Why Has My Solar Output Dropped? A Troubleshooting Guide

Bottom line up front: If your solar output has dropped, the cause is almost always one of five things: seasonal change, soiling (dirt/lichen/bird mess), new shading, an inverter fault, or a string that has dropped out. Around 80% of "my solar is broken" calls turn out to be a perfectly clean panel array doing exactly what it's supposed to do in mid-winter Wellington gloom, or a slightly grumpy inverter that needs a power cycle. The rest are genuine faults worth a service call. This guide walks you through how to tell the difference, what you can safely check yourself, and when it's time to ring your installer.

Noticing a drop in solar output is unsettling. You paid good money for this system, and seeing the numbers slip on your app can feel like watching a smoke detector chirp at 2am: something's clearly off, but you're not quite sure how serious. The good news is that most output drops in New Zealand homes are easily explained and just as easily fixed.

This article is for homeowners with a grid-tied rooftop PV system who've spotted that their production is lower than expected. We'll cover the five most common causes, the safe DIY checks anyone can do from the ground, and the clear signals that mean it's time to stop poking and call a professional.

What "Output Dropped" Actually Means for NZ Homeowners

Before we troubleshoot, let's define the problem properly. "My output has dropped" can mean several different things, and they each point in different directions:

• Drop compared to last year, same month: this is the most useful comparison. If June 2024 produced 180 kWh and June 2025 produced 120 kWh, that's a real signal.
• Drop compared to last month: often just seasonal. NZ solar production swings dramatically between summer and winter, sometimes by a factor of 3 or 4.
• Drop compared to the brochure number the installer quoted: this might never have been realistic, especially if you're in Dunedin or Invercargill. Quoted figures are annual averages.
• One panel or string showing far less than the others: this is the one to take seriously. It almost always points to a real hardware or shading issue.

NZ irradiance varies hugely by region and season. NIWA's solar radiation data shows that Auckland in December typically receives more than three times the solar energy per day that it does in June. A "drop" from January to July is not a fault; it's physics. Sense-check your concern against the season first.

The Five Most Common Causes of Output Drop in NZ

1. Seasonal change (the most common "false alarm")

If you're looking at a winter month and worrying, take a deep breath. According to EECA and NIWA irradiance data, a typical North Island roof produces roughly 4 to 5 kWh per installed kW per day in summer, and 1.5 to 2.5 kWh per installed kW per day in winter. South Island figures sit slightly lower in winter, particularly in Otago and Southland.

A 6 kW system that pumped out 30 kWh on a clear January day might only produce 8-10 kWh on a cloudy July day, and both numbers are completely normal. Compare year-on-year, not month-on-month, when judging whether something's actually wrong.

2. Soiling: dirt, bird droppings, pollen, salt and lichen

Solar panels are self-cleaning to a degree (rain does most of the work), but NZ has a few specific dirt problems that can knock 5-15% off output:

• Bird droppings: concentrated and shading. One well-placed gull deposit can take a panel offline.
• Pollen and pine sap: a problem in pine-belt areas of the Bay of Plenty, Nelson and Canterbury during spring.
• Salt spray: coastal Northland, Wellington's south coast, Kāpiti, Tasman; builds up faster than you'd think.
• Lichen and moss: shaded edges of panels, particularly in damp Waikato and Taranaki conditions.
• Volcanic dust and ash: occasional issue across the central plateau and downwind.

For a thorough walk-through of when and how to clean, see our guide on cleaning and maintaining solar panels in NZ. The short version: never climb onto a wet roof yourself, and most modern panels need professional cleaning at most once every 2-3 years unless you have a specific soiling problem.

3. New shading you might not have noticed

Shading is the silent output killer. When your installer designed the system, your neighbour's pittosporum was 2 metres tall. Three years on, it's 5 metres and casting an afternoon shadow across your western string.

Shading is non-linear. A single panel shaded by 20% can drag a whole string down by far more than 20% if you have a string inverter without optimisers. Walk around your property at three different times of day (morning, noon, late afternoon) and look up at the panels. If anything new is casting a shadow that wasn't there before, you've likely found your culprit.

4. Inverter faults

The inverter is the brain of your system, and it's also the component most likely to misbehave. Common symptoms include:

• Red or amber warning light on the inverter casing (green is healthy on most brands)
• The inverter app showing "offline", "fault", or a specific error code
• Production showing as zero on a sunny day
• One MPPT input showing zero while others produce normally (string drop-out, more on this below)

Inverters carry a typical warranty of 10-12 years on string inverters and up to 25 years on micro-inverters (Enphase, for example). If your inverter is throwing genuine fault codes and it's still in warranty, this is a warranty claim, not a DIY job.

5. String drop-out

If you have a string inverter (the most common setup in NZ), your panels are wired in series into one or two "strings". If one string disconnects, due to a loose DC connector, a tripped DC isolator, or a single failed panel, that entire string can show zero output while the other string runs normally.

On your inverter app, you'll see one MPPT (maximum power point tracker) input doing nothing while the other is producing as expected. This is one of the clearest signals you have a real fault, and one of the more dangerous ones to investigate yourself.

What You Can Safely Check Yourself (From the Ground)

Before you ring anyone, run through this DIY checklist. Everything here is safe to do from the ground, in shoes, while standing on solid earth. Never climb onto your roof. Never open the inverter casing. Never touch DC cabling.

The 10-minute safe check

1. Open your monitoring app. Look at today's production curve. Is it a clean dome shape, a jagged line, or flatlined? A clean dome on a sunny day is healthy.
2. Compare year-on-year. Same month last year, how did the system perform? If it's within 10%, this is probably normal seasonal or weather variation.
3. Look at the inverter. From outside or wherever it's mounted (often in the garage). Note the colour of the status light. Green is healthy on Sungrow, Fronius, Goodwe, SolarEdge and most others. Amber or red means a warning or fault.
4. Check the Wi-Fi connection. A surprising number of "my solar has stopped working" cases are actually "my router has changed and the inverter can't phone home". The panels are producing fine; you just can't see the data.
5. Look up at the array from the ground. Use binoculars if you have them. Are there visible bird droppings, leaves, branches, or lichen patches on the panels?
6. Note the date of any service work, roofing work, or storms. If the drop started after a recent event, that's your strongest clue.

For a deeper dive on reading your app data properly, our guide on how to monitor your solar production walks through what each chart actually means.

The safe "soft reset"

If your inverter is showing a fault code and your installer suggests it over the phone, you can usually safely do a power cycle: turn off the AC isolator (the switch in your switchboard), wait two minutes, turn it back on. Do not touch the DC isolator unless your installer specifically walks you through it on the phone. DC current on a sunny roof is genuinely dangerous and is the cause of the rare-but-real solar fire incidents reported by Fire and Emergency NZ.

When to DIY vs When to Call Your Installer

Here's the simple rule: anything from the ground, you can check. Anything on the roof, in the inverter casing, or involving DC cables, you cannot.

Stop and call your installer if:

• Inverter is showing a red light or a persistent fault code after a soft reset
• One string or MPPT input shows zero while the other works fine
• You smell burning, see scorch marks, or hear arcing/buzzing from the inverter
• You see physical damage to a panel (cracked glass, delamination, water ingress)
• The system has been producing zero for more than 48 hours of sunlight
• You've had a recent storm with hail or extreme wind
• The roof needs cleaning and you don't have the right safety gear (you don't, and that's fine)

If your system is still inside its workmanship warranty (typically 5-10 years for the install) or hardware warranty (25 years for panels, 10-12 for string inverters), get the installer back first. Don't pay a third party to fix something that should be covered. Our guide on how to make a solar warranty claim in NZ walks through the process.

What This Means for You (by Persona)

The ROI Pragmatist

An output drop matters to you because every kWh lost is a kWh you're buying back from Genesis, Mercury, Contact or your retailer of choice. A 10% sustained drop on a 6 kW system that previously produced 8,500 kWh per year is 850 kWh lost. At an average retail rate of around 33c/kWh (check the Commerce Commission's quarterly retail pricing data for current figures), that's roughly $280 a year you're hemorrhaging. Worth a service call.

If you're recalculating expected ROI because output has shifted, our Solar System Cost & ROI Calculator lets you plug in updated production figures and see where you actually stand.

The Tech-Savvy Optimiser

You're probably already deep into the monitoring app. The thing to watch is per-string and (if you have optimisers or micro-inverters) per-panel data. Look for a panel that consistently underperforms its neighbours by more than 5%; that's diagnostic. Modern inverter platforms (Enphase Enlighten, SolarEdge Monitoring, Sungrow iSolarCloud, Fronius Solar.web) all let you set production alerts. Turn them on.

The Eco-Conscious Family

An output drop doesn't change the embodied carbon of your panels. They're still doing their environmental job. But it does mean you're pulling more from the grid, which in NZ is around 80-85% renewable but not 100% (the gap is largely natural gas peakers and Huntly's coal). Fixing the drop quickly keeps your household emissions profile where you want it.

What Installers Won't Always Tell You

Three honest truths from the trade:

1. "Tier 1" panels still degrade. All silicon panels lose 0.4-0.8% of output per year. Over 10 years, that's a 4-8% reduction baked in. This isn't a fault; it's the spec sheet. A 6 kW system installed in 2015 producing 5-6% less today than it did new is performing exactly as designed.

2. Some installers oversell expected output. The quoted annual production figure is often based on optimal pitch, optimal azimuth, zero shading and average-to-best regional irradiance. Real-world figures land 5-15% below brochure for most NZ installs. If your "drop" is actually just reality catching up with an over-optimistic quote, that's a sales problem, not a system problem.

3. Wi-Fi/monitoring drop-outs aren't faults. If you replaced your router, changed your Wi-Fi password, or switched broadband provider, your inverter has probably lost contact with the cloud. The system is producing fine; you just can't see it. Most inverter brands have a phone-app re-pairing process that takes 10 minutes.

Frequently Asked Questions

Why is my solar producing less in winter even on sunny days?

The sun sits lower in the sky and the days are shorter, so panels see less direct light at a steep angle. Even on a clear winter day, total daily kWh will be roughly 40-60% of a clear summer day. This is normal physics, not a fault.

Should I clean my panels myself?

Generally no. Roof work in NZ is governed by WorkSafe height regulations, and a wet pitched roof is one of the most common serious accident scenarios. For routine rain-cleaning it's fine to leave nature to it; for bird droppings, lichen or pine sap, hire a professional solar cleaner or roof-access tradie with proper harness gear.

My inverter app says "offline" but my power bill seems normal. What's happening?

Almost certainly a Wi-Fi or monitoring connection problem, not a production problem. The inverter is still working; it just can't report data to the cloud. Check your router, then follow your inverter brand's re-pairing instructions in their app.

How much output loss is "normal" before I should worry?

Compared to the same month last year, a drop of more than 10-15% with similar weather is worth investigating. Year-one to year-ten, expect a gradual 4-8% decline baked in from panel degradation.

Can a tripped circuit breaker stop my solar?

Yes. The AC isolator in your switchboard can trip during storms or grid disturbances. Look for a switch in the "off" position labelled "solar", "PV" or "inverter". You can safely switch it back on. If it trips again immediately, stop and call your installer.

My output dropped after a storm. What should I check?

Look up at the array from the ground for visible damage (cracked panels, dislodged mounts). Check the inverter status light. Reset the AC isolator if it has tripped. If anything looks damaged or the inverter throws a fault, document with photos and contact your installer; storm damage is often covered by home contents insurance even where it isn't covered by panel warranty.

How long should an inverter actually last in NZ?

String inverters typically carry a 10-year warranty and last 10-15 years in service. Micro-inverters carry up to 25 years. Coastal salt-air locations (Northland, Kāpiti, Tasman) sometimes see shorter inverter lifespans, which is worth factoring into long-term planning.

Will my buy-back rate change if my system produces less?

The rate per kWh stays the same; you just have fewer kWh to export. To see live rates across NZ retailers and how they apply to your situation, the Dynamic Tariff & Buy-Back Engine on NZ Solar Centre keeps current figures.

Is it worth getting a paid solar health check?

If your system is more than 5 years old, has never been professionally inspected, and you've noticed a real output drop, yes. A typical health check from a certified installer runs $150-$350 and covers DC isolator function, connector tightness, inverter diagnostics, and a roof inspection. It often pays for itself in recovered production.

Where to Go From Here

If you've worked through the safe checks and your system seems fine, congratulations: you've just learned how to read your own production data, which puts you ahead of most solar owners in the country. Bookmark this page for the next time something looks off.

If you've found a real issue, your first call is your original installer. Have your monitoring screenshots ready, your install date handy, and a clear description of what changed. Most genuine faults are fixable in a single visit.

For ongoing care, our solar system ownership guide is the central resource for everything that happens after the installers drive away. And if you're researching solar for the first time and want to make sure whatever you buy is set up to be easy to look after, our three free quotes from vetted installers service is the smartest place to start.