Additionally, renewable energy depends on variable wind, sun and exogenous conditions that are intermittent and more difficult to schedule than conventional fossil fuels. Given the limited and still developing electricity storage technology' at utility scale, it is economical to integrate intermittent renewables into larger, flexible electricity systems. The fossil fuel-based generation can be backed down when renewables are available, and with an electricity grid system that covers large geographical areas, the available average renewable resources would be easier to schedule.