Super El Niño winter solar panels: underperforming, or just the weather?

A south-facing roof in London makes about 3.7% of its year in December, whatever El Niño does. NOAA does put “a greater than 90% chance of a very strong event” on winter 2026-27, but that is about the tropical Pacific. For Britain, the Met Office says only that El Niño years are “one factor that can increase the risk of colder winters in the UK”, and colder is not the same as darker. What tells a dark December from a dead string is your own daily production, held against the weather your address actually had.

What NOAA actually says about the winter of 2026-27

NOAA’s Climate Prediction Center has an El Niño Advisory in force. Its discussion of 10 September 2026:

“El Niño is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27.”

It also puts a 75% chance on a historic event for October to December 2026, stronger than any El Niño since 1950.

NOAA’s forecast is about the Pacific. For your street, the Met Office’s own forecasts beyond five days are written for the UK as a whole.

What the Met Office says El Niño means for a UK winter

The Met Office’s explainer has one sentence on British winters:

“El Niño years are one factor that can increase the risk of colder winters in the UK.”

It is a general statement about El Niño years, not an outlook for this one. Each El Niño is different, the Met Office adds, and not every impact turns up every time.

How much do solar panels produce in December in the UK and Ireland?

Far less than in summer, and the reason is day length and the angle of the sun rather than anything happening in the Pacific. What a 1 kWp system tilted 35° and facing south makes in London and Dublin, according to PVGIS, the European Commission’s free solar calculator, in kWh per kWp installed:

• December is about 3.7% of the London year and about 3.5% of the Dublin year.
• November, December, January and February together come to about 17.4% of the London year and about 17.4% of the Dublin year.
• London’s best month, July, is roughly 3.3 times its December. Dublin’s best month, May, is roughly 3.6 times its December.

So a December that looks alarming next to June is not a symptom of anything. Roughly a sixth of your year’s electricity arrives across four months, and the other five sixths arrive in the other eight. That is the normal shape of a pitched, south-facing UK or Irish roof, and it does not need an explanation from the Pacific.

Storms are the other thing owners notice in winter. The 2026/27 storm-naming season, run by Met Éireann with the Met Office and KNMI, opened on 1 September 2026 and runs until 31 August 2027; a storm is named when it could have medium or high impacts in one of the three countries. What to check on a system once one has passed through is in the guide on solar panels after a storm, and snow is covered in snow on solar panels.

Solar panels underperforming, or just a dark December?

A low month looks like a fault, and a fault looks like a low month. That confusion costs money, and it has nothing to do with El Niño. In winter the two explanations for a disappointing number (the weather, and something broken) produce the same figure in your app, and the weather is always the more comfortable of the two.

Here is an owner working through it in public, on a UK forum:

“Since installation the generation figures have been slightly lower than before but I put this down to cloudier weather and deterioration of the solar panels.” “However when I checked the inverter I noticed that whilst string 2 was working fine, string 1 had the correct voltage (360v) … but the ampage was zero and hence not generating any output. The inverter appeared to be working fine and had no error messages.”

One of the inverter’s strings, a whole string of panels, was showing the correct voltage and zero current, which means no output from that string. The inverter itself showed no error message. And the owner’s first explanation was cloudier weather, which was a perfectly reasonable thing to think.

The second pattern is the bill arriving a season late:

“I didn’t notice anything amiss until September, 2021 when I got an unexpected power bill. When I looked at the Solar Edge monitoring data, power production starts to noticeably drop first in March, 2021 and I thought that the problem started around then.” “Monthly production for these two most recent years is down 10 to 50%.”

The decline ran for six months before the owner noticed it, and what told them in the end was a bill, not the monitoring platform they already had.

And the winter version of the same story, from a German forum, where the discovery came from a meter reading in December:

“Auf Grund der Ablesung im Dez. 2013 habe ich festgestellt, das die Anlage viel mehr hätte bringen müssen.” “Bei der Prüfung wurde festgestellt, das 2 Strings auf Grund verschmorter Stecker nicht gearbeitet haben.” “Ich habe insgesamt ein Ertragsausfall von ca. 4000€.”

In English: “On the basis of the December 2013 meter reading I established that the system should have been producing a great deal more.” · “On inspection it was found that 2 strings had not been working, because of burnt connectors.” · “In total I have a lost yield of about €4,000.”

If solar production has dropped: the check that tells weather from a fault

An unusual winter is exactly the winter in which a real fault hides best. The headlines promise something out of the ordinary, a low December looks like the thing the headlines promised, the app shows a plausible number either way, and what finally tells an owner is usually an annual settlement, a meter reading or a chance glance at the app, weeks or months later. Because nobody can say what this winter will bring, “it was probably the weather” is not an answer anyone can check without a baseline.

The method is ordinary, and you can run it by hand:

1. Work in daily kWh, not monthly totals. Your app shows a day-by-day figure, and most monthly reports carry one too. A monthly total averages the days before a fault together with the days after it, so a fault that starts mid-month is already half smoothed out of the number by the time you read it.
2. Take one low week and look up the weather that week actually had over your address. Not last December: last December had its own weather, and comparing two different winters tells you about the winters, not about the roof.
3. Compare against a reference for your own roof. PVGIS will give you a monthly figure for your exact location, tilt and orientation; divide it down to a day, then read it against the sunshine that day had. The arithmetic is written out in how to check your solar production against the weather.

Solmiro runs that check every day and e-mails you when one of three things happens: production has stopped, data has stopped arriving, or output is well below what the weather over your roof allowed. None of those needs a weather forecast. Connecting is free; how it works.

If production has stopped and has not come back, the order to check things in is in solar inverter offline and not producing. If it is a shortfall rather than a stop, solar panels producing less than expected is the one to read.

The fault is cheap to have in December and expensive to keep until June, and December is when it is hardest to see.

What winter looks like on a real roof

Even in the darkest months, a 6.09 kWp Enphase roof in Almere kept producing: December 2024 came to 51 kWh, December 2025 to 80 kWh, January 2026 to 91 kWh and February 2026 to 184 kWh. The best day of December 2025 was 4.8 kWh, against 42.2 kWh on the roof’s best day, in June 2024, and its brightest month, May 2025, made 965 kWh: nineteen times the darkest. That is the normal winter shape.