- 1Network constraint — power cannot get from here to there.
- 2Oversupply — more generation than demand at that moment.
- 3System stability — the grid needs certain services to stay balanced.
Three reasons output gets reduced.
01
Constraint: the wires are full
The most common reason is a network constraint. Generation is often concentrated where wind and sun are strongest, and demand is often somewhere else. The cables between them have a maximum capacity, and when that is reached, output has to come down.
02
Oversupply: more than the country needs right now
At times of low demand and high output — a mild, windy night, for instance — generation can exceed what the whole system is using. Electricity has to be produced and consumed simultaneously unless something stores it, so output is reduced.
03
Stability: keeping the system within safe limits
The grid needs frequency and voltage held within tight limits. Occasionally the operator changes the mix of what is generating to keep those services available. This is a smaller cause than constraint, but it is a real one.
04
What actually reduces it
Three things help: building more network capacity, moving demand to the hours when power is abundant, and storing energy close to where it will be used. The first is slow and expensive. The second and third are where flexibility and battery storage come in.
For residents
It is the reason 'use electricity at a different time' is a genuine environmental and financial lever, not just a way to pay a lower rate.
For housing providers
Local constraint patterns should shape where a pilot is sited. A portfolio in a constrained area is a stronger candidate than one in an unconstrained area.
Assumptions and limitations
- Causes are described qualitatively; the balance between them changes by region and by year.
Sources
- New proposals to cut bills with community batteries
Department for Energy Security and Net Zero, 4 June 2026
Where we are today