Its all to do about demand and supply, with a little bit of market intervention in.There is more Generation than demand even on the coldest day. This is more pronounced over summer. Therefore Generation will never be operating at 100%.Power stations have to go on outage to maintain their equipment.EU has limited amount of generation coming from coal power stations because its dirty.Wind gets big subsidies to generate and its free. Coal costs money to generate so if their is limited demand, coal will not generate as won't make enough money to cover costs. therefore coal just fills in the gaps when the wind isn't blowing or when its really cold