If a large fraction of electricity generation comes from renewable sources, how can the electricity system be kept in balance?

This question has been addressed by many research groups, and a large body of information is available.

Most research deals with wind generation.  You can see real-time wind generation data amongst other sources on Elexon's Balancing Summary Page; the fuel-type table is updated every five minutes. 

Please note that this only covers around one third of the wind farms connected to the GB system.  The remaining two-thirds of total wind generation is counted as negative demand.  For a very rough view of the instantaneous total wind generation, simply multipy the generation shown by three. 

Some other sources of information are:

• David Milborrow's article for New Power UK, Is wind power reliable? (2009) and follow-up commentary hosted at the Claverton Energy Research Group;
• Reducing the cost of system intermittency using demand side control measures; a study by IPA Consulting, Econnect and Flexitricity (2006);
• Matching Renewable Electricity Generation with Demand: an assessment for the Scottish Government of the real-time correlation between renewable energy resources and electricity demand in Scotland, by the University of Edinburgh's Institute for Energy Systems (2006);
• The Royal Commission on Environmental Pollution's 22nd report, Energy - the changing climate (2004);
• Quantifying the system costs of additional renewables in 2020, also known as the “SCAR report” by Ilex (now Pöyry Energy Consulting) and Professor Goran Strbac (2002);
• The British Wind Energy Association's summary of UK wind energy statistics (updated regularly).

Most research on renewable energy balancing acknowledges several key points:

1. Generation capacity is not the same as fuel consumption.  Wind power can reduce fossil fuel burn without necessarily reducing the amount of fossil fuelled generation capacity available.  See David Milborrow's article and the University of Edinburgh study.
2. Predictability is valuable, even though it is not the same as controllability.  The more accurate and long-range the prediction, the less fossil fuel is wasted in keeping capacity available.  The study by IPA, Econnect and Flexitricity addresses this point.
3. Some forms of renewable generation - biomass, anaerobic digestion, geothermal - are controllable.  Some of these are quantified by the Royal Commission on Environmental Pollution.
4. Electricity is coupled with other energy uses, including heat and transport.  This is considered by the Royal Commission on Environmental Pollution.