Where applicable, the alternative fuels to gas-based generation (such as diesel, fuel oil or coal) are mainly stored on the site of CCGTs. The period of load duration depends on a number of factors, including the ability to restock back-up fuels, generator characteristics, the type of gas supply contract - either firm or interruptible - and expectations about the variability of electricity prices as these determine the installation of back-up facilities (BERR, 2006). However, when information on this indicator remains disaggregated, the overall picture for the national energy system may be less clear.