As I have read, that power supply should be equal to demand. As it can't be stored .So basically this means power is generated in real time.

Consider an area where power comes from a Substation A, which receives power generated from power plant B.Of this area suppose each device is currently switched off till now power plant B should not generate anything.

Now I switched on appliances, then at the same time power should be generated by powerplant and transmitted also in real time. So how they Know when to generate? Or correct me If I am wrong somewhere. This might seem silly but I am bit confused.

  • \$\begingroup\$ You are forgetting here that things are done in a grid, there isn't one powerplant responsible for your house. \$\endgroup\$ – PlasmaHH Aug 27 '18 at 11:33
  • \$\begingroup\$ If the system cannot respond very quickly to changes in demand (such as with steam powered systems) then the schedulers have to predict changes in demand and ramp up but venting steam until load comes on. See Tea time in Britain causes predictable, massive surge in electricity demand. \$\endgroup\$ – Transistor Aug 27 '18 at 11:37
  • \$\begingroup\$ Also there are power plants designed to have a very rapid and short reponse time, have a look for Dinorwig power station in Wales, UK. \$\endgroup\$ – Solar Mike Aug 27 '18 at 12:13
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    \$\begingroup\$ How does a battery know how much power to supply to a torch when the button is pressed. If you can figure that out, you are most of the way to answering your own question. \$\endgroup\$ – Andy aka Aug 27 '18 at 12:14
  • \$\begingroup\$ ”Substation A, which receives power generated from power plant B” No, it will receive power for every power station connected to that grid. Also, read up on inertia. The grid has very little of it but far more than any home appliance you have at home. The grid will have a response time in the second range in case of hydropower. \$\endgroup\$ – winny Aug 27 '18 at 12:38

In order to meet changes in demand, the generation authority always has more generators running at any time than is needed to meet the present consumption.

Assume we just have rotating generators. The instant a load is switched onto the grid, the current that flows creates a braking torque in the generators, and they start to slow down. The instantaneous energy needed to power our load comes from the kinetic rotational energy stored in the generators.

As the generators start to slow down, the speed regulators kick in, and increase the mechanical power to the generators to maintain their speed.

The generation authority would notice that supply A was starting to get close to its maximum, so would start up supply B, just in case there was a further increase in demand. If that increase happened before B was up to speed and connected to the grid, then you would have at best a voltage sag (brownout) as A failed to cope, and at worst a power cut.

If a further increase did not happen, then B has been started, not supplied any power, and been a net cost. The margin between the maximum capacity of all the running generators, and the present consumption, is known as the 'spinning reserve'.

This is why forecasting is a very important part of running an electrical supply. You need to be able to guess from the weather, and from the TV schedules, when, and how big, the load demand spikes will be, to have enough, but no more, extra capacity already running to cope. Too much spinning reserve will reduce the profits. With too little spinning reserve, an unexpected demand spike could leave you having to explain an embarrassing and unnecessary power cut.


To look at power generation in an elementary way, assume there is one small generator and one load. Assume that the generator is driving by an engine. The engine and generator can be running without a load. The generator produces voltage but no current is flowing. The engine is burning some fuel. but it is producing only enough torque to overcome its own friction and the friction of the generator. The only power produced is used to supply losses and power the control system. When you switch on a light, current flows instantly. The generator's electrical load increases resulting increased load torque on the engine. The inertia of the rotating parts supply the initial energy causing the speed to decrease sightly. The speed decrease is detected by the regulator which opens the engine throttle. It is much like the situation when you are driving a car down a level road and suddenly encounter a hill.


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