Field performance and seasonal energy yield of a residential photovoltaic prosumer in Romania over three years

In the quiet corners of homes across Europe, a quiet revolution is taking place. Instead of relying solely on electricity generated far away and shipped through the grid, more families are installing solar panels on their roofs to make their own power. These households are becoming "prosumers," a blend of producer and consumer, creating energy when the sun shines and using it immediately. For scientists and engineers, understanding how these systems perform in the real world is crucial. While computer models can predict how much energy a solar array should generate based on weather patterns and location, the actual performance on a specific roof can differ due to dirt, shadows, or the way the equipment handles heat. In regions with distinct seasons, like Central and Eastern Europe, knowing exactly how much power a home produces month by month helps homeowners and policymakers understand the true value of solar energy. It reveals whether the sun's rhythm matches the family's needs and if the system is aging gracefully or struggling against the elements.

A team of researchers from Transylvania University in Brașov, Romania, decided to look closely at one such home to see how it fared over three years. They studied a residential solar installation that was connected to the national power grid but operated in a unique way: it produced exactly as much electricity as the household consumed, with no power sent back to the grid and none drawn from it. This perfect balance meant the home was entirely self-sufficient. The researchers gathered data from a digital monitoring system attached to the solar panels, collecting production numbers for every single month from January 2023 through January 2026. Their goal was to move beyond simple estimates and provide a clear, measured record of how a real solar system behaves in Romania's temperate climate, which features warm summers and cold winters.

The results painted a vivid picture of the solar year. The system produced a total of 9,250 kilowatt-hours over the three full years of monitoring, with an average of about 3,083 kilowatt-hours per year. The production followed a predictable, unimodal curve, rising steadily from the low light of winter to a bright peak in the summer and then falling back down as autumn arrived. The summer months, specifically July through September, were the heavy lifters, contributing between 33 and 37 percent of the total annual energy. When combined with the spring months of April through June, the warmer half of the year accounted for nearly 70 percent of all the electricity generated. In contrast, the winter months produced the least, with December often marking the lowest point of the year. The highest single month recorded was July 2023, when the system generated 420.7 kilowatt-hours, while the lowest was December 2024, with just 57.9 kilowatt-hours.

One of the most striking findings was the consistency of the system's shape, even when the total amount of power changed. The researchers compared the production patterns of 2023, 2024, and 2025 and found that the curve of the year remained remarkably stable. The system did not suddenly fail in the winter or spike unexpectedly in the spring; it simply rose and fell with the seasons in the same way each year. However, the total amount of energy did vary. The year 2025 saw a noticeable drop in production, generating about 9.7 percent less energy than the baseline year of 2023. The researchers investigated whether this drop was caused by the solar panels getting dirty, the equipment breaking down, or the panels simply wearing out over time. They ruled out the idea that the panels were degrading significantly, as the natural wear and tear of solar technology is usually a slow, gradual process that would not cause such a sharp, single-year drop. They also determined that dirt accumulation could not explain the entire loss.

Instead, the evidence pointed to the weather itself. The pattern of the 2025 decline showed that the winter months actually produced more power than in previous years, while the summer and autumn months produced less. This specific shift suggests that the skies were cloudier during the summer of 2025, reducing the sunlight available to the panels, while the winter was clearer. Statistical tests confirmed that there was no long-term downward trend in the system's health; the drop was a temporary fluctuation caused by the natural variability of the climate, not a failure of the technology. The system remained healthy and stable, simply responding to the changing sky above it.

Perhaps the most remarkable aspect of this study was the home's ability to match its energy production with its energy use perfectly. For every single month of the three-year period, the amount of electricity the solar panels generated was exactly equal to the amount the household used. There was zero electricity imported from the grid and zero electricity exported to it. This complete self-consumption is rare and difficult to achieve, as solar power is strongest during the day when many people are at work, and weakest at night when families are home. The fact that this home managed to balance its needs entirely with its own solar production suggests that the system was sized perfectly for the household's specific habits. It demonstrated that in Romania, a well-designed solar setup can allow a family to live entirely off their own roof power without needing to rely on external sources or complex storage batteries.

The study also addressed a common challenge in solar research: knowing the exact size of the solar array. The researchers did not have the official nameplate capacity of the panels, which is usually measured in kilowatts. Without this number, they could not calculate the standard efficiency metrics used by engineers. However, by comparing the actual energy produced against known weather data for the region, they were able to estimate the system's size with high confidence. Their calculations suggested the system was likely between 2.7 and 3.0 kilowatts. This estimate placed the home's performance squarely within the expected range for similar systems in Central Europe, confirming that the installation was operating at a level consistent with the local climate and technology standards.

Ultimately, this research provides a rare, detailed look at how a residential solar system performs in the real world over multiple years. It shows that while weather can cause significant year-to-year swings in total energy production, the underlying behavior of a healthy system is stable and predictable. The study confirms that solar energy in Romania is a reliable resource that follows the seasons, and that with the right sizing, a home can achieve total energy independence from the grid. By documenting these patterns with precision, the researchers have offered a valuable template for others to understand the true potential of solar power in their own homes, proving that even without perfect weather, the sun can power a home consistently and effectively.