A network that has to stay balanced
The electricity grid is the connected system of power stations, wires, substations and control rooms that delivers electricity to homes and businesses. Its defining feature is that electricity is hard to store in large quantities cheaply, so for the most part the amount put into the network has to match the amount taken out at every moment. If supply and demand drift apart, the frequency of the alternating current moves away from its target value, and operators have to correct it quickly or equipment can disconnect to protect itself.
Grid operators therefore work all the time to forecast demand, line up enough generation and keep reserve capacity ready. Most people only notice the system when it fails, which is a sign of how well the balancing normally works.
The path from generator to socket
Electricity moves through several stages.
- Generation. Power stations and other sources produce electricity. These include gas, nuclear, wind, solar, hydro, biomass and others, depending on the country. The renewable energy sources article describes how each of those works.
- Transmission. High-voltage lines carry power over long distances. Using high voltage reduces losses along the way.
- Distribution. Substations step the voltage down, and local networks carry it to streets and buildings.
- Your home. A meter records how much you use. Some homes also feed electricity back to the grid, for example from solar panels.
A small amount of energy is lost along the wires, so a unit generated is slightly more than a unit delivered.
What makes demand change
Demand is not constant. It follows patterns:
- Time of day. Typically it is low overnight, rises in the morning, and peaks in the early evening when people cook, heat or cool their homes and switch on lighting and appliances.
- Season. Cold winters or hot summers can raise demand through heating or air conditioning, depending on the climate and how homes are heated.
- Weather and events. Cold snaps, heat waves and big events can shift demand from usual patterns.
Operators use weather forecasts and historical data to predict these swings, but there will always be some error that has to be handled in real time.
How supply keeps up
Different sources play different roles.
- Always-on or baseload plants run steadily for long periods. Nuclear is an example in countries that have it.
- Flexible plants, such as gas turbines and some hydro, can be turned up or down fairly quickly to follow demand.
- Variable renewables, such as wind and solar, produce power depending on weather and daylight. They have low running costs and no direct emissions in operation, but their output cannot simply be dialled up on demand.
- Imports and exports. Neighbouring countries or regions are often linked by interconnectors, which can move power in either direction when one side has a surplus.
The operators decide which plants to use largely by cost and availability, turning on cheaper sources first and calling on more expensive ones to cover the peaks. The details differ between countries and markets.
Why the grid mix changes through the day
Because different generators are online at different times, the carbon content of each unit of electricity changes. On a sunny, windy afternoon with moderate demand, a large share may come from renewables. In a still, dark evening peak, more may come from gas or other dispatchable plants. The mix also varies from season to season, and from country to country.
This is why electricity is not equally “clean” at every hour, and why the numbers used in a footprint calculation are usually averages. It also explains the weakness of annual-matching claims in some green tariffs, because a supplier can match your total yearly use without matching the time of day you use it.
Storage and flexibility
As wind and solar grow, the challenge shifts from producing enough electricity to having it where and when it is needed. A few tools help.
- Batteries can store power for minutes to hours and respond very quickly. They are useful for smoothing short-term swings and shifting solar power into the evening, though storing for days or weeks is much harder and more costly with current technology.
- Pumped hydro stores energy by pumping water uphill and releasing it later. It is well established but depends on suitable geography.
- Other storage such as heat stores, compressed air and hydrogen is in use or being developed, with varying costs and efficiency.
- Demand flexibility means moving some use to times when supply is plentiful. Examples include charging electric vehicles overnight, running a dishwasher at a set time, or pre-heating a home. Smart home devices and time-of-use prices are used to make this easier.
- Stronger networks with more interconnection can help move surplus power to where it is needed.
Storage has its own trade-offs, including losses during charging and discharging, materials in batteries and the cost of building it. No single option solves everything, and most grids are expected to use a combination.
What this means at home
The grid does not require you to change anything, but understanding it can inform some choices.
- Using less electricity helps in every case, since it lowers demand on the system at all hours.
- Moving flexible tasks away from the evening peak can help the grid and sometimes your bill, if your tariff rewards it. Whether it reduces emissions depends on what is generating at those times in your area.
- Standby power adds a constant background load across the year, which is small per device but adds up.
- The grid’s mix is changing over time in most places, so appliances that run on electricity, such as heat pumps, tend to get cleaner as the grid does.
Your own energy meter readings can show when you use the most power, which is a practical starting point.