Solar panels keep working through an Irish winter, but they produce roughly a quarter of what they do in May. A 4 kWp array in Dublin generates about 137 kWh in December against 495 kWh in May, and November through February together account for about 17% of the year's output. Nothing is broken when your app shows 4 kWh on a wet December Tuesday. That is what the physics allows, and the annual payback figures every installer quotes already assume it.
- December output: about 34 kWh per kWp installed, so roughly 137 kWh from a 4 kWp system
- Versus May: 124 kWh per kWp, about 3.6 times as much
- Nov to Feb combined: roughly 17% of the annual total. May to August is 47%
- Why: Dublin gets 7 hours 34 minutes of daylight on the winter solstice against 17 hours 5 minutes in June, and the midday sun sits only 13 degrees above the horizon
- Cold is not the problem. Panels are marginally more efficient in cold air. Daylight is the limit
- The winter gap between counties is wider than the annual one: Donegal makes 78% of Wexford's annual output but only 53% of its December output
Month by Month: What an Irish Roof Produces
The figures below come from PVGIS, the European Commission's Joint Research Centre model, for Dublin at 53.35°N. They assume the most common Irish residential setup: crystalline silicon panels, due south, 35 degree pitch, with 14% system losses for wiring, inverter and soiling.
| Month | Per kWp installed | 4 kWp system | Daily average, 4 kWp | Share of year |
|---|---|---|---|---|
| January | 37 kWh | 148 kWh | 4.8 kWh | 3.8% |
| February | 53 kWh | 211 kWh | 7.5 kWh | 5.4% |
| March | 86 kWh | 344 kWh | 11.1 kWh | 8.9% |
| April | 108 kWh | 432 kWh | 14.4 kWh | 11.1% |
| May | 124 kWh | 495 kWh | 16.0 kWh | 12.7% |
| June | 117 kWh | 469 kWh | 15.6 kWh | 12.1% |
| July | 113 kWh | 454 kWh | 14.6 kWh | 11.7% |
| August | 103 kWh | 413 kWh | 13.3 kWh | 10.6% |
| September | 89 kWh | 356 kWh | 11.9 kWh | 9.1% |
| October | 63 kWh | 252 kWh | 8.1 kWh | 6.5% |
| November | 45 kWh | 179 kWh | 6.0 kWh | 4.6% |
| December | 34 kWh | 137 kWh | 4.4 kWh | 3.5% |
| Year | 973 kWh | 3,890 kWh | 10.7 kWh | 100% |
Two things jump out of that table. May, not June, is the best month in Ireland, because June brings more cloud and higher panel temperatures. And the drop into winter is front-loaded: you lose more between September and November than you do between November and December, because by December there is not much left to lose.
The daily average column is the one that causes panic. A 4 kWp system averaging 4.4 kWh a day in December means plenty of individual days under 2 kWh, and a few bright frosty ones over 8 kWh. An average is not a floor.
Why the Drop Is So Steep
Three things compound, and the third is the one people forget.
Daylight length. Dublin gets 7 hours 34 minutes between sunrise and sunset on 21 December, against 17 hours 5 minutes on 21 June. That alone more than halves the window.
Sun angle. At solar noon on the winter solstice, the sun in Dublin is about 13 degrees above the horizon. In June it reaches about 60 degrees. Low-angle light travels through far more atmosphere and strikes a 35 degree roof at a poor angle, so each hour of winter daylight is also weaker than a summer hour.
Cloud. Irish winter weather is not just dark, it is overcast. Panels do run on diffuse light through cloud, at roughly 15 to 25% of rated output, which is why the December figure is 34 kWh per kWp and not zero. But a December low-pressure system parked over the country for a week will hold you near the bottom of that range.
If your roof faces east or west rather than south, the winter penalty is proportionally worse than the annual one, because the sun spends so little time anywhere near your orientation. Our guide to solar panel direction in Ireland works through the full orientation maths.
Cold Weather Helps. It Is the Dark That Hurts
This trips up nearly everyone. Photovoltaic panels are rated at a cell temperature of 25°C, and current N-type panels quote a power temperature coefficient near −0.29% per degree: LONGi's Hi-MO 7 and JA Solar's DeepBlue 4.0 Pro both publish exactly that figure. Older p-type panels sat closer to −0.4%. Either way, the sign is what matters. Above 25°C output falls, below it output rises slightly. A crisp 4°C January morning is thermally better for a panel than a muggy 26°C July afternoon.
So a cold snap is not bad news for generation. A bright, still, frosty day in late January can outproduce a grey, drizzly day in April. What kills winter output is hours of usable light, not the thermometer.
Snow that settles on panels stops generation completely until it clears, because the cells are covered. In most of Ireland this costs a handful of days a year at most, and a 30 to 35 degree pitch sheds wet snow quickly once any light gets through and warms the glass. Do not climb up to clear it. The generation you would recover is worth a couple of euro, and a wet winter roof is exactly how people end up in hospital.
The County Gap Widens in Winter
Everyone knows the south east does better than the north west over a year. What is less well known is how much that gap stretches in December, again using PVGIS at the same settings.
| Location | Annual, per kWp | December, per kWp | December, 4 kWp |
|---|---|---|---|
| Wexford town | 1,089 kWh | 35 kWh | 141 kWh |
| Cork city | 1,007 kWh | 32 kWh | 129 kWh |
| Dublin | 973 kWh | 34 kWh | 137 kWh |
| Letterkenny, Co. Donegal | 845 kWh | 19 kWh | 75 kWh |
Over a full year Letterkenny produces 78% of what Wexford does. In December it produces 53%. A Donegal system is not a bad investment, the annual figure is what pays the loan, but a Donegal owner should expect a deeper winter trough than the national numbers suggest, and should not benchmark themselves against a neighbour's app screenshot from Waterford.
For the full annual picture by county, see do solar panels work in Ireland.
What Winter Output Is Worth in Money
Generation matters less than what you do with it. In summer a typical house exports a large share of what it makes. In winter almost nothing is spare, because output is low exactly when the house is drawing most: dark mornings, dark evenings, heating on.
Take that Dublin 4 kWp system in December, 137 kWh. Assume 85% is used in the house and 15% exported. Those split assumptions are estimates, not measurements, and your own figures will differ, but the shape holds.
| December | May | |
|---|---|---|
| Generated | 137 kWh | 495 kWh |
| Used in the house (assumed) | 116 kWh (85%) | 198 kWh (40%) |
| Exported (assumed) | 21 kWh | 297 kWh |
| Saved on imports at 35c/kWh | €40.60 | €69.30 |
| Export credit at 19.5c/kWh | €4.10 | €57.90 |
| Total value | €44.70 | €127.20 |
December is worth about a third of May, not a quarter, because the units you keep are worth more than the units you sell. Electric Ireland and SSE Airtricity currently pay 19.5c/kWh under the Clean Export Guarantee and Energia 18.5c/kWh, against an import price nearer 35c. Every unit you self-consume in winter is worth almost twice an exported one. Current supplier rates are compared on our Clean Export Guarantee rates page.
One practical consequence: your export income, and therefore the €400 annual tax disregard, is earned almost entirely between March and September. Winter contributes very little to it.
Four Things Worth Doing Before the Clocks Go Back
1. Move flexible loads to the middle of the day. The winter generation window is roughly 10am to 3pm and it is narrow. A dishwasher or washing machine run at 1pm on a Saturday will pick up solar. The same cycle at 7pm will not, and 7pm is the most expensive part of the day on most smart tariffs.
2. Check for new shading. A 13 degree winter sun casts shadows three or four times longer than a 60 degree summer one. A hedge, a chimney or a neighbour's extension that never touches your array in July can sit across it for most of a January morning. Watch the roof at 11am on a bright winter day, not in summer.
3. Expect the diverter to go quiet. A solar immersion diverter only has something to divert once the house's own demand is met. From November to January there is often no surplus at all, so the water tank stops getting free heat. That is normal and it comes back in spring.
4. Switch your battery strategy from solar to tariff. This is the big one. A home battery charged from a winter roof will barely fill. A battery charged from a cheap night rate and discharged across the expensive evening peak earns its keep in exactly the months solar cannot. Our guides to night rate electricity and the best electricity plans for solar owners cover how to set that up, and battery storage in Ireland covers the economics.
What is not on the list is cleaning. Irish rain handles panel soiling for most homes, and a winter roof is the worst possible place to prove otherwise. See solar panel cleaning and maintenance for the cases where it genuinely matters.
Is Winter a Bad Time to Install?
No, and it is often the best time to organise it. The work takes a day or two and the panels start earning whenever they go up. What matters is where you are in the calendar when the good months arrive.
The paperwork is the slow part. Your installer must apply to ESB Networks to connect the system before installing it, and SEAI says that application takes at least 4 weeks, or 20 working days. Once you have an SEAI grant offer you have 8 months to complete the works. Start the process in November and you are generating for the whole of the February to September stretch that carries 80% of the year. Start it in April and you have already missed a chunk of it.
Installer order books are also shorter in winter than in the spring rush, so quotes come back faster.
The SEAI Solar Electricity Grant pays €700 per kWp on the first 2 kWp and €200 per kWp on the next 2 kWp, capped at €1,800 for a 4 kWp system, and SEAI has confirmed the €1,800 maximum stays in place for 2026. The home must have an MPRN and have been built and occupied before 2021. Full detail on our SEAI solar grant guide.
Get winter-honest numbers from three installers
Ask each quote to state the monthly generation profile, not just the annual total, and to show what you would produce in December. An installer who will only quote an annual figure is hiding the shape of it. Free quotes from SEAI-registered installers, no obligation.
Frequently Asked Questions
Yes, at reduced output. A 4 kWp array in Dublin produces about 137 kWh in December and 148 kWh in January, against 495 kWh in May, according to PVGIS modelling for a south-facing roof at 35 degrees. That is roughly 4.4 kWh on an average December day. Panels run on daylight rather than direct sun, so they generate through overcast weather at about 15 to 25% of rated output, but the short days and low sun angle mean the four winter months deliver only around 17% of the annual total.
About a quarter of peak-month output. December in Dublin yields roughly 34 kWh per kWp installed against 124 kWh per kWp in May, a factor of 3.6. November through February combined account for about 17% of annual generation, while May through August account for about 47%. The transition months matter more than people expect: October is already down to 63 kWh per kWp and September to 89 kWh.
No, cold slightly improves it. Panels are rated at a 25°C cell temperature and current N-type panels quote a power temperature coefficient near −0.29% per degree, with older p-type models closer to −0.4%, so output rises marginally as the cells get colder. A bright, frosty January day can outproduce a warm, hazy summer one. The winter drop in Ireland is caused by shorter days, a low sun angle and heavier cloud, not by temperature.
Settled snow blocks light and stops generation until it clears, but in most of Ireland this affects only a handful of days a year. A typical 30 to 35 degree roof pitch sheds wet snow quickly once daylight warms the glass. Clearing it yourself is not worth it: the recovered generation is worth a euro or two, and climbing onto a wet or icy roof carries real risk. Leave it.
Yes, but for a different reason in winter than in summer. From November to February there is rarely enough surplus solar to fill a battery, so its value comes from tariff arbitrage instead: charging on a cheap night rate and discharging across the expensive evening peak. Set up that way, a battery earns most of its winter return from the electricity plan rather than the roof. Homes on a flat 24-hour rate get far less out of it.
Usually not. The install itself takes a day or two in any season, and the paperwork is the slow part: your installer's connection application to ESB Networks takes at least 4 weeks or 20 working days, and an SEAI grant offer must be used within 8 months. Starting in late autumn or winter means the system is commissioned before the February to September stretch that carries the bulk of annual output. Waiting until spring risks missing part of it, and installers are busier then.
Because a 4 kWp system averages about 4.4 kWh a day in December, and individual overcast days can fall below 2 kWh. That is normal. Worth checking only if output stays near zero on a bright day, if one string or panel group reads zero while others generate, or if new shading has appeared. The winter sun sits about 13 degrees above the horizon at midday in Dublin, so obstructions that never shade the roof in summer can cover it for hours in January.
Considerably less, and the gap is wider than the annual figures suggest. PVGIS puts Letterkenny at 845 kWh per kWp a year against Wexford's 1,089 kWh, which is 78%. In December the same comparison is 19 kWh against 35 kWh, or 53%. The north west has a deeper winter trough, so a Donegal owner should judge the system on its annual yield rather than on a December reading.
Generation figures modelled with PVGIS v5.2 (European Commission Joint Research Centre) for crystalline silicon panels, due south, 35 degree tilt, 14% system losses. Daylight lengths from NOAA sunrise and sunset calculations for Dublin. Grant terms from SEAI, checked 17 September 2026. Electric Ireland and Energia export rates read from their own tariff pages on 17 September 2026; the SSE Airtricity rate is from our SSE Airtricity microgeneration page, last checked 12 September 2026. Temperature coefficients from LONGi and JA Solar published datasheets.