The language of the grid
A few distinctions make electricity figures much easier to read.
Reviewed 2 November 2026 · Republic of Ireland
- MW — megawatt
- A unit of power: the rate of producing or using energy at a moment. One MW is 1,000 kilowatts.
- MWh — megawatt-hour
- A unit of energy. As an arithmetic example, a constant 1 MW for one hour produces 1 MWh.
- GWh — gigawatt-hour
- 1,000 MWh. This index uses GWh to make complete-day totals easier to compare.
- System demand
- The demand series published by the system operator. It is the denominator used in this index, and is not an individual household meter reading.
- Wind-to-demand ratio
- Wind generation divided by system demand, multiplied by 100. Always compare the same region and period.
- Capacity
- A generator’s rated power. It is different from its actual output and from energy generated over time.
- Forecast and actual
- A forecast is a prediction. Actual is the operational estimate of what happened. This index uses actual readings.
- Constraint
- A limit linked to network capacity or local operating conditions.
- Curtailment
- A system-wide restriction on renewable output. Dispatch-down reports provide the fuller breakdown.
- Normalised wind
- An annual statistical series which adjusts for variation in wind conditions. It is different from an instantaneous operational reading.
- Provisional data
- Figures that may be corrected later. Refreshing the index can therefore change a historical value.