Capacity factor is the percentage of a power plant's maximum possible output that it actually delivers over a given period. If a 100 MW wind farm generated electricity at full power every hour of the year it would produce 876,000 MWh. If it actually produced 262,800 MWh - because the wind does not blow at full strength all the time - its capacity factor would be 30%. Think of it as a batting average for power stations: it tells you how hard a generator is actually working compared to its theoretical maximum.
This metric is essential for anyone trying to understand Ireland's energy transition. A country can install gigawatts of wind and solar capacity, but if the capacity factor is low, the actual electricity delivered will be a fraction of the nameplate figure. Conversely, a high capacity factor means each megawatt of installed capacity is working harder and delivering more value. In Ireland, where wind energy provided 33% of all electricity in 2024 from just over 5 GW of installed capacity, the capacity factor of wind farms is one of the most important numbers in energy planning.
🌿 Ireland's Wind Capacity Factor
Ireland's onshore wind fleet has historically achieved a capacity factor of approximately 28-30%, among the highest onshore figures in Europe thanks to the country's Atlantic exposure and consistently windy climate. In the five years to 2019, the recorded average was 28.3%. This means Ireland's 5 GW of onshore wind capacity delivers roughly the same energy output as 1.4-1.5 GW of always-on generation would.
For context, a modern gas-fired combined cycle plant in Ireland might achieve a capacity factor of 50-70% depending on how often it is called to run, while solar PV in Ireland typically achieves 10-12% given the country's limited direct sunlight hours. Offshore wind promises significantly higher capacity factors - the Hywind floating project in Scotland recorded 56.8%, and Irish offshore sites on the Atlantic coast are expected to achieve 40-50% or higher. This is because wind speeds increase substantially at sea, and the relationship between wind speed and energy output is cubic - even a small increase in average wind speed produces a large jump in electricity generated.
🌿 Why Capacity Factor Matters for Ireland's 2030 Targets
Ireland's Climate Action Plan targets 9 GW of onshore wind, 5 GW of offshore wind and 8 GW of solar PV by 2030, aiming for 80% of electricity from renewable sources. But these headline capacity figures only tell half the story. The actual electricity each technology delivers depends on its capacity factor, and the numbers vary enormously:
- 🌿 Onshore wind at 28-30% - 9 GW would generate roughly 22-24 TWh per year
- 🌿 Offshore wind at 40-50% - 5 GW would generate roughly 17-22 TWh per year
- 🌿 Solar PV at 10-12% - 8 GW would generate roughly 7-8 TWh per year
These ballpark figures show why offshore wind, despite having a lower installed capacity target than onshore, could contribute almost as much energy in practice. They also explain why solar PV, despite its rapidly growing installed base - Ireland added 0.53 GW of solar in 2023 alone and had over 1.6 GW installed by early 2025 - delivers a smaller share of total generation than its nameplate capacity suggests. Over the last decade, Ireland added onshore wind capacity at an average rate of 0.26 GW per year, though this slowed to 0.14 GW per year over the most recent five years according to SEAI's Energy in Ireland 2024 report.
🌿 Factors That Affect Capacity Factor
Several factors determine a power plant's capacity factor beyond the raw resource availability. Wind farms in Ireland's western counties (Kerry, Galway, Mayo) tend to achieve higher capacity factors than those in the midlands because of stronger, more consistent Atlantic winds. Turbine technology also plays a role - modern turbines with larger rotors relative to their generator rating can capture more energy in moderate wind conditions, boosting their capacity factor even if their peak output is the same as older models.
Capacity factor is also affected by curtailment - if the grid cannot accept all the power a wind farm can produce, the turbines are instructed to reduce output, lowering their effective capacity factor. Grid constraints and the SNSP limit of 75% both contribute to this problem. For solar farms, shading, panel orientation and Ireland's high latitude all constrain the achievable capacity factor. Energy storage does not improve a generator's capacity factor directly, but it can capture surplus output that would otherwise be curtailed, effectively improving the value of each installed megawatt.
🌿 Frequently Asked Questions
🌿 Is a higher capacity factor always better?
Not necessarily. A high capacity factor means a generator is running close to maximum output most of the time, which is ideal for baseload plants. But in a renewable-heavy system, different technologies complement each other. Wind and solar have lower capacity factors individually but when combined with storage and demand flexibility they can meet demand collectively. The goal is matching supply to demand across the whole system, not maximising any single generator's utilisation.
🌿 Why is offshore wind capacity factor higher than onshore?
Wind speeds at sea are stronger and more consistent than on land because there are no hills, trees or buildings to create friction and turbulence. Offshore turbines also tend to be much larger, with hub heights of 100 metres or more accessing even faster winds. The relationship between wind speed and power output is cubic, so a 20% increase in wind speed translates to a roughly 73% increase in energy captured. Irish Atlantic offshore sites are expected to achieve capacity factors of 40-50% or more.
🌿 What capacity factor does Ireland need to meet its 80% renewable electricity target?
Ireland's total electricity demand was 33,791 GWh in 2024 and is growing. Meeting 80% from renewables requires roughly 27 TWh of renewable generation. Achieving this depends on hitting the installed capacity targets for wind and solar and maintaining strong capacity factors. Any shortfall in capacity factor - due to curtailment, grid constraints or poor wind years - directly reduces the renewable share and increases reliance on gas.