In the UK every supplier meters its power to the grid, both in and out.
All consumers have their power metered too.Some large consumers have their own generation capacity and the nature of their operations mean that they cannot easily change how much they produce(such as steel mills or large CHPS plats).They have import/export meters installed.
Power suppliers will guaruntee to supply their consumers using the network which makes a charge based on the power transfer.
The power suppliers will have several points of generation and will generally use the cheaper ones first and as demand rises the cost of generation rises.
Suppliers will buy power from a wholesale pool, as the demand rises then the cost of generation rises so the pool price rises.
Some generators have sophisticated monitoring systems that automatically bring in their additional generation sets when the prices per unit becomes viable.
Consumers agree to various tariffs, they will agree to a peak demand price and if they exceed this they pay for the extra power at a premium.The monitoring equipment has a needle that moves to the peak position and stays there and until reset once every month or quarter.This means that one minutes worth of excess demand can cost a consumer a month or more of peak demand penalty.
By limiting the peak demand of consumers in this way suppliers can gear up for a baseline generation capacity more efficiently without having to buy as much power from the pool which is usually more expensive than their own capacity.
Consumers will also pay for maximum capacity, lets say a consumer uses say 10MW but needs security of supply, like in a hospital, then the consumer will pay an extra fee in case they suddenly need more power for a short time - bit like an electrical overdraft facility.This means that the network supplying the hospital has an installed capacity of maybe 20MW and that costs more money to maintain and more capital investment.
Consumers will also pay for priority supply, again imagine a hospital, where the supplier will guaruntee supply.This matters when cascade failures occur, a fault may develop in the network somewhere which sucks in power from all over, this sudden overload causes the nearest generators to cut out, reducing capacity to supply and placing a greater burden on the other generators, which in turn shut down and so on.The fault may last only a fraction of a second but it can be enough to start a cascade shutdown.
To counter this the network has load shedding equipment which will cut off users in a strict priority basis.
Consumers will be penalised for their load characteristics through the use of power factor metering.This is not easy to explain but basically some users have plant that ends up using more capacity than the power that is actually consumed, so although they use 10MW of power they utilise 15MW of generation and network capacity.This encourages consumers to manage the characteristics of the consumption.
Sometimes it is worth it for the supplier to buy say 50MW of power from a relatively expensive source rather than run a 600MW generator at only a fraction of its capacity so you can see unusual peaks and troughs in the pool price.If that demand went up by another 200MW the pool price might actually fall as the more efficient 600MW unit is brought on line and the supplier wants to load it up to its best operating point.
There is much much more but I think I’ve illustrated how complex the electrical generation market can be.