There are two aspects to the reliability of electrical supply. First, the ability to produce a consistent supply to meet baseload demand – the level which is required all the time. Second, the flexibility to increase output to meet predicted peaks.
The output of an electricity supply containing a high proportion of intermittent energy sources can be difficult to predict or control. This can make matching electricity supply to consumer demand problematic.
Electricity supply must be matched to demand on a minute-by-minute basis, because electricity currently cannot be stored on a large scale for any significant length of time. If the electricity generated were to fall short of demand and no spare generating capacity was available, the National Grid might have to resort to electricity rationing.
Intermittent energy sources are those whose output is variable and subject to factors outside the operating company's control.
Wind and solar energy are considered intermittent and therefore unpredictable because their electrical output depends on environmental conditions – the speed of the wind and the amount of sunlight reaching the solar panel.
The higher the proportion of intermittent energy sources in a country's energy mix, the lower the overall predictability of its electricity supply becomes. Which means energy sources that can provide backup to ensure electricity demand can be met if the sun isn’t shining or the wind isn’t blowing will also be required.