How to cut demand charges at a charging depot

If your site sits on an all-volumetric schedule with no demand component, this entire article is misdirected effort and your money is better spent on energy timing. If your tariff carries a ratchet, the opposite is true and the case for a hard cap gets much stronger, because a ratchet turns a single failure into eleven months of billing.Take the schedule code off the bill, find that schedule in the tariff book, and read the definition of billing demand as well as the rate.

The delay belongs in the equipment, not in a routine. Most chargers can hold a start time, and most vehicles can be told to begin charging at a set hour from the vehicle side. Asking drivers to plug in at staggered times works until the first busy week.Staggering has a hard limit that is worth understanding, because it is the reason it is a first step and not the whole answer. Groups cannot overlap, so the number of groups you can run is the window length divided by the time one vehicle takes. In the example below that is twelve hours divided by four and a third, which is two groups. Beyond that you need the ports running simultaneously at reduced power, which is what a cap does.

A cap does what staggering cannot: it lets every port run at once at reduced power, which fits more vehicles into the same window than sequential groups do. Total energy divided by the window length gives you the minimum average power the site has to deliver, and any cap above that floor works.It has a second benefit that shows up on the capital side rather than the operating side. Where the relevant editions of NEC Articles 625 and 750 are adopted and the authority having jurisdiction accepts the approach, a service can be sized against the maximum an energy management system permits rather than the sum of the connected equipment. That is frequently the difference between fitting inside the existing service and paying for a new one.The question to ask the vendor, and to test at commissioning, is what the cap does when something fails. A cap that lifts when the controller loses contact with the chargers is not a cap; it is a default.

A hard cap is blind. It knows the site total and nothing about the vehicles, so under a tight cap it slows everything equally, including the van that leaves at 04:00 for a long route. A managed platform pulls departure times and state of charge, usually from your telematics, and allocates the same limited power in the order the schedule requires.It earns its subscription in three situations: mixed departure times, vehicles that must leave at a specific state of charge, and more vehicles than ports, where sequencing has to be automatic because nobody is moving cables at 02:00. If your yard is twenty identical vans that all leave at 06:00, a cap does most of the same work for no recurring cost.Price it at the port count you will eventually have, not the one you are installing. Per-port-per-year pricing scales with the build, and a number that looks small against eight ports looks different against forty.

Storage is the right answer to a peak you cannot move: mid-shift DC charging, a yard where vehicles return unpredictably, or a site whose service is genuinely full and where a line extension is worse value than a battery. It is the wrong answer to an overnight peak that a $2,000 controller would have flattened.Two things decide the sizing, and people usually only think about the first. Power decides how many kilowatts you can shave. Energy decides how long you can shave them for. Shaving 90 kW across a four-hour peak needs 360 kilowatt-hours of usable capacity, and a battery with the right power rating and the wrong energy rating will stop shaving halfway through and let the meter record the peak anyway.The arithmetic that decides whether it pays: kilowatts shaved, times your demand rate, times twelve. At the illustrative $18 per kilowatt used above, shaving 90 kW is $19,440 a year. That figure has to carry the capital cost, the O and M, the footprint, the fire-code and permitting work, and the degradation over the asset's life. We do not publish an installed cost per kilowatt-hour for storage, because we have not sourced one we would stand behind, and a made-up number here would flatter a decision worth six figures.

Derating the ports permanently means every session is slow whether the yard is busy or empty, which wastes the capacity you own on quiet nights and can push you past the window on busy ones. A site cap lets ports run at full rate when there is room and back off only when there is not.The related mistake is capping below the floor. If twenty vans need 1,000 kilowatt-hours and the window is twelve hours, the site must average 83 kW. A cap of 70 kW does not save you money; it sends vans out short and produces an angry conversation with dispatch that ends with somebody disabling the cap.