Basically, based on breaker size. If you have a 400A breaker you might pay $2,000 / month demand charge. Usage might be small (sometimes these loads are spikey). It's not uncommon for folks to then pay more attention to peak load if you pay based on breaker size effectively.
this is a form of two-part pricing[0], which is a compromise between simple (single) pricing and continuous pricing (aka perfect price discrimination) to maximize value capture with minimal complexity.
two-part pricing also tends to make markets (from the demand-side) more rational and efficient, but that's not often the reason it's employed, which is why regulation is often needed (particularly in monopoly markets).
A-10 rates for example in PG&E land have a demand charge -
https://www.pge.com/en_US/small-medium-business/your-account....
Basically, based on breaker size. If you have a 400A breaker you might pay $2,000 / month demand charge. Usage might be small (sometimes these loads are spikey). It's not uncommon for folks to then pay more attention to peak load if you pay based on breaker size effectively.