By definition base load is simply the lowest demand ever found on the power grid during regular operations. its everything that runs 24 hours a day in simple terms, in reality though that still involves some balance since for example a refrigerator runs all the time but will draw more power during the day when the door keeps being opened then it will at night.
Base load changes over time as the economy expands and contracts, and it can be lowered intentionally, but only by reducing consumer energy consumption. Its generally less then half of peak load. Also since the USA, India and China all lack efficiently interconnected power grids, so does Japan and more then one other small country too, base load has really managed on a regional basis, peak load even more so.
The term intermediate load is used for power demand which is regular, but not for the entire day, such as office lights that are on 9-5 can be considered an intermediate load, rather then a peak spike. You know they'll happen, you know that's load that will be sustained for a serious amount of time and can plan power plant operations with this in mind. Peak loads are somewhat predictable, but much more volatile and weather dependent. So peak load generally has to be supplied by quick reaction systems, which pretty much comes down to hydro and simple gas turbines, while intermediate can be met by a somewhat wider range of plants, and base load is what you want nukes and coal powering since both can take hours to come online. Then you have that rat bastard wind, which can supply a random fraction of base load randomly, and solar which matches up with intermediate demand very nicely, unless you have electric heating in winter (also near any heat system draws some electric anyway)
In the US peak load is around 800 GW, I think base load is around 350 GW but I don't recall when I read that. In any event such figures have limited value, since the grid must have a large reserve of capacity to make up plants being offline for repairs made worse by lack of interconnection (which increases required reserves) and variable fuel and weather conditions. Hot conditions for example can drive down generation from nuclear and some coal plants because they cannot draw enough cooling water without boiling local waterways. This all means it can be more useful to work in terms of actual energy consumption.
"This cult of special forces is as sensible as to form a Royal Corps of Tree Climbers and say that no soldier who does not wear its green hat with a bunch of oak leaves stuck in it should be expected to climb a tree"
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