Specifying the highest available AC rating is the default and frequently the wrong answer, because it consumes supply capacity that could have served additional bays.
The decision turns on two things the specification sheet does not mention: how long vehicles stay, and what their onboard chargers actually accept.
The vehicle caps the rate
On AC, conversion happens inside the vehicle and its onboard charger sets the ceiling. A high-rated wall unit serving vehicles that accept less delivers less, while still requiring the supply headroom its rating implies.
Establishing what the vehicles at this site actually accept is a short exercise that frequently changes the specification and reduces the connection requirement.
Dwell decides whether rate matters
Where vehicles stay for hours, both ratings deliver a full charge and the higher one simply finishes sooner and then sits idle. The extra capacity purchased was never converted into energy delivered.
Where dwell is short, rate matters directly and the higher rating delivers meaningfully more. The boundary sits somewhere around the point where the lower rate can no longer fill the typical need within the typical stay.
Bays versus rate on a fixed connection
A given supply serves roughly three times as many bays at the lower rating as at the higher one. On a site where dwell is long, that trade almost always favours more bays: more drivers served, higher occupancy, better utilisation of the same connection.
The exception is a site where drivers arrive expecting a quick top-up and would not wait, which is a different use case and frequently a DC one.
Cost per bay and cost per driver served
Higher-rated units cost more per bay, and they also consume more of the supply, which is the scarcer resource on most sites. Comparing cost per bay understates the difference; comparing cost per driver served over a day reflects it.
That second measure is the one that matters commercially and it is rarely the one used during procurement.
Single phase and three phase
The lower rating is achievable on a single-phase supply, which many sites already have. The higher rating requires three phase, which some sites do not, and obtaining it is a supply upgrade rather than an equipment choice.
Where three phase is not present, this decision has already been made by the site.
A workable default
For workplace, residential, hotel and depot charging, where dwell is measured in hours, the lower rating with more bays serves more people on the same connection.
For retail with visits of one to three hours, the higher rating earns its supply consumption. Below an hour, neither AC rating is the answer.