Base load is the minimum level of electricity demand that must be met continuously, 24 hours a day, 7 days a week. This baseline includes power for essential services, refrigeration, industrial processes, and background consumption that never drops to zero. Utilities design their generation systems to reliably meet base load with power sources that run continuously and cheaply, traditionally including nuclear, hydroelectric, and coal plants. Peak load above base load is met by flexible sources that can ramp up and down. As renewable energy grows, managing base load becomes more complex since solar and wind generation varies with weather and time of day.