Solar Panels Not Producing Enough: The Diagnostic Order That Actually Finds the Problem

Step 1: Check monitoring before you check anything else

Start here every time. Your installer's monitoring platform — Enphase Enlighten, SolarEdge, mySunPower, or a similar app — is a separate data connection from the actual power production, usually running over your home Wi-Fi. When that connection drops, the app can show flat or missing data for days while the panels keep producing normally. Open the app and check the last-reported timestamp before assuming anything is wrong with the system itself.

What you can see also depends on the equipment. Microinverter systems (Enphase) and power-optimizer systems (SolarEdge) report production panel by panel, so a single failing unit shows up directly in the app. A conventional string inverter without optimizers (SMA, Fronius, and similar) only reports at the string or whole-system level — it cannot show you which individual panel is underperforming, only that total output is down. If you have a string-inverter system, isolating a single bad panel requires a technician with a clamp meter, not just a login to an app.

California's contractor disclosure law lets a solar installer include, at the board's discretion, "a calculator that calculates performance of solar projects to provide solar customers the solar power system's projected output" (Business & Professions Code §7169(c)) — this item is optional supporting information, not a mandatory disclosure, so not every installer provides one. If yours did, that document is worth pulling now; if it didn't, or you don't know what monitoring platform you have, that's still worth resolving before you go further — it tells you what data you actually have access to.

Step 2: Rule out new shading

The single most common cause of a real, gradual production decline is shading that didn't exist at install — usually a tree that's grown, a neighbor's new structure, or a satellite dish or vent pipe that got added to the roof. Walk the property at 9am, noon, and 3pm on a clear day and look for shadows crossing the array that weren't there when the system went in. Compare against your original install photos if you have them.

The U.S. Department of Energy's homeowner guide notes that panels perform best on south-facing roofs with a 15-40 degree slope, and that shading is one of the primary variables installers should have accounted for in the original design (DOE/EERE, "Homeowner's Guide to Going Solar," energy.gov, as of 2026-09-05). If shading is new since install, there's no California statute that forces a neighbor to trim a tree for your solar system's benefit — that's a private negotiation, not a code violation. On systems without microinverters or optimizers, partial shading on even one panel can drag down the output of its entire string, which is why this step comes before you assume a hardware failure.

Step 3: Soiling — dust, pollen, and time since the last rain

Dust, pollen, and bird droppings reduce output gradually and are easy to miss because the loss creeps in over weeks. This is more pronounced in the Central Valley and desert regions during long dry stretches between rains, and less of an issue near the coast. Industry practice puts typical annual soiling losses in the low single digits in low-dust coastal areas, climbing meaningfully higher in dusty inland conditions without rain for months — these are trade-standard figures, not numbers this brief could verify against a primary NREL or CEC document this session, so treat them as directional rather than exact.

The practical test doesn't require a percentage table: look at the panels, and compare production in your app right before and right after a rinse or a real rainstorm. If output visibly recovers after cleaning, soiling was the cause and a periodic cleaning schedule (or waiting for the next rain) is the fix — not a service call.

Step 4: Inverter faults

An inverter problem typically announces itself two ways: a red or blinking status light on the physical unit, and an error code either on the inverter's own display or flagged in the monitoring app's alerts tab. If your app has an alerts or notifications section you haven't checked, check it now — many systems surface fault codes there automatically.

The detail worth knowing before you're surprised by it: inverters commonly carry a 10-12 year manufacturer warranty, meaningfully shorter than the 25-year warranty on the panels themselves. If your system is more than a decade old and the inverter fails, replacement may be an out-of-pocket cost unless you bought an extended warranty at install. This is standard industry practice across manufacturers, not a California-specific rule, so check your own paperwork for the specific term you were sold.

Step 5: String outages — the failure your app might not show you

A "string" is a group of panels wired together feeding one input on the inverter. When an entire string goes down — from a bad connection, a tripped breaker, or a wiring fault — production drops by roughly the proportion that string represents (for example, about 50% if you have two strings and one dies), even though every individual panel in that string might be fine.

On a microinverter or power-optimizer system, this is usually visible directly in the app because each panel reports separately, so a whole string going dark is easy to spot. On a plain string-inverter system, you'll see the aggregate production drop but nothing pointing to which string or connection failed — that diagnosis requires an electrician or your installer testing each string directly. If your production dropped sharply and stayed down at a fairly constant lower level (rather than fluctuating with weather), a string outage is a strong candidate.

Step 6: Panel-level failures and your warranty coverage

Individual panel failure is governed by two separate warranties bundled into your original paperwork: a workmanship warranty (typically 10-25 years, covering defects in manufacturing) and a separate power/performance warranty, which guarantees the panel won't degrade below a specified output curve over 25 years — commonly starting in the high 90s percent of rated output in year one and declining toward the high 80s percent by year 25, though the exact numbers vary by manufacturer and this brief could not pull current figures for your specific installed panel model this session. Before assuming a percentage, find your panel's brand and model on the original contract and pull its current datasheet directly from the manufacturer.

Isolating a single bad panel, again, requires microinverter or optimizer-level monitoring, or a manual per-panel test if you have a plain string inverter. If a panel is confirmed underperforming within its power-warranty window, the manufacturer — not necessarily your installer — is who owes you the remedy, which matters if your original installer is out of business.

Step 7: Was the original production estimate ever realistic?

If you've worked through steps 1-6 and the system is functioning exactly as installed — no faults, no shading, no soiling, no dead strings — but it has never come close to the number you were shown at the sale, the problem may be the estimate itself, not the equipment. The Department of Energy's own guidance warns homeowners against relying on generic online calculators and recommends a custom, site-specific estimate from the installer (DOE/EERE, energy.gov, as of 2026-09-05). The industry-standard modeling tool for these estimates is PVWatts (NREL/DOE), which factors system size, tilt, azimuth, shading, and an assumed system-loss percentage into its output — worth knowing exists, even though this brief could not independently confirm PVWatts's current default assumptions this session.

California law gives you a paper trail to check this against, though it's a narrower trail than it might seem. B&P §7169(b) does require every solar contract's cost, payment-terms, and cancellation-rights disclosures to appear on the front or cover page in 16-point boldface type — but the production-calculator output itself is governed by the separate, discretionary §7169(c), which the board may allow an installer to include as supporting information, not a mandatory front-page item. In practice, if your installer did give you a production-calculator printout as part of the sale, it should be in your contract file somewhere, boldface or not, and it's worth pulling and comparing to what you were told verbally. If the number in it doesn't match, or if no such document exists at all, that's a documentable gap worth raising with the installer or CSLB — not proof of fraud, and not a statutory violation on its own, but a specific, citable point rather than a vague complaint that "it doesn't make what they said."

Production guarantees — what they cover, and how a claim actually works

Whether you have a production guarantee at all depends heavily on how you paid. Guarantees are a standard feature of third-party-owned systems — leases, power purchase agreements (PPAs), and subscription plans — not of systems bought outright with cash or a loan, where your only protection is the separate manufacturer equipment warranties described above, not a whole-system output promise. With the federal 25D tax credit having ended December 31, 2025, more California buyers are likely to end up in a TPO/PPA structure going forward (which can still reach the commercial 48E credit through the third-party owner), which means more homeowners will actually have a production guarantee to lean on — worth confirming which category you're in.

As one concrete example of how these guarantees work in practice: Sunrun's lease and "Protection Plus" plans promise the system will produce "at least 90% of estimated production over its lifetime," and if it falls short, Sunrun "will pay you the difference" between estimated and actual output, based on monitoring data collected roughly every six hours (Sunrun, "Your Guarantee," sunrun.com, as of 2026-09-05). This is Sunrun's specific contract language, cited as an example of how TPO guarantees are typically structured — not a claim that every installer offers identical terms. Sunrun doesn't publish a formal claims form; the guarantee runs on its own monitoring flagging a shortfall and the company reaching out, with a customer service line as the fallback if it doesn't. Check your own contract for the specific percentage, payout mechanism, and any exclusions before assuming Sunrun's numbers apply to you.

If you own your system outright and have no production guarantee, your paths are narrower: the Song-Beverly Act's implied warranty (Civil Code §1791.1) only runs 60 days to one year after purchase, too short for most underperformance discovered years in. Longer term, you're relying on the manufacturer's written equipment warranties, a CSLB complaint against the installer (patent defects: 4-year window; latent defects: 10 years, under B&P §7091), or a demand under the Unfair Competition Law (B&P §17200) if the original sales estimate was demonstrably misleading. Note that CSLB's Solar Energy System Restitution Program is no longer accepting new claims — its appropriated funds are depleted — so a contractor's $25,000 license bond (B&P §7071.6(a), a pool shared across all of that contractor's claims, not a per-homeowner guarantee) and small claims court are the more realistic remaining options for owned-system disputes.