Why Has My Solar Output Dropped? A Troubleshooting Guide

If your solar output has dropped, the single most common culprit is seasonal change, not a fault. A well-sized system in most of New Zealand produces roughly three to four times more in mid-summer than in mid-winter, because NIWA data shows winter daily sunshine and sun-angle fall away sharply, especially south of Taupo. So before you panic, check the date. After that, the usual real faults, in rough order of likelihood, are soiling (dirt and lichen), new shading, a tripped or faulting inverter, and a dropped string of panels. The quick test: if today's output is well down on a sunny day compared to the same week last year, something is genuinely wrong and worth chasing. If it's just lower than summer, that's physics doing its job.

First, rule out the obvious: is it actually a fault?

Solar output naturally swings with the seasons, the weather, and even the time of day. The mistake we see constantly is a homeowner comparing a grey July morning to a brilliant January afternoon and assuming the system is broken.

NIWA's long-term sunshine records make the scale of this clear. Places like Nelson, Blenheim and Whakatāne top the country at well over 2,400 sunshine hours a year, while Invercargill and parts of the West Coast sit closer to 1,600 to 1,700 hours. But the within-year swing matters more for troubleshooting: in winter the sun is lower, the days are short, and clear-sky output can be a third of a clear summer day even with spotless panels.

So the first question is always the same: are you comparing like with like?

• Same time of year? Compare this June to last June, not to last December.
• Same weather? A high-cloud day can halve output without anything being wrong.
• Same time of day? Output peaks around solar noon and tapers off morning and evening.

If you've got a monitoring app, this is exactly what it's for. If you're not sure how to read yours, or you haven't set one up, we walk through the major platforms over here: how to monitor your solar production. Year-on-year comparison is the fastest way to separate "winter" from "fault".

The quick gut-check that beats guessing

On the next genuinely clear, cloudless day, look at your system around midday. A healthy grid-tied system should be producing close to its rated output in good sun (a 5kW array won't usually hit a flat 5kW because of inverter and temperature losses, but on a clear cool day it should get into the 4kW-plus range). If it's sitting at half that on a perfect day, you've confirmed a real problem worth investigating.

Cause 1: Soiling (dirt, lichen, bird mess and pollen)

This is the most underrated cause in New Zealand, and the easiest to fix. Panels lose efficiency when they're dirty, but the losses are rarely uniform, which is what makes soiling sneaky.

A film of general dust and pollen across the whole array might cost you a few percent. The real damage comes from concentrated, opaque soiling: a stripe of bird droppings, a patch of lichen creeping up from a damp corner, or leaf litter trapped along the bottom edge of the panels. Because panels are wired in series, a single heavily shaded or fouled cell can drag down the output of an entire string far out of proportion to the area it covers.

New Zealand conditions make this worse in specific places:

• Lichen and moss thrive in the damp, humid north: Northland, Auckland's leafier suburbs, the Coromandel and Bay of Plenty.
• Pollen and farm dust build up fast on Canterbury and Waikato roofs, particularly in a dry nor'west spell.
• Salt haze near the coast leaves a fine film that rain doesn't fully clear.
• Bird traffic under trees or near a TV aerial is a perennial offender.

The good news: rain does most of the cleaning, and a gentle clean once or twice a year is usually plenty. The important bit is doing it safely and without voiding your warranty, which is more nuanced than it sounds (high-pressure washers and abrasive pads can wreck the anti-reflective coating). We cover the right way, including when to leave it to a pro, in our piece on cleaning and maintaining solar panels.

Cause 2: New shading you didn't have before

Here's the one almost nobody checks: shade that wasn't there when the system was installed. Trees grow. Neighbours build. The sun sits lower in winter, so shadows that miss your roof in summer can fall right across it in June and July.

Think about a 1960s weatherboard place in Mount Eden with a neighbour's poplar over the back fence. In December the sun is high and the poplar shades nothing. By midwinter the sun arcs low across the north and that same tree throws a shadow over the bottom row of panels from about 2pm. The owner sees afternoon output collapse and assumes the inverter is dying, when really it's a tree and a season doing exactly what trees and seasons do.

Because of the series-wiring effect, even partial shade on one or two panels can knock out a whole string. This is precisely where the hardware design matters:

• String inverters (a single inverter for a group of panels) are most vulnerable: shade one panel and the whole string suffers.
• Microinverters and power optimisers isolate each panel, so a shaded one only drags down itself, not its neighbours.

If your system was sold to you with optimisers specifically to handle a shading problem, and output is still tanking, that's a flag the optimisers may have failed or were never commissioned properly. Worth raising with your installer.

How to check for shading yourself

Walk outside on a sunny day, midmorning and again midafternoon, and look at the array. Note any shadow creeping across panels. Then check your monitoring: if output drops off a cliff at a consistent time of day rather than fading gradually with the sun, shading is the prime suspect. A genuine fault tends to look different (see below).

Cause 3: Inverter faults and dropouts

The inverter is the brain of the system, converting the panels' DC into the AC your house uses, and it's the component most likely to fail in the system's lifetime. Panels routinely carry 25-year performance warranties, while string inverters are typically warranted for 5 to 10 years (some now offer 10 to 12). That gap tells you where the weak link usually is.

Symptoms of an inverter problem:

• Output is zero on a sunny day when it shouldn't be.
• A red or orange light, or no lights at all, on the inverter unit.
• An error or fault code showing on the inverter screen or in the app.
• The system cutting in and out, or repeatedly restarting.

One genuinely common and harmless cause of an inverter "dropout" is worth knowing, because it saves a needless service call. Grid-tied inverters are required to shut down automatically if the grid voltage drifts outside set limits. This is an anti-islanding safety feature, and it's mandated under New Zealand's connection standards so the inverter can't backfeed a line that linesmen think is dead.

The quirk: on a long rural feeder, or at the end of a street with a lot of solar, the local voltage can climb on a sunny day. When it crosses the upper threshold, your inverter trips off to protect itself and the network, then reconnects once voltage settles. You see midday output stuttering or cutting out on the sunniest days, which feels completely backwards. If that's your pattern, it's often a grid voltage issue, not a faulty inverter, and the fix usually involves your installer logging the data and your lines company (Vector, Orion, Aurora, Powerco, whoever services your patch) adjusting the network or your inverter's settings within allowed limits. This is the kind of fault that gets misdiagnosed for months, so it's worth knowing it exists.

The DIY inverter reset (and the limits of it)

Most manufacturers have a documented restart sequence, usually: turn off the AC isolator, then the DC isolator, wait a couple of minutes, then bring the DC back on followed by the AC. Your installer's handover pack or the manufacturer's manual will give the exact order for your unit. A reset clears the occasional software glitch.

If a reset doesn't fix it, or a fault code returns, stop there. Do not open the inverter or touch DC wiring. The DC side of a solar array can sit at hundreds of volts whenever the sun is up, there is no off switch on the panels themselves, and it does not behave like household AC. This is licensed-electrician territory, full stop.

Cause 4: A dropped string or dead panel

If your system has multiple strings (common on larger or split-roof installs, say panels on both the north and west faces), one string can drop out while the other keeps working. The tell-tale sign is output that's settled at roughly half, or some other clean fraction, of what you'd expect, consistently, in good sun.

Causes include a tripped DC breaker, a failed connector, water ingress at a junction, rodent damage to cabling (yes, really, especially in roof spaces), or a single failed panel taking its string down. Per-panel monitoring, if you have optimisers or microinverters, will often point straight at the dead unit. Without it, an installer needs to test each string.

This is not a DIY fix. It's diagnosis and repair on live DC, and it's exactly what your workmanship and product warranties are there for.

Cause 5: The boring ones people forget

• You changed your habits, not your system. If you're looking at your power bill rather than generation figures, remember that solar savings depend on self-consumption. New EV charging overnight, a teenager home all summer, or a heat pump running harder can lift your grid import even while the panels produce exactly as before.
• A clipped or undersized inverter. If your panels are oversized relative to the inverter, peak midday output gets "clipped" at the inverter's ceiling. This is normal and usually deliberate, but if you weren't told, it can look like a cap on performance.
• Monitoring gone offline. Sometimes the panels are fine and it's the Wi-Fi or the monitoring gateway that's dropped out. Zero in the app, normal on the meter, is a connectivity problem, not a generation one.
• Daylight saving and clock drift. Occasionally an inverter's internal clock slips, so production looks shifted in time. Cosmetic, not a real loss.

When to DIY-check and when to call your installer

Here's the honest dividing line. You can and should do the no-tools, no-risk checks yourself. Anything involving wiring, the DC side, or opening hardware is a job for a licensed installer or electrician.

Safe to do yourself

• Compare output to the same period last year in your app or on the meter.
• Check the weather and time of day before assuming a fault.
• Look for visible soiling, lichen, leaves or bird mess from the ground.
• Walk the property morning and afternoon to spot new shading.
• Read the inverter's status lights and any fault code (note it down, photograph it).
• Perform the manufacturer's documented restart sequence, once.
• Check your Wi-Fi and monitoring connection.

Call a professional

• A fault code that persists or returns after a reset.
• Output stuck at zero, or a clean fraction (half, a third) on a sunny day.
• The inverter cutting in and out around midday (likely a grid voltage issue to log).
• Any sign of water, scorching, melted connectors, or a burning smell. This last one means switch off the AC isolator and call straight away.
• A suspected dead panel or dropped string.
• Anything that would require touching DC cabling, connectors or the inverter internals.

Before you call, gather the details: the date your system was installed, the brand of panels and inverter, photos of any fault codes, and a screenshot of the output drop from your app. Installers can often diagnose remotely if your inverter is online, which saves a call-out fee.

What a fair repair should cost, and who pays

This is where knowing your warranty position matters. A quality New Zealand install should come with three separate warranties:

• Panel performance warranty: commonly 25 years, guaranteeing the panels still produce a set percentage of rated output.
• Inverter product warranty: typically 5 to 12 years depending on brand and whether you paid to extend it.
• Workmanship/installation warranty: from the installer, often 5 to 10 years, covering the labour and the install itself.

If your inverter dies inside its warranty term, the part is usually covered, but labour and call-out may not be, depending on the installer's terms. That gap surprises people. A standard inverter swap, where you're paying labour, generally runs a few hundred dollars; a full out-of-warranty inverter replacement on a typical home system can be $1,500 to $3,000-plus installed, which is exactly why the inverter warranty length is worth scrutinising before you buy.

If the fault is genuinely the installer's or manufacturer's responsibility, don't pay for it. We've laid out exactly how to lodge and pursue a claim, including your rights under the Consumer Guarantees Act, in our walkthrough on making a solar warranty claim in New Zealand. Keeping good records from day one makes all the difference here.

A worked example: the Christchurch system that "broke" in winter

Take a 6.6kW array on a brick-and-tile home in Halswell, on the Orion network. In February the owner watched it pump out close to 40kWh on clear days and felt chuffed. Come July, daily output sat around 12 to 14kWh and they were convinced something had failed.

Nothing had. NIWA's figures for Christchurch show the dramatic seasonal swing in usable sun, and the low winter sun angle plus shorter days fully explain a drop of that size. A like-for-like check (clear July day this year versus clear July day last year) showed output within a whisker of the previous winter. The system was perfectly healthy.

The lesson: winter lows are not faults. The number that tells you something is wrong is a year-on-year drop on comparable days, not a summer-to-winter drop. If you want to sanity-check what your system should be producing across the seasons, and how that flows into savings, our cost and ROI calculator lets you model it for your region.

Keeping output where it should be

Most output problems are prevented rather than fixed. A light annual once-over keeps the easy stuff at bay:

• Glance at your monitoring monthly so you spot a drop early, not six months and several hundred kilowatt-hours later.
• Clear leaf litter and check for lichen each spring and autumn.
• Keep an eye on growing trees and book a trim before they reach the winter sun line.
• Note your inverter's warranty expiry in your calendar.
• Keep your install paperwork, serial numbers and commissioning data somewhere you can find it.

If you'd like the full picture on looking after a system over its life, from monitoring to servicing to warranty admin, we keep it all together in our ownership and aftercare guide.

Frequently Asked Questions

Is it normal for solar output to halve in winter?

Yes, and often it drops further than that. The lower winter sun angle and shorter days mean clear-sky winter output can be a fraction of a clear summer day, with the effect more pronounced the further south you live. NIWA's sunshine and solar radiation data show the seasonal swing clearly. A winter low is only a concern if it's well below the same month in previous years.

Can dirty panels really make that much difference?

General dust costs a few percent, but concentrated soiling like bird droppings or lichen can hurt far more than the area suggests, because panels are wired in series and a single fouled cell drags down a whole string. Rain handles most cleaning, but a gentle once or twice yearly clean keeps things tidy. Avoid high-pressure washers and abrasive pads, which can damage the panel coating and void your warranty.

My inverter shows zero on a sunny day. What do I do?

Check the inverter's status lights and screen for a fault code and note it down. Try the manufacturer's documented restart sequence once (AC off, DC off, wait, DC on, AC on, in the order your manual specifies). If it stays at zero or the fault returns, call your installer and do not open the unit or touch any DC wiring, which stays live whenever the sun is up.

Why does my system cut out around midday on the sunniest days?

This often points to high grid voltage rather than a fault. Grid-tied inverters must shut down if voltage drifts outside set limits, an anti-islanding safety requirement under New Zealand's connection standards. On feeders with lots of solar, voltage can climb at peak generation and trip the inverter. Your installer can log the data and work with your lines company to resolve it.

Should I clean the panels myself or pay someone?

A single-storey roof with safe access and light soiling can often be rinsed gently from the ground or with a soft brush and plain water. Anything involving climbing, a two-storey roof, or stubborn lichen is better left to a professional, both for your safety and to protect the panel surface and your warranty.

How long should my inverter last?

String inverters typically carry 5 to 12 year warranties and are the component most likely to need replacing during a system's life, while panels usually carry 25-year performance warranties. Budgeting for one inverter replacement over the life of the system is sensible. Note that the part may be covered under warranty while labour and call-out are not, so check the fine print.

Could the problem just be my power bill rather than the panels?

Quite possibly. Your bill reflects how much grid power you still buy, which depends on self-consumption. If you've added EV charging, a new heat pump, or simply have more people home, your grid import can rise even though the panels produce exactly the same. Check your actual generation figures in the app before assuming the system is underperforming.

What records should I keep to make a future warranty claim easier?

Keep your install date, the brand and serial numbers of your panels and inverter, the commissioning report, and your warranty documents from both the installer and the manufacturers. Screenshot any fault codes and output drops as they happen. Good records turn a drawn-out dispute into a quick claim.

The bottom line

Most "my solar's broken" moments turn out to be the season, the weather, or a bit of lichen. Start by comparing like with like: this winter against last winter, on clear days, at the same time. If output is genuinely down year-on-year, work through the likely causes in order, soiling and shading first (both safe to check yourself), then inverter status and string behaviour.

Keep the line clear in your head: eyes and apps are yours, anything with wiring or DC voltage belongs to a licensed installer. Know your warranty position before you pick up the phone, and keep your paperwork handy. Do that, and a dropped output figure becomes a quick diagnosis rather than a long worry.

Where to go from here

From here, it's worth getting comfortable reading your monitoring app so you catch dips early, and bookmarking how to lodge a warranty claim if you ever need to. A little attention now saves a lot of kilowatt-hours later. And if you'd like the wider view on keeping a system healthy for the long haul, our ownership and aftercare guide pulls the whole picture together.