Clearer skies lifted May irradiance well above average across coastal East Asia, with Japan, Korea, eastern China, Taiwan and the northern Philippines all seeing stronger solar resource, according to analysis using the Solcast API. A persistent high was the main driver, pushing coastal irradiance above long-run May averages, while a late-month rainband reduced irradiance inland across parts of southern and central China.
Japan saw some of the strongest increases in the region. Tokyo recorded May irradiance about 15% above its 2007–2025 average, while Osaka was 14% above average, making it Osaka’s sunniest May in the Solcast record. Seoul was more modest, at 6% above average, but still sat within the broader band of unusually warm and settled conditions across Japan and the Korean Peninsula. The clear skies that cause these increases also contributed to record May heat across northern China, Mongolia and Japan, with multiple locations breaking records. High temperatures were less good news for PV operators in these regions due to efficiency decreases from the heat.
The sunny coastal corridor extended from eastern China through Taiwan and into the northern Philippines. In the north, Manchuria remained about 9% above average, while
further south, Shanghai recorded May irradiance 13% above average, while the wider Yangtze Delta was around 6–10% above average. This corridor extended across the Taiwanese and Luzon Straights with Taipei and Manila seeing similar increases to Shanghai, more than 10% above average, helped by reduced storm activity.
Unlike some irradiance increases we’ve seen recently in Eastern China, where reduced aerosols played a significant role, last month’s increase we primarily due reduced cloud- cover with clear sky irradiance only a few percent above normal.
A contrast to the sunny conditions came late in the month. Around the last third of the month, a slow-moving rainband brought heavy rainfall across southern and central China as moist ocean area converged, including 24-hour totals near 100mm. Resulting in severe damage and lost lives, the net solar for the month in the affected areas were more mixed however with areas either a few percent above or below the average intermixed. Small deviations above and below average was also true for South East Asia.
Multiple national weather agencies including NOAA, Australia’s Bureau of Meteorology, and the China Meteorological Administration have forecast a strong to very strong El Niño peak later in 2026. Already tropical Pacific Ocean sea surface temperatures have been at record highs since April, including above recent El Niño periods. Both agencies forecast . For May, however, the story was less about El Niño and more regional: reduced cloud cover pushed coastal East Asia above average, while other areas were more mixed.
Solcast produces these figures by tracking clouds and aerosols at 1-2km resolution globally, using satellite data and proprietary AI/ML algorithms. This data is used to drive irradiance models, enabling Solcast to calculate irradiance at high resolution, with typical bias of less than 2%, and also cloud-tracking forecasts. This data is used by more than 350 companies managing over 300 GW of solar assets globally.
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