Solar production drops every year between September and December. The sun is lower, days are shorter, and weather is worse. That is expected and it is not a fault.
The difficulty is that a genuine problem, a failed optimizer, a tripped breaker, a shaded string, looks the same on a monthly bill. Distinguishing them requires comparing the right things, and most systems give you the tools to do that if you know where to look.
Compare Against the Right Baseline
Do not compare month to month. October will be lower than July on every system in the northern hemisphere. That tells you nothing.
Compare the same month year over year. If last October produced a certain amount and this October produced substantially less under similar weather, that is a signal.
Compare against the production estimate in your original proposal, which usually gives monthly expected output. Comparing actual to modeled by month is the cleanest check available.
Compare panels or strings against each other in your monitoring app. On systems with per panel monitoring, one underperforming panel in an array where the rest match is the clearest possible diagnostic. On string inverter systems you have less resolution, but a comparison between strings still helps.
Normal panel degradation is gradual, typically a fraction of a percent per year, and it is written into the manufacturer's production warranty. A sudden drop is not degradation.
The Common Causes, Roughly in Order
Soiling. Dust, pollen, bird droppings, and in some regions ash or agricultural dust. In most climates rain handles this adequately. In dry regions, or under a tree, or at a low tilt where water does not sheet off, buildup is measurable. Cleaning is generally low risk if done from the ground with a soft brush and water, and it is worth paying for rather than walking on a roof.
Shading. Trees grow. A system designed around a tree line five years ago may be shaded now, and partial shade on one panel can affect more than that panel depending on the system architecture. Check what the array looks like at midmorning and midafternoon rather than at noon.
Leaves and debris collecting at the lower edge of the array or in the gap between panels and roof.
Snow. Panels shed snow as they warm, and clearing them is generally not worth the risk. Do not use anything abrasive or metal.
A tripped breaker or disconnect. More common than people expect, and the first thing to check when production goes to zero rather than merely low.
Inverter fault. String inverters have a service life shorter than the panels, often in the range of ten to fifteen years, and replacement is a foreseeable cost. Microinverters and optimizers fail individually and show up as one dead panel.
A monitoring outage. Sometimes production is fine and the reporting is not. If your monitoring platform lost its internet connection, the app shows nothing while the system runs normally. Check the utility bill against the app before assuming a hardware fault.
Utility side changes. If your billing changed, your bill may look worse while production is unchanged. Rate structure and export credit changes are covered in net metering changes are reshaping home solar quotes.
The Sequence to Work Through
- Open the monitoring app and look at production by panel or string, not just the total.
- Check whether it is zero or low. Zero at one panel points to hardware. Low across everything points to weather, soiling, or shading.
- Look at the array from the ground. Soiling, debris, and new shade are all visible.
- Check the inverter for fault lights or error codes, and check the AC and DC disconnects and the breaker.
- Compare to last year, same month.
- Check your monitoring connection if the data itself looks wrong or stale.
- Call the installer if a specific panel or string is down, if the inverter shows a fault, or if year over year output is meaningfully off with no visible explanation.
Warranties Are Layered
A solar installation typically carries several separate warranties with different terms, and knowing which one applies determines who you call.
- Panel product warranty covering defects
- Panel production warranty guaranteeing output stays above a declining threshold over decades
- Inverter warranty, often shorter than the panel warranty, and frequently extendable at purchase
- Workmanship warranty from the installer, covering the installation itself
- Roof penetration warranty, sometimes separate
The general structure is covered in understanding workmanship and material warranties. The practical complication is that if your original installer is no longer in business, the manufacturer warranties usually survive but the labor to diagnose and replace does not. Another company will service the system, and they will charge for it.
Maintenance Worth Doing
Solar is genuinely low maintenance, which is a fair claim, but it is not no maintenance.
- Check monitoring monthly. Five minutes. It is the whole maintenance program for most systems.
- Keep vegetation trimmed back from the array.
- Clear debris from around and under the array where it collects.
- Have the roof inspected on the same cycle you would otherwise. A roof under panels is harder and more expensive to repair, which is why should you replace your roof before going solar matters at purchase time.
- Budget for an inverter replacement if you have a string inverter and the system is approaching a decade old.
- Check any critter guard at the array perimeter, since nesting under panels is common and causes both shading and wiring damage.
If You Are Still Shopping
Production estimates in proposals are modeled, not measured, and modeling assumptions differ between companies. Ask what shading analysis was performed, what degradation rate is assumed, and what the monthly expected output is rather than only the annual figure. How to read a home solar quote walks through separating the layers of a proposal, and what solar payback looks like without the federal credit covers the economics as they now stand.
If you want quotes for home solar, tell us about your home and we will connect you with installers serving your area. It is free to use, with no obligation to hire.