Last month, Alejandro Diego Rosell – energy consultant, professor and analyst with more than a decade in the photovoltaic sector – discovered something that does not fit with what we all believe about solar energy: his balcony produced the highest generation day of the year and also a day of absolute zero. Same month, same installation, but opposite results.
The paradox is not a flaw: it is exactly how a solar balcony works in a real city. And what his case reveals dismantles many of the myths of urban self-consumption.
The solar balcony phenomenon. The explanation begins with a phrase that Diego repeats in the interview he gave us in Xataka: “The real performance depends more on the angle, shadow and geometry of the building than on the calendar month.” Its panels are installed almost vertically, an unusual orientation on roofs but very common in Spanish apartments. And this completely alters the classic pattern of solar production.
- Record day: 2.35 kWh on a cold, clear day in November.
- Zero day: November 15, with 0% apparent production.
And why? It is precisely because of the combination of verticality and battery. Your installation now works with plug-and-play batteryand that introduces a little-known phenomenon: “The battery needs a minimum current to start charging. If the output is too low, it does not accept it and does not send anything to the microinverter either.” In other words, some energy is generated, but it is so little that the battery does not activate and the system does not account for it. That minimum production is left out of the records, which causes some days to appear as “zero” even though they really are not.
Position matters. Alejandro Diego’s experience uncovers several lessons that almost no one knows before installing one of these kits. On the one hand, a vertical panel performs better in winter. “In winter the sun is so low that it looks at you from the other side of the street,” says the energy analyst. And it makes physical sense because the sun, being low, affects almost perfectly on a vertical panel and the cold makes for better performance. In fact, this idea is not anecdotal, verticality is beginning to be adopted even in professional installations, as is the case of the company Over Easy Solar in the Valencian Community.
On the other hand, shadows are the great invisible enemy. “Shadows travel,” insists the energy consultant. A railing that barely touches the glass panel in June can ruin 20% of the day in January. A neighbor’s awning can cut entire hours of production. And tall buildings create cast shadows that move like clockwork.
The batteries and the fine print. Here we come to the kit question: “It’s not plug and play.” The Master in Renewable Energies (MERME) professor details that Plug-and-play domestic batteries help—they shift consumption, allow prolonged injection, improve peak utilization—but they also bring surprises: very low production simply does not enter the system, there are efficiency losses in the charge-discharge cycle, and they weigh more than people imagine.
In a market where Ikea, EcoFlow, Zendure or even electric ones are launching batteries “for everyone”, this clarification matters.
Urban photovoltaics are unpredictable. If there is one thing that Diego is clear about after almost a year measuring every watt that enters his balcony, it is that photovoltaics in the city do not follow the rules that one imagines from the outside. In its installation, the data changes abruptly depending on the angle of the sun, the presence of shadows or even the type of cloud cover. And there is no need to go into theories: you see it in your daily life.
In December, For examplehas reached more than 2 kWh in a single day. It seems counterintuitive—especially considering that December is one of the months with the fewest hours of daylight—but the explanation is simple: the low sun hits a vertical panel almost head-on and the cold improves the electrical performance of the module and the microinverter. However, in April – with longer days and clear skies – there were days that did not even reach 1.5 kWh. “The angle of the sun changes everything,” he explains. In spring the sun begins to rise, hits the panel from above and the verticality penalizes more than intuition suggests.
The clouds also influence. This opens another chapter: even small passing clouds can reduce production in a matter of seconds, because they block direct light—the one that really triggers the generation—and leave only the diffuse light, much less usable in such an angle-dependent installation. When the sky is completely covered, the situation is even clearer: production usually sinks to 5–10% of the daily potential, figures that the consultant has seen repeated over and over again.
These same extreme oscillations are common in the thousands of solar balconies installed in Germany: very good days, very bad days and a performance that depends more on urban physics – shadows, orientation, tall buildings that cut off the sun at different times – than on the calendar or the general weather.
The conclusion, in Diego’s own words, is that a solar balcony is educational, useful and surprisingly efficient for its size, but not magical. It produces, yes, but it produces according to the physical reality of the building, not according to the mental idea that many have before installing one.
The real barriers to installing one. In Spain there is a particular ecosystem: plug-in kits are limited by law to 800W, neighborhood communities may require permission if they are on a façade or railing and the regulations require electrical protections and, sometimes, a bidirectional meter. Alejandro Diego had no problems with his community—”from the street you can hardly see it”—but he admits that in other buildings it can be a bottleneck.
On the other hand, in countries like Germany, the regulation explicitly protects the right to install them. The result has been more than 1.5 million of kits operating and half a million installed in just one year.
The false fear of saturating the network. One of the most repeated fears in neighborhood communities is that, if many residents install solar balconies, the building’s network could become saturated. The analyst dismisses it without nuance: “Very unlikely or bordering on impossible.” The microinverters in these kits are limited by law to between 300 and 800W, a minimum power compared to the peaks that any community already supports due to elevators, water pumps, garages or appliances running at the same time. Even a hundred neighbors producing simultaneously would generate a smaller electrical impact than the start of an elevator.
The paradox is that, far from “stressing the network”, these small systems tend to lighten it, cutting the demand of each home during sunny hours. And that relief translates into money: according to Diegoa well-oriented solar balcony can save between 10% and 25% of the bill, depending on habits and base consumption. In his own case, in March – with 400 W and without battery – he paid €48.90 in Madrid; Without self-consumption, it would have been around €65. With increasingly cheaper panels and typical paybacks of 4 to 6 years, the equation is no longer just an energy one but also an economic one.
Where urban photovoltaics is evolving. According to the energy consultant, the next thing will be: more efficient microinverters, plug-and-play batteries with AI and specific panels for facades and bifacials.
These forecasts fit with the BIPV (Building Integrated Photovoltaics) wave: transparent solar windows, bifacial solar railings, bifacial vertical panels which work best with diffused light and even cruise ships with photovoltaic balconies capable of covering almost half of the consumption of each cabin. With the EU mandating zero-emission buildings from 2030, every façade will be an energy opportunity.
Tips before buying. Alejandro Diego Rosell has pointed this out in four points:
- Look at the angle of your balcony in winter, not summer.
- Choose an approved microinverter.
- Start with one or two panels and expand.
- Use a ammeter clamp to avoid spillage.
A close up from the balcony. The story of Diego’s solar balcony shows that the energy transition begins small: on a balcony, in a two-kilo panel that is hung almost like a flower pot. It is not a machine of instant promises, but it is a real piece of the urban energy puzzle: it produces, teaches, saves and changes the way we understand electricity at home.
And perhaps, in a few years, when the facades are invisible generators and the windows produce electricity without anyone noticing, we will remember that it all started with something as simple as a solar balcony.
Image | Marco Verch
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