EV fleet charging and demand charges: why depot bills spike and how to flatten them

A fleet operator electrifies a depot, models the energy cost per mile, and finds it comfortably below diesel. Then the first full electricity bill arrives and the number is far higher than the energy math predicted. The culprit is almost always demand charges, driven by dozens of vehicles plugging in at once. The energy was cheap. The peak was not.

How simultaneous charging creates a peak

Fleet charging concentrates a lot of power into short windows. The US DOE Alternative Fuels Data Center notes that fleet electrification greatly increases electricity consumption, especially when several vehicles need to charge simultaneously at high rates, and that this can incur demand charges, a fee applied to your greatest power draw during peak periods on top of the energy rate. Individual chargers are large: DC fast charging equipment can deliver up to 500 kW per unit. A handful running at once creates a multi-hundred-kilowatt spike that sets your billed demand for the whole month.

That spike is expensive because of how demand is billed. Demand charges bill the highest short-interval power draw, often a 15-minute average, and per the Clean Energy Group and NREL they can account for 30 to 70 percent of a commercial customer's monthly bill, with nearly all medium and large commercial customers obligated to pay them. A depot is squarely in that category.

Why demand charges dominate at low utilization

Early in a fleet rollout, chargers sit idle much of the day, so a large fixed demand charge is spread over relatively few kilowatt-hours. The effect can be extreme. An RMI and EVgo analysis found that demand charges can make up as much as 90 percent of the monthly bill of operational public DC fast chargers, and that DC fast charging cost reached $1.96 per kWh during summer months in some 2017 locations. The lower the utilization, the more each kWh has to carry of that fixed peak cost.

The exposure is widespread because demand charges are common and large. NREL, in work hosted by the Clean Energy Group, surveyed more than 10,000 US utility tariffs and used a threshold of 15 dollars or more per kilowatt to identify where demand charges are high enough to matter, estimating that roughly 5 million commercial customers sit at or above that level. A depot that concentrates its load into short charging windows is exactly the profile that hits those charges hardest.

Managed charging and load staggering

The fix is to spread the same energy over more time so the peak never spikes. The DOE notes that networking allows smart charging, scheduling charging events to stagger vehicles and take advantage of off-peak rates. The savings are real. RMI reports that slower, managed charging can reduce a depot's peak load by as much as a third, that one Southern California fleet cut its peak by 1 MW through managed charging, and that charging during off-peak hours can save fleets up to 30 percent in charging costs, while being forced to charge during peak hours can raise annual electricity costs by as much as 40 percent.

• Stagger plug-in and start times so vehicles do not all ramp at once.
• Cap the number of chargers allowed at full power simultaneously.
• Shift charging into off-peak and overnight windows where rates and grid load are lower.
• Sequence chargers so one finishes before the next begins where dwell time allows.
• Pair charging with on-site storage to shave the residual peak.

Using interval data to size and schedule

You cannot manage a peak you cannot see. Interval meter data and charger telemetry, aligned on the same clock, reveal the coincident peak, which vehicles and sessions contributed to it, and how much headroom the service has. Reading that shape is the same skill covered in reading load profiles: baseload and peak, applied to a depot. With a clean profile you can size the electrical service correctly, set charging windows that respect both operational dwell time and rate periods, and verify that the managed-charging logic actually flattened the curve rather than just moving the spike.

The Canadian context

Fleet electrification is scaling under policy pressure, so this cost problem is about to be much more common. A Transport Canada study notes Canada's 2030 Emissions Reduction Plan sets an aspiration for 35 percent of total medium- and heavy-duty vehicle sales to be zero-emission by 2030, moving toward 100 percent of new truck and bus sales where feasible by 2040. Every depot that electrifies inherits the demand-charge problem, and the ones that plan for managed charging from day one avoid building a peak they then have to pay down.