What a roof in La Costera produces, with data and no hot air

How many kWh does one kilowatt peak produce a year?

To compare roofs, counting panels is useless: what counts is how much yearly energy each installed kilowatt produces. In Xàtiva, a south-facing roof at the right pitch delivers around 1,500 kilowatt hours per kilowatt peak per year according to PVGIS, the European Commission's reference tool, which we use in every study.

In practice, across the comarca we work within a range of 1,350 to 1,500 depending on orientation, pitch and shading. That is the range behind our calculator and every study, so we never sell expectations the roof cannot meet.

What adds and what subtracts on your roof?

Four factors rule that number, in this order:

• Orientation: south is optimal. An east-west pitched roof produces around 15 to 18 percent less over the year, though it spreads production better between morning and evening.
• Shading: a chimney, the neighbour's house or the mountain at dusk weigh more than they seem, because they do not just subtract the shaded area but the whole string's output if the design does not account for it.
• Pitch: at our latitude the yearly optimum sits near 35 degrees, but the area's typical clay-tile roofs come close and the real difference is usually small.
• Dirt and heat: summer dust and high temperatures trim production. They are the two factors you can actually manage after installing, with sensible cleaning and proper panel ventilation.

Why is August not the best month?

It surprises almost everyone: the best production months per kilowatt are usually not the hottest ones. In extreme heat, panels lose efficiency, and the long mornings of May, June and July more than make up for August's punishing sun. That is why a properly sized installation is calculated on the twelve-month profile, not with summer in mind.

From the estimate to the meter

PVGIS estimates; the meter rules. Our job is to make the two figures match: first with a study built on your actual roof, its shading and orientation measured, and then with daily monitoring, which compares what your installation produces against what it should produce on that specific day. When the two curves drift apart, something is up, and we look at it before you notice it on your bill.

If you want your roof's number rather than a brochure's, that is exactly the first sheet a study delivers.

Two roofs on the same street, two different meters

Around Xàtiva, PVGIS puts the reference yield between 1,350 and 1,500 kWh per kilowatt peak per year. That range is the frame for the comarca, but on a single street you will find roofs sitting at the top of it and roofs that never get there, with identical panels. What separates them is hardly ever visible on a June site visit: it shows in December.

The winter sun runs low, and an obstacle that casts no shadow at all in summer can eat the first or last hours of the day for months. The house at the foot of the castle hill produces like any other in August, but in December the hillside steals its morning. The stair housing on a flat roof, which throws a hand's width of shade in June, knocks out a whole row of panels by mid afternoon in January. And the neighbour's cypress, harmless-looking on the visit, crosses the slope from side to side in winter.

That is why a serious simulation uses the sun's path across the whole year, not the impression from the day of the visit. And that is why two quotes with the same number of panels can be two very different installations: one is designed around the winter geometry and the other never looked at it.

Why does production flatten out at midday?

There is a detail almost nobody explains at handover: the panels' peak power is usually higher than the inverter's rated power, and that is a design decision, not an oversight. Panels almost never deliver their theoretical maximum: temperature, sun angle and dirt see to that. Sizing the inverter for a peak that shows up a few hours a year would mean overpaying the rest of the time.

The consequence shows in the app: on clear, cool spring days the production curve climbs, hits the ceiling and stays flat for a while around midday. That plateau alarms plenty of owners, who ring up convinced something has failed. It has not: the inverter is trimming a brief surplus, and what is lost there is a small slice of the year in exchange for properly sized equipment.

What matters is telling the cases apart. A high, clean plateau in spring is design. A curve that is flat but low, or saw-toothed at midday, is something else: new shade, dirt or a fault worth checking. That distinction is exactly the kind of work daily monitoring does for you: not glancing at one day's graph, but knowing what shape each month should have and flagging it when it bends.

Frequently asked questions