Why are the counties so different?
Three main factors play into the regional variations in solar power output across Ireland: latitude, altitude, and local climate.
Latitude
How far north or south you are has a significant effect, with further south being more productive. This is because counties in the south of Ireland receive more direct sunlight and longer daylight hours throughout the year, which boosts solar energy generation potential compared to counties further north.
Local climate
Local climate is surprisingly important. Cloud cover is the main driver here that brings variety across the country. Yes, temperature has an effect (cooler is better), but again there's not enough difference here in Ireland for that to be significant.
County Cork leads all counties nationally at 8% above the national average — Ireland's most southerly county benefits from more direct sunlight year-round, making latitude the dominant factor there. Dublin is another strong performer at 6.4% above the national average, where the consistently clearer east-coast climate gives panels an extra boost despite being further north than Cork.
Kerry is an interesting case: despite sitting at a similar latitude to Cork, its more westerly position exposes it to greater Atlantic cloud cover, resulting in -3.3% below the national average. It's a good illustration of why local climate — not just latitude — matters.
The datasets used in such calculations have huge amounts of historical data, using things like satellite records of cloud cover, to try and predict the likely solar energy available at every location across the country. It's never spot on, but still, they are very impressive tools that are available and have a surprising amount of localisation built in.
Altitude
Altitude does have some effect. But, here in Ireland, the variation in altitude isn't significant enough to have a noticeable impact on solar output.
