I'd update the units; good call. You may have that confused? Traditional supercapacitors have had lower power density and faster charging/discharging. Graphene and hemp somewhat change the game, AFAIU.
It makes sense to put supercapacitors in front of the battery banks because they last so many cycles and because they charge and discharge so quickly (a very helpful capability for handling spiky wind and solar loads).
You're right that it makes sense to put supercapacitors in front of the battery banks for the reasons you said.
My understanding is that there's usually a curve over time t that represents the charging rate from empty through full.
> Charge and discharge rates are often denoted as C or C-rate, which is a measure of the rate at which a battery is charged or discharged relative to its capacity. As such the C-rate is defined as the charge or discharge current divided by the battery's capacity to store an electrical charge. While rarely stated explicitly, the unit of the C-rate is [h^−1], equivalent to stating the battery's capacity to store an electrical charge in unit hour times current in the same unit as the charge or discharge current.