Overnight or opportunity charging for a fleet?

Overnight depot charging is almost invisible on the demand line. The site is otherwise asleep, the load is flat and spread across ten hours, and a controller can cap it at whatever the service can carry.Opportunity charging is the opposite. It adds a large, short, unpredictable draw on top of a site already running its lights, its HVAC and its compressors. It does not just add to the peak, it usually sets it, and a peak set once is billed for the whole month.Which is why this decision belongs with your account representative and your rate schedule rather than with dispatch. Take the schedule code off the bill, download that tariff sheet, and find three things: the demand charge per kilowatt, the hours the peak period covers, and whether the schedule has a ratchet.

We publish no national demand charge figure, because there is not an honest one. Rates vary by utility, by rate schedule, by season and by service voltage, and the difference between neighbouring territories is larger than the difference between a good and a bad charging strategy. The table below is a sensitivity — find your own rate on the tariff sheet and read across.Note what it implies about sequencing. Two DC ports that never run simultaneously cost one port's worth of demand. Two that overlap for a single interval cost two. On a demand-billed tariff, sequencing software pays for itself faster than any hardware decision on the project.

Both are legitimate, and most depots end up doing some of each. The distinction worth holding onto is which side of the capital-versus-operating line each choice sits on, because that is what a business case is made of.

If a vehicle's daily energy exceeds what the dwell window can deliver, mid-day charging is the only way the route runs at all.When the vehicles do not come home. Long-haul, regional and some municipal duty cycles park somewhere other than your yard, and depot charging cannot reach them.When the yard is out of space. A depot that physically cannot fit the port count the overnight arithmetic requires can sometimes substitute power for parking positions.When shifts run back to back. A vehicle handed from one driver to the next at three in the afternoon has whatever window exists between them, and that is a mid-shift window whether you call it opportunity charging or not.In all four cases the demand charge is a cost of doing business rather than an avoidable error. The point of running the tariff arithmetic first is to know which of the two you are looking at.

Peak windows are defined in the tariff and commonly fall on weekday afternoons and early evenings, with different definitions by season — but the only version that matters is the one on your schedule. A top-up at ten in the morning and the same top-up at four can carry very different prices on the same tariff.A site power controller that holds total demand under a set kilowatt ceiling protects the bill regardless of how many ports get plugged in, and it does it without anyone in dispatch having to think about it. Configure it before energisation, because a peak set during commissioning still counts.Two ports sharing a demand ceiling deliver almost the same energy across an afternoon as two ports running simultaneously, at half the peak.Ask whether the utility offers an EV-specific commercial rate schedule. Some do, and some of those reduce or restructure demand charges for qualifying charging load. Whether one exists in your territory is a utility-specific fact, and it is exactly the kind of question a load letter conversation should surface early.None of this involves buying different chargers. It is configuration, scheduling and rate selection, which is why it should be settled before the equipment order rather than after the first quarter's bills.