article.

Walk outside and look at the inverter during daylight hours when the panels should be generating. The Queensland guidance treats a red or orange light during the day as a warning sign; check your manual for what each indicator and error code means on your model. The U.S. Department of Energy’s design basics note that inverters are expected to need replacement at least once in the 25-year lifetime of a PV array, so an aging inverter can be a source of low production. If your inverter shows a fault, do not attempt to open it or reset it beyond what the manual allows; contact your installer. Learn more about inverter types and behavior on our Home Inverters page.

What to Do If You Spot a Problem

If you notice any of the following, the Queensland guidance advises asking a licensed electrician, an installer accredited by Solar Accreditation Australia (or the equivalent in your region), or a New Energy Tech Consumer Code approved seller to inspect the system: a red or orange inverter light during the day, excessive hard-to-remove debris on panels, panels that look loose or damaged, cracked or damaged isolation switch or inverter cases, or wiring that appears damaged or frayed. The inspector should check that panels are clean, secure and undamaged, vents are free of debris, switches are undamaged, wiring is intact, fittings and cables are secure, isolator switches are not blocked, and emergency shutdown procedures are clearly displayed. They should also perform electrical checks and review the inverter display for recorded faults. In the United States, any work involving home wiring or the grid must be done by a licensed professional with the necessary permits and utility approval.

Quick Reference: Causes, Data Patterns and Actions

Cause How It Shows in Data What to Do
Cloudy or rainy weather Production dips on those days, recovers when clear Check local weather history; no action needed
Seasonal change Gradual rise and fall over months, lower in winter Compare to same month in previous years; normal
High temperatures Production lower than expected on very hot sunny days Normal; the DOE notes cells perform better when cooler
Shading (new or seasonal) Production drops at specific times of day or months Check for tree growth or new structures; consider trimming or microinverters
Soiling (dust, bird droppings) Gradual decline over weeks or months, especially in dry periods Inspect from ground; if heavy buildup, have a professional clean panels (never go on the roof yourself)
Panel degradation Very slow decline year over year (around 0.5%/year) Compare annual totals; if much faster, have system inspected
Inverter fault Sudden drop to zero or very low, inverter shows red/orange light or error code Contact your installer; do not attempt internal repairs
Monitoring or communication issue Data missing or flatlined but inverter appears normal Check internet connection; contact installer if persistent

Comparing Your Output to an Expected Range

The PVWatts calculator, provided by the National Laboratory of the Rockies (formerly NREL), estimates the energy production of a grid-connected PV system based on 30 years of historical weather data at your location. The tool’s output includes an expected range that is intended to provide an indication of the variation you might see from year to year. If your actual annual or monthly production consistently falls below the low end of that range, and weather or shading do not explain the gap, it may be time to have a professional inspect the system. Remember that PVWatts uses typical system assumptions and does not account for every site-specific detail, so use it as a benchmark rather than a precise prediction.

Frequently asked questions

How do I know if my solar production is low because of weather or a real problem?

Start by comparing your daily or monthly production to local weather records. If output drops on cloudy days and recovers when the sun returns, weather is the likely cause. You can also check your system’s output against the expected range from the PVWatts calculator for your location. A persistent shortfall that cannot be explained by clouds, season, or known shading is worth investigating further.

Should I clean my solar panels myself?

Not from the roof. The Queensland Government’s solar maintenance guidance states that you should conduct visual inspections from the ground and never attempt to repair electrical issues yourself. Only a qualified professional should go onto the roof or perform close-quarters inspections. If you see heavy soiling, contact a licensed electrician or accredited installer to clean the array safely.

How much production loss is normal as panels age?

According to a review by Jordan and Kurtz at NREL, the median degradation rate for flat-plate terrestrial modules is about 0.5% per year. A gradual decline of this size is expected. If your system’s output is falling significantly faster, have it inspected for faults or damage.

For more detailed troubleshooting steps, visit our Production Issues page, or explore the broader Home Solar Troubleshooting section.

Sources