An electric power system is a network that supplies, transfers, and uses electric power. It includes generators that produce electricity, a transmission network of high-voltage wires that transport power over long distances, and a distribution network that delivers electricity to consumers. Electricity supply and demand must be balanced at all times since electricity cannot be economically stored at large scales. Different generation technologies like fossil fuels, nuclear, and renewables have varying operating characteristics that impact their predictability, flexibility and variability. While 100% renewable electricity systems may be technically feasible long-term, transitioning energy systems will face challenges around balancing supply and demand with variable renewable resources.