Probably most people never give a thought to where the power comes from to switch on the light or their TV. Engineers are primarily responsible for ensuring that the right number of power stations are available to supply exactly the right amount of electricity to match demand. If supply exceeds demand then energy needs to stored, for instance at the Dinorwig storage scheme [ http://www.fhc.co.uk/dinorwig.htm ]; however if demand exceeds supply then someone’s lights will dim or go out until an additional power station can be switched on or the output increased from one that is running. The latter is a relatively quick process but switching on a power station takes longer than half time in a televised football match when everyone switches on the kettle or makes some toast.
You can see how national demand in the UK varies in real-time at the National Grid website [ http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Demand/demand24.htm ]. There is a similar “national electricity meter” for Spain [ https://demanda.ree.es/demandaEng.html ], which also shows the blend of energy sources being used.
Blending sources such as fossil fuels, hydro, nuclear, solar, tidal and wind is the key to a cost-effective sustainable energy supply with the diversity to adapt to unexpected circumstances.