Many commercial electricity tariffs include a component based on the highest demand recorded in a period rather than on energy consumed. Charging can set a new peak in minutes and pay for it for months.
This is the mechanism by which a charging site with modest utilisation can be unprofitable despite a healthy margin on every unit of energy sold.
How the charge is incurred
Demand is typically measured as an average over a short interval, and the highest such interval in the billing period sets the charge. A brief coincidence of several vehicles starting together can establish a peak that persists in the bill long after.
The consequence is that the cost is driven by the worst moment rather than by typical behaviour, which is the opposite of how energy cost behaves.
Why charging is particularly exposed
Charging load arrives in large steps rather than gradually. Several vehicles connecting within the same interval produces a sharp increase that the building's other loads do not smooth.
It is also correlated: at a workplace, arrivals cluster; at a depot, vehicles return together. The load is therefore both large and synchronised, which is the worst combination for demand charges.
Limiting the site total is the direct control
A site-wide allocation that caps total charging draw prevents a new peak being established regardless of how many vehicles connect. That converts an unbounded exposure into a known one.
Setting that cap requires knowing the building's existing peak, because the charging allowance sits on top of it. Without that measurement the cap is a guess.
Ramping rather than stepping
Starting several sessions simultaneously at full rate creates a step. Staggering starts and ramping to the target draw spreads the increase across intervals and can avoid setting a peak that the same energy delivered more gradually would not.
This costs nothing in delivered energy where dwell is long, which is most AC charging.
Shift outside the building peak
Where charging can be scheduled, moving it into periods when the building draws least means charging load fills a trough rather than adding to a peak. On sites with long overnight dwell this eliminates the exposure almost entirely.
Workplace charging cannot use this, which is why workplace sites need the site cap rather than scheduling.
Confirm the tariff before modelling anything
Whether a demand component applies, how the interval is defined, and whether the peak resets monthly or persists all vary. These determine how much the measures above are worth, and they are specific to the connection.
Establish this with the supplier at the start. A charging business case built without knowing the tariff structure is built on an assumption that can invert the result.