What about winter? What a La Costera solar roof honestly produces

The sceptic's question, which is a good question

When somebody from Canals or Ontinyent asks what solar panels produce in winter, they are not being difficult: they are asking exactly the question that should be asked. A solar installation pays for itself with the whole year's production, and anyone selling it by showing only the month of July is telling half the story. Winter exists, it produces less, and any serious study puts that on the table from day one.

The good news is that a La Costera winter is a kind one for photovoltaics. This is not a climate of whole weeks under a leaden sky: it is a Mediterranean valley where winter sunshine is a regular visitor, even if it shows up for fewer hours and lower in the sky. Understanding exactly what happens on the roof between November and February is the best vaccine against inflated expectations and against unjustified pessimism, the two classic and symmetrical mistakes.

Shorter days, but sunshine on most of them

The first winter factor is obvious: there are fewer hours of daylight and the sun crosses the sky lower down, so the panels receive less energy each day. There is no way around that and no technology changes it. What does change from one climate to another is how many of those short days are sunny ones, and there this district plays with an advantage: winter high-pressure systems bring long runs of clear skies, with a low but clean sun that the panels harvest from midday into mid afternoon.

What works in favour of a local roof in winter can be summed up briefly:

• Plenty of clear days between one weather front and the next, typical of an inland Mediterranean winter.
• A low but steady sun through the middle hours, when the house usually has consumption.
• Cool temperatures which, as we shall see, suit the panels well.
• Rain concentrated in episodes rather than in whole weeks of closed sky.

Cold is not the enemy: the panel performs better

Here comes the part that most surprises people hearing it for the first time: cold weather is good for a photovoltaic panel. Solar cells convert light into electricity more efficiently when they are cool, and they lose efficiency as they heat up, which is precisely what happens to them on July afternoons in full sun. A clear, cold January morning in Moixent, with the panel at a low temperature, is a scenario in which every ray of sunshine is put to especially good use.

This does not mean a January day produces more than a June one, because hours of sunshine rule and summer has far more of them. It means something more modest but real: winter does not deliver two punishments, only one. There is less light, yes, but the light that arrives is converted at a good rate. That is why clear winter days deliver production figures that surprise more than a few owners when they check their inverter app at midday.

Valley fog: in Xàtiva we know it well

La Costera is valley fog country: winter mornings when Xàtiva, Canals and l'Alcúdia de Crespins wake up under cover while the hillside next door sits in sunshine. That fog is the local factor with the biggest influence on winter production, and it deserves telling without drama. First: fog does not switch the panels off. The diffuse light that filters through keeps generating electricity, less than under a clean sky, but not zero. Second: typical valley fog is the kind that leaves, lifting mid morning or by midday, after which the day behaves like any other sunny day.

The practical effect is that morning production is trimmed and the bulk of the day's output concentrates in the middle hours and early afternoon. It is one of the reasons the orientation and shading of each specific roof matter so much: a roof that clears early towards the south does not produce like one boxed in that only sees direct sun late in the day. That is a conversation for a study of your roof, not for a leaflet.

December and January: less, but not zero

December and January are the weak months of the solar year, and denying it would be taking people for fools. The days are at their shortest, the sun runs low and the fog plays its part. But from there to imagining the panels standing idle is a huge leap: they keep producing every day, with very respectable peaks on clear days and smaller contributions on overcast ones. The house still runs a fridge, a washing machine, lighting and, increasingly, some heating load, so that winter production easily finds somewhere useful to go.

Besides, the weak months number two or three, not six. February already recovers clearly, and November tends to post better figures than people imagine. Judging an installation by how it behaves in the darkest week of the year is the mirror image of the mistake made by judging it on August alone.

Systems are sized for the whole year, not the worst day

A grid-connected installation is not designed to survive the 21st of December: it is designed so that the balance of the full year works out. In the Xàtiva area, the PVGIS database places annual production between 1,350 and 1,500 kWh per installed kWp per year, with figures around 1,500 for a south-facing roof in good conditions. An east-west roof is by no means ruled out: it yields between 82 and 85 per cent of what it would give facing south, and in exchange it spreads production more evenly between morning and afternoon. For context, 2025 sector data from APPA puts the average Spanish installation at 5.5 kW with a typical payback of 6 to 7 years, before any tax deductions.

The month-by-month split of that annual production is precisely what the study of each roof shows, with its real orientation, pitch and shading, and there is no sense in quoting generic monthly figures here that would not be yours. A serious study should show you, at the very least:

• The estimated month-by-month production of your specific roof, calculated with PVGIS on its real orientation and pitch.
• Shading from nearby buildings, hills or trees and how it shifts through the seasons.
• Your consumption by time band, to see how much production you will use directly.
• The complete annual balance, winter included and not hidden inside an average.

Electric heating plus solar: what to expect, said honestly

The next logical question is whether the panels can carry the heating. The honest answer is: partly, and it depends on how you heat. Heating demand concentrates in the months of lowest production, and a large share of the consumption falls at night, when the panels produce nothing. Anyone expecting electric heating at no cost at all in January is heading for disappointment, and we would rather say so before than after.

That said, the combination makes a great deal of sense when planned well. A heat pump running during sunlight hours can lean substantially on the panels' production, above all on those clear winter middays that are so common here. The strategy is to align consumption with the sun as far as possible:

It is not magic, it is management: January heating will cost less with panels than without them, but it will still show on the bill. Anyone telling you otherwise is telling you a story.

• Heat the house through the middle hours of the day and reach the evening with that warmth banked.
• Programme the water heater, washing machine and dishwasher into the sunny window.
• Favour a heat pump over plain resistance radiators, which consume far more for the same warmth.
• Check the insulation: every draught through old window frames eats production.

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