Total demand is not constant, but you can represent total demand as a sum of a given constant demand plus variable demand. Say for instance the average demand varies from 200-350 kWh a year. You could run nuclear power plants to generate 200 kWh worth of electricity, and use solar/wind for the remaining 0-150 kWh demand. It would be fairly efficient to have nuclear provide a base load of some kind while solar and wind vary to meet the full demand.