Four Years of Real Data Reveal How a Ugandan Island Solar-Diesel Mini-Grid Really Performs

On Bugala Island in Uganda’s Kalangala district, a hybrid power plant has been quietly keeping the lights on for thousands of households since 2014, and a new four-year analysis of its actual operating data now offers one of the most detailed real-world portraits yet of how a solar-diesel-battery mini-grid performs in rural Sub-Saharan Africa. The study, published in Frontiers in Energy Research, examined monthly energy records from the 1.6-megawatt Kalangala plant between July 2019 and June 2023, providing rare empirical evidence at a time when most performance assessments of off-grid systems rely on simulations or short monitoring windows.

The plant, located in Bukuzindu village about 17 kilometers from Kalangala town, combines a 0.6-megawatt solar photovoltaic array with 1.0 megawatt of diesel generation, battery storage, and a distribution network stretching across the island. It was built at an estimated cost of 17 million US dollars and currently supplies more than 5,523 domestic customers and 67 commercial connections, including hotels and small enterprises. Its solar array consists of 2,832 modules rated at 235 watts peak each, split into units feeding grid-connected inverters and MPPT charge controllers, while a battery bank of two strings of 240 lead-acid cells, each rated at 686 ampere-hours at 2 volts, provides roughly 658.56 kilowatt-hours of storage.

The analysis revealed that the plant generated an average of 203.1 megawatt-hours of electricity per month, equivalent to roughly 2,436 megawatt-hours annually. Of that total, 174.1 megawatt-hours on average reached consumers, 22.4 megawatt-hours were lost in generation and transmission, 0.96 megawatt-hours were consumed by the plant’s own auxiliary equipment, and 5.4 megawatt-hours went into charging the battery bank. In other words, about 86 percent of everything the plant produced was actually delivered to the island’s homes and businesses, a figure the researchers consider acceptable for a system of this scale and configuration.

Beneath those headline numbers lies a more granular picture of how the two generation sources shared the burden. The solar array produced between 49.2 and 83.3 megawatt-hours per month, averaging 65.5 megawatt-hours, while the diesel generators ranged from 85.02 to 240.18 megawatt-hours, averaging 137.15 megawatt-hours. Solar’s monthly share of total generation swung from 23.67 percent in November 2020 to 86.12 percent in June 2020, with an average contribution of 49.69 percent. The researchers observed that solar’s contribution was inversely proportional to diesel output, exactly the complementary behavior a hybrid design is meant to achieve: when the sun does not deliver, the generators step in.

The demand pattern on the island explains much of this balance. Most consumers are residential households, so electricity demand peaks in the evening after sunset, precisely when solar output falls to zero. The diesel generators therefore carry the evening load, while the solar array covers daytime demand. Notably, the variability coefficient, the ratio of standard deviation to average output, was 10.29 percent for solar, 18.48 percent for diesel, and just 11.57 percent for the combined hybrid system, indicating that the plant as a whole delivered remarkably stable output across the study period. That stability matters for forecasting and for planning future expansion on an island whose electricity demand continues to grow.

The study also documented a striking demand spike in November 2020, when transformer-metered energy peaked. The authors attribute this partly to Uganda easing COVID-19 restrictions on 9 November 2020, which allowed businesses, particularly in the tourism sector that anchors the island’s economy, to resume operations. The plant manager also reported that a palm oil processing factory requested continuous supply after its own steam turbine broke down, though the researchers note there is no publicly available evidence confirming that event drove the increase.

On the technical performance side, the solar array achieved an annual average performance ratio of 65.6 percent, meaning roughly two-thirds of the theoretical energy available from incident sunlight was converted into useful electricity after all system losses. Its capacity factor stood at 13.4 percent, constrained by the fundamental intermittency of solar resources: generation is limited to daylight hours and further shaped by cloud cover, seasonal variation, module temperature, and system losses. The diesel generators posted a capacity factor of 18.88 percent, reflecting their role as a dispatchable backup rather than the primary energy source, an operating strategy that reduces fuel consumption, cuts emissions, and extends generator lifetime. The mini-grid as a whole achieved a capacity factor of 17.39 percent.

Losses emerged as a key finding with direct policy implications. The combined generation and transmission loss averaged 11.03 percent, ranging from 8.66 percent in November to 13.21 percent in February. Breaking this down, generation losses averaged 3.13 percent while distribution losses averaged 7.90 percent. The distribution figure is notably efficient compared with Uganda’s national grid, which recorded average distribution losses of 17.74 percent between 2021 and 2025. However, the mini-grid’s transmission line loss of 7.9 percent was nearly double the national grid’s average transmission loss of 4.42 percent over the same period, and the researchers trace this to a structural mismatch: the island’s 33-kilovolt network stretches approximately 144 kilometers, while published studies suggest 33 kV lines are typically suitable for distances of roughly 30 to 60 kilometers. As demand grows, higher loading on the low-voltage transmission line drives up current flow and, with it, resistive losses.

The authors recommend that the Ugandan government and electricity-sector stakeholders conduct a detailed feasibility study to upgrade the island’s principal transmission network from 33 kV to 66 kV, supported by load-flow, voltage-drop, fault-level, and economic analyses. Raising the transmission voltage would reduce the current needed for a given power transfer, cutting resistive losses, improving voltage regulation, and providing headroom for the island’s projected demand growth. They frame this not as a quick fix for current loss levels but as part of a long-term transmission reinforcement strategy to keep the island’s infrastructure reliable as its economy expands.

Compared with other systems, the Kalangala plant holds its own. Its performance ratio and capacity factor exceed those of an off-grid mini-grid studied in southern Ethiopia, which achieved 42 percent and 13.0 percent respectively, though they fall short of Uganda’s own Soroti solar plant, which recorded a performance ratio of 75.84 percent and a capacity factor of 19.07 percent. The researchers attribute the gap partly to lower average daily solar radiation at Kalangala, 4.94 kilowatt-hours per square meter per day versus 5.85 at Soroti, and partly to dust accumulation from the plant’s proximity to a busy road, inverter conversion losses, battery storage losses, and additional power electronics losses inherent in the hybrid configuration. The study’s limitations include the inability to assess the levelized cost of electricity or payback period, since the plant’s distribution tariff has only been declared through the first quarter of 2022. The authors call for updated tariff data and further techno-economic analyses of hybrid renewable energy systems across Uganda, arguing that such evidence is essential for planning, designing, and scaling the decentralized systems that will carry the country toward its Vision 2040 target of 52.4 gigawatts of installed capacity, with solar photovoltaics expected to supply roughly 12 gigawatts of that total.

Subject of Research: Technical performance evaluation of a solar PV-diesel-battery hybrid mini-grid in Kalangala, Uganda

Article Title: Performance of a solar PV-diesel generator-battery mini-grid using actual data: a case of Kalangala power plant in Uganda

Article References: Performance of a solar PV-diesel generator-battery mini-grid using actual data: a case of Kalangala power plant in Uganda. (n.d.). https://doi.org/10.3389/fenrg.2026.1785084

Image Credits: AI Generated

DOI: 10.3389/fenrg.2026.1785084

Keywords: solar PV, mini-grid, hybrid energy system, Uganda, rural electrification, performance ratio, capacity factor, transmission losses, battery storage, diesel generator, Kalangala, renewable energy