Your solar is producing less than expected. Is it a fault?

Nearly every solar owner eventually stares at the app and thinks this seems low. The trouble is that "low" is a feeling, and a feeling is hard to argue with an installer.

This walks through four explanations in order of how likely they are. Most people stop at the first or second.

1. You're comparing against a number that was never true

The most common cause of apparent underperformance is a wrong expectation. The figure a salesperson once quoted is usually calculated for an ideal south-facing roof at the optimal tilt, with no shade and no losses. Your roof isn't ideal — and that needn't be anyone's fault.

The differences that surprise people most:

• Orientation. East–west instead of south typically costs 15–20 % a year. That isn't a defect, it's geometry.
• Tilt. A flat roof yields somewhat less at our latitudes than the optimal 35° or so.
• Losses. Cabling, the inverter, panel temperature and soiling together take 8–14 % even from a perfectly healthy installation.

Before investigating a fault, you need a number that applies to your roof. That can be calculated from location, tilt and orientation — which is exactly what our quick check does.

2. It's simply a bad month

Year-to-year weather variation is larger than most people expect. Two consecutive years on the same system routinely differ by 5–10 %, and individual months by 30 %. An overcast June is not a fault.

This is why a single weak month is never judged on its own. The signal comes from several weak months in a row, or from a full year below expectation.

Winter is a category of its own. In December and January the deviation from any model is so large that almost nothing can be concluded from it — which is why we treat winter months far more leniently than the rest of the year.

3. Something is shading the roof and you don't know it

Shade behaves quietly. It doesn't arrive overnight, it grows — a tree over five years, a neighbour's extension over one summer. And because panels in a string are wired in series, shading one panel can drag down the whole string.

The giveaway: output drops only at certain times of day, or only in part of the year. In summer, with the sun high, everything may look fine; in winter, when it tracks low across the sky, even a low obstacle starts to matter.

Whether your site has a horizon problem can be established before anyone climbs on the roof — our shading check needs nothing but an address.

4. An actual fault

Only a minority of cases get this far. Real faults usually have a sharper signature than a gradual "somehow less":

• A disconnected or failed string. Output drops in a step, often to roughly half on a two-string system. It shows up when strings are compared against each other — on a clear midday they should be similar, assuming the same panel count and orientation.
• An inverter fault. The inverter usually knows and reports an error code. It distinguishes a recoverable fault (a reaction to grid fluctuation, clears itself) from a permanent one (it won't).
• An outage. A month with practically no output where there should be some. The easiest to spot, and unfortunately sometimes found a quarter late, because nobody was looking.

What to actually do

The order that saves the most time:

1. Establish the expected output for your roof, not a generic brochure figure.
2. Look at a whole year, not one month.
3. Check for shade — the most common cause nobody expects.
4. Only then call service — and arrive with numbers, not a feeling.

That last point matters more than it looks. A complaint backed by "it seems low to me" and one backed by a year-long comparison against a model for your location carry entirely different weight.

Where Slunora helps

The quick check compares your annual or monthly output against what your location should deliver, and says whether the gap sits inside normal variation or outside it. It takes minutes and needs nothing beyond the figures already in your inverter app.

If you have a SolaX, you can connect the inverter and the check then runs by itself — including comparing individual panel strings and surfacing faults the inverter reports about itself.

And above all: if the data doesn't point at a fault, we say so. A verdict of "no apparent problem" is as useful as a fault found — it saves you a phone call that would have led nowhere.

(Does your output instead look suspiciously high, or catastrophically low? That extreme case has a different cause — covered in your solar is producing far more than expected.)