EV Charging FAQ: Why the kW Number on the Charger Isn't a Revenue Guarantee
A 150 kW or 350 kW rating tells you what a charger can deliver under ideal conditions, not what it will deliver to every vehicle, every session. For CPOs, that gap between advertised ceiling and real-world delivery isn't just a technical detail. It drives support tickets, site host complaints, and driver churn, even when the hardware is working exactly as designed.
This FAQ breaks down why charging speed varies, and more importantly what it actually costs a CPO when that variation isn't explained upfront.
1. What does a charger's kW rating actually mean?
It's a ceiling, not a promise. A 150 kW charger is capable of delivering up to 150 kW under the right conditions the right vehicle, the right battery state, the right temperature. It doesn't mean every session will hit that number, and treating the label as a guarantee is where most driver complaints originate.
2. Why does a session come in under the charger's rated power?
Usually because of the car, not the charger. Every EV has its own limit on how much power it can take in, if a car maxes out at 100 kW, it won't go faster than that even on a 350 kW charger. Charging also slows down as the battery fills up, and in very hot or cold weather. Site power-sharing or charger derating can slow things too, but that's less common.
3. Why does charging slow down near the end of a session?
Most vehicles charge fastest at low states of charge and taper as the battery approaches full, to protect long-term battery health. The U.S. Department of Transportation notes this pattern directly. DC fast charging is quickest early in a session and slows as the vehicle nears 100%. A session that looks sluggish at 85% is often working exactly as intended, not malfunctioning.
4. Does that tapering actually cost the CPO money?
Yes, in a specific way: stall turnover. A driver who doesn't understand the charging curve tends to stay plugged in well past the point of fast charging, occupying a stall that could be serving the next customer. Site economics are usually priced around expected session length, a driver camping near 100% quietly erodes that math.
5. Is derating the same thing as a broken charger?
No, and this is worth separating clearly, since it's a smaller factor than most people assume. Derating is a charger protecting its own components, under high heat, sustained load, or site power constraints by reducing output. It's a protective function, not a fault. It's a real metric worth tracking, but it explains a minority of "slow charging" complaints compared to vehicle-side limits.
6. Can shared power across a site affect delivered speed?
Yes. Some sites distribute available power across multiple active dispensers, so a charger's maximum rating may not be available if several stalls are drawing power simultaneously. This isn't inherently a problem, it's an efficient use of site capacity, but it does mean the number on the charger and the number in a given session can diverge for reasons that have nothing to do with equipment health.
7. So when is underperformance actually a real problem?
When it's repeatable and can't be explained by vehicle limits, battery state, or normal power sharing. If a charger consistently underdelivers across multiple compatible vehicles under similar conditions, that's worth investigating as a hardware, installation, or site power issue. The useful question isn't "is the charger online", it's "is the charger delivering the power this vehicle, at this state of charge, should be getting."
8. What should CPOs be tracking instead of just uptime?
Uptime confirms a charger is reachable. It says nothing about whether the session was commercially productive. A more useful set of metrics includes average delivered power by vehicle category, session duration relative to state-of-charge at plug-in, derating frequency as a distinct metric from downtime, and failed or underperforming sessions broken out from genuine faults. This is the data that actually explains complaints and revenue gaps that uptime reports miss entirely.