kW vs kWh: What's the Difference?

Two units, one letter apart, and constantly confused. Here's the plain-English version, and why the difference decides both your charging speed and your bill.

Think of it like water. Kilowatts (kW) are how wide the pipe is: the rate energy can flow. Kilowatt-hours (kWh) are how much water ends up in the bucket, the total amount delivered. A wider pipe fills the bucket faster. It doesn't change how big the bucket is.

kW is Rate. kWh is Amount.

Kilowatt (kW) measures power: how fast energy is being delivered right now. A 7 kW home charger delivers energy roughly ten times faster than a 3-pin plug.

Kilowatt-hour (kWh) measures energy: the total delivered over time. Your electricity bill is priced in kWh. Most EV batteries hold 40–100 kWh, and a typical HGV pack runs considerably higher.

One Decides Your Charging Time. The Other Decides Your Bill.

A vehicle's battery capacity is fixed in kWh (say, a 60 kWh van battery). How long it takes to fill depends entirely on the charger's kW rating. Put that same 60 kWh battery on a 7 kW home charger and it takes roughly 8–9 hours. Put it on a 150 kW DC rapid charger and it's closer to 25–30 minutes. Same energy delivered. Very different time.

This is the whole calculation behind sizing a depot or fleet site: how much energy, in kWh, the fleet needs per day, and how much power, in kW, you need on-site to deliver it in the time available between shifts.

Approximate figures assuming a flat charging rate. Real-world times vary with battery chemistry, temperature, and state of charge. See the note on charging curves below.

A Higher kW Rating Doesn't Always Mean a Faster Charge

Charging isn't delivered at a flat rate for the whole session. Most batteries accept power fastest in the middle of their charge and taper off as they approach full, to protect battery health. A 400 kW charger and a 150 kW charger can end up delivering a similar amount of energy in the last 20% of a charge, because the battery itself is the bottleneck, not the charger. This is why fleet operators plan charging windows around usable charging speed, not just the number on the charger's spec sheet.

Sizing a Depot: kWh Demand, Then kW Capacity

Work out the total kWh your fleet needs to add per day, then divide by the charging window between shifts to get the kW you need on-site. With multiple vehicles charging at once, that on-site capacity is usually shared across bays through a Master Unit and dynamic load balancing, rather than each bay having its own full-power connection, since grid connections sized for peak simultaneous demand are the single biggest line item in most depot projects.

Does a higher kW charger always charge faster?

Not for the whole session. Batteries accept power fastest in the middle of a charge and taper off near full to protect battery health, so a 400 kW and a 150 kW charger can end up delivering similar energy in the last 20% of a charge.

How do I work out how many kW I need on-site?

Start from the kWh your fleet needs to add per day, then divide by the charging window you actually have between shifts. That gives the average kW draw, before accounting for simultaneous vehicles and any dynamic load balancing across bays.