Charging a fleet has two costs, and they behave nothing alike. The energy cost follows the miles driven: so many kilowatt-hours per van per day, bought at whatever the energy rate is. The demand cost follows something else entirely — how many vehicles draw power in the same fifteen minutes.
That second cost is the one that surprises people, because the vehicles do not change and the miles do not change. Only the timing does.
The Alternative Fuels Data Center says it directly: DC fast charging equipment is more likely to trigger demand charges than Level 1 and Level 2 charging. The worked example below shows why, and why even slow charging can do it if every van plugs in at once.
The site
A depot with a daytime building load. The fleet returns in the evening.
Starting point
Before any chargers are installed.
- Building peak, mid-afternoon420 kW
- Building load, 6 pm to 5 am150 kW falling to 80 kW
- Demand charge, facility maximum$15.00 / kW-month
Monthly demand charge today$6,300
The afternoon sets billed demand. The evening and night sit well below it — which is capacity already paid for. Rates illustrative.
The fleet: 20 vans, each needing about 60 kWh a night, parked from 6 pm to 5 am. Twenty Level 2 chargers at 19.2 kW each.
Case 1: everyone plugs in at six
With no control, every charger draws its full rating from the moment the vans arrive.
Unmanaged evening charging
All twenty chargers start together.
- Chargers × rating: 20 × 19.2 kW384 kW
- Building load at 6 pm150 kW
- (New facility maximum)534 kW
- Increase over the afternoon peak114 kW
Added demand charge per month$1,710
The chargers finish in a little over three hours and then sit idle for eight. The demand charge is priced on those first fifteen minutes. Figures illustrative.
The energy is the same as it would be under any schedule. The $1,710 is purely the price of doing it all at once.
Case 2: the same energy, spread across the night
The fleet needs 20 × 60 = 1,200 kWh, and it is parked for eleven hours. That is an average of about 109 kW.
Managed charging with a power ceiling
Combined draw capped at 150 kW.
- Energy required per night1,200 kWh
- Hours available11
- (Average power needed)109 kW
- Ceiling set on the chargers as a group150 kW
- Building load at 6 pm plus the ceiling300 kW
- Afternoon peak, unchanged420 kW
Added demand charge per month$0
The ceiling leaves headroom for late arrivals and a colder night. The fleet charges entirely inside capacity the site already pays for. Figures illustrative.
This is what the Federal Energy Management Program describes as smart charge management: adjusting charging power or shifting sessions, and enforcing a power ceiling across multiple chargers. The mechanism is the same one described in demand limiting controls, applied to a load that is unusually easy to move, because a parked van does not care when its battery fills.
Case 3: two fast chargers at lunchtime
Now add two 150 kW DC fast chargers for midday top-ups, used by a few vans around 1 pm — the hour the building peaks.
Midday fast charging
Two DC fast chargers used during the building peak.
- Building peak420 kW
- Two fast chargers at full output300 kW
- (New facility maximum)720 kW
- Increase in billed demand300 kW
- (Added demand charge per month)$4,500
- Energy delivered: 2 sessions × 40 kWh × 22 days1,760 kWh
Demand cost per kWh delivered$2.56
On top of the energy rate. A few sessions in the wrong fifteen minutes turn fast charging into the most expensive energy on the site. Figures illustrative.
The point is not that fast chargers are a mistake. It is that their demand cost has to be priced before they are bought. One session inside the peak interval is enough to set the month, as the fifteen-minute demand interval explains, and under a ratchet it can set a floor on the months after it too — see ratchet clauses.
The separate-meter question
The Alternative Fuels Data Center notes that some utilities offer time-of-use rates or other incentives for charging infrastructure, and a separate meter is often the way onto them. It is worth modeling, and it is worth modeling both ways.
On a separate meter, the chargers get their own peak. Unmanaged, the twenty Level 2 chargers in Case 1 would bill 384 kW on their own account: 384 × $15.00 = $5,760 a month, against the $1,710 they added when their load sat partly inside the building's headroom. A dedicated account only wins if its rate structure is genuinely better for charging — lower or no demand charges, or time-of-use energy pricing that rewards overnight charging — and if charging is managed either way. Time-of-use rates covers how those windows are set.
What to do before the chargers arrive
- Pull interval data for the site and find the headroom: the gap between the monthly peak and the load in each hour the fleet will be parked.
- Size the energy, then the power. Kilowatt-hours per night divided by hours parked gives the average power the fleet actually needs.
- Specify managed charging in the purchase, with a site-level power ceiling, not as a later upgrade.
- Price fast charging separately, at the hours it will really be used, including its effect on any ratchet.
- Ask the utility about charging rates and metering before choosing where the chargers connect, as the Alternative Fuels Data Center advises for fleet planning.
The fleet's energy is set by its routes. Its demand charge is a choice made when the charging system is specified, and it is far cheaper to make that choice before the first van comes home.
