Why/how does using less electricity at home cause the power plant to generate less electricity?

Question

I was always under the assumption (wrongly I suppose) that power plants would generate predetermined set levels of electricity, based on the number of connections on the grid, and time of day, and that they would only generate less if there was a significant drop in wattage used. It just seems more efficient.

But clearly I have no idea how they work really. I have a very basic knowledge of AC power.

Answer 1

Power plants only generate power thats going to be immediately used, electrical energy cannot be efficiently stored.  Therefore if you turn a light off at home or something, the local power plants generators make a small but finite adjustment to the fuel / steam feed that causes the generators to rotate.

Answer 2

The voltage in the electrical system is similar to the water pressure in a garden hose. The amperage is similar to the water flow. The generator is like a water pump trying to keep constant pressure. If we open the nozzle on the garden hose there will  be like a demand on water (flow) and the pump (generator) have to move more water to keep the pressure(voltage) constant. If we close down the nozzle the pump can slow down to. This is very simplified but do illustrate how it works.

In a hydro power plant this is regulated with the water flow thru the turbines. The turbines runs the generators. Mounted on the same driv shaft (axle). The generators in use have to keep constant rpm's to make the desired 60Hz frequency. And the amonut of currency (Amp's)that are used in the network (based on the power demand or how many lamps and stoves we turn on) is regulated  by how much Amp's that's used to electro magnetize (the magnetic power) the rotor of the generator. F there is no demand on electricity we store the energy in the dam above the hydro power plant. That electro magnetic power is what forces the electrons to move in the power lins to our bulbs and stoves. Voltage from the generators is fairly low and amperage is very high. This will at power plant be transformed into very high voltage and more moderate amperage. This to minimize resistance losses in the power line network and lower gauge wiring (less expensive and less heavy) can be used.

Near consumer areas, cities, industries etc, voltage will be transformed, in local transformer stations, down to, what ever the local system require, like 220 volts for regular housing.

Long distance power lines can be 100-500KiloVolts and maybe 1000 Amps, many hundreds of Mega Watts

Local power lines can be around 220V-10 KV

To not bore  you to death I wont mention the 3-phase system, with Neutral and Ground

Answer 3

A power plant generates to maintain the line voltage, as less power is consumed the line voltage increases and to maintain the set value the generators is slowed down burning less fuel, if the voltage gets high enough they can shut a generator down till the voltage drops again.

Answer 4

Some power plants are "base load" they operate at 100% capacity.  Others are peak or load following, as trollazoid described, and a few are used as synchronous condensors to modulate frequency and voltage.

All are sychronized to the grid.  Meaning they must operate at a preset frequency (60 Hz) and voltage (120 V).  Only the amperage produced changes, and this affects the total wattage produced.

The grid must only produce as much power as is demanded (accounting for some losses due to transmission losses and shorts).  The easiest way to visualize the grid is as a balloon full of tiny holes, each hole being a load such as an appliance and the plants are air entering the balloon.  The idea is to keep the balloon constantly at exactly the same size.

As each load is reduced, or hole plugged, the supply of air must be matched to keep the ballon size constant.  Therefore as each appliance or light is turned off it means the power plants need to produce less power.