I’m sure the original comment had incorrect units as used, but this explanation that cumulative units “can’t peak” seems wrong.

If you consider the total stored energy (Wh) over time of a solar-battery system under load, there certainly will be peaks or, in other words, maximal excess capacity of the system.

So no, it’s not impossible to define a unit of Whp as such. “Cumulative” and “momentary” values are not exclusive and also do not have any bearing on whether a function of such values has maxima and minima.