I would opt for levels of concern in power usage: low concern (desktop or mobile that is charging), concern (mobile), high concern (mobile on low battery). Then

I would opt for levels of concern in power usage: low concern (desktop or mobile that is charging), concern (mobile), high concern (mobile on low battery). Then we could streamline it more, because websites know when you are on mobile already and give single flag: low power. It is is similar in any other case of low resources. There probably is not much of a difference for a website if user is low in power than if it is low in memory. There should be then just one flag/event really: low resources (catch-all for battery, memory, CPU time, storage etc.).