Baseload refers to the minimum constant level of electric demand created by a customer or a group of customers. For a utility or a system operator, baseload is the minimum amount of demand that the system must meet.  

Baseload demand for a single customer facility


Baseload demand for the ERCOT (Texas) ISO for one day in April


Baseload generation refers to electric generators that are normally operated continuously across a 24-hour period to meet the system’s baseload demand. Typical baseload generation types vary by region but often include nuclear power, run-of-the-river hydro, some coal or natural gas steam turbines, cogeneration units, and some renewable resources such as geothermal or biomass steam turbines. Baseload units are often those with the least flexibility (nuclear), those that need to be running at minimum levels because they will need to be ramped up quickly later in the day (coal and natural gas steam), and renewables with non-variable output (run-of-the-river hydro, geothermal, and biomass). 


Generation used in CAISO (California ISO) to satisfy load. Note the use of nuclear power, biopower, and geothermal as baseload units. 
Source of graph: Source: CAISO Third Quarter 2020 Report on Market Issues and Performance