kW vs kWh: the difference, explained

A kilowatt (kW) measures power: how fast energy is flowing right now. A kilowatt-hour (kWh) measures energy: how much has flowed in total. Power is a rate; energy is an amount. Run one kilowatt of power for one hour and you have used one kilowatt-hour of energy.

The pairing works like speed and distance: kW is the speedometer, kWh is the odometer. The two numbers answer different questions, and confusing them is a common unit error in energy coverage, spec sheets, and casual conversation.

The arithmetic

Energy equals power multiplied by time: kW × hours = kWh. A 1.5 kW space heater running for two hours uses 3 kWh. A 100-watt lightbulb (0.1 kW) left on for ten hours uses 1 kWh. Ten kWh could be one appliance drawing 10 kW for an hour, or a steady 1 kW for ten hours; the energy total is the same, the power profile completely different.

One consequence: "kilowatts per hour" is almost never what anyone means. Kilowatts already describe a per-time rate. If a phrase needs "per hour", the unit was probably supposed to be kWh.

Batteries and EVs: why both numbers matter

A battery spec sheet lists a kW rating and a kWh rating, and they describe different limits. The kW figure is how fast it can charge or discharge; the kWh figure is how much it holds. A Tesla Powerwall 3, for instance, stores 13.5 kWh and delivers up to 11.5 kW: it can run close to a whole house at once, but at that rate it empties in a bit over an hour.

Divide energy by power and you get duration, the number that tells you what a battery is for. Grid-scale projects are quoted the same way in bigger units: a 100 MW / 400 MWh battery is a four-hour resource, sized to soak up cheap midday solar and cover the evening ramp (the shape covered in our duck curve explainer). A battery with high power and short duration is built to catch price spikes and stabilize frequency; long duration at modest power is built to shift energy across hours.

The same split runs through EVs. An EV battery holds energy (a typical pack is 60 to 100 kWh), and a charger delivers power. A home Level 2 charger supplies roughly 7 to 11 kW, so an empty 75 kWh pack refills overnight at the top of that range. A 250 kW fast charger can, in principle, do it in under twenty minutes, though real cars taper the rate as the pack fills. Range anxiety is a kWh problem; charging-stop length is a kW problem.

Your bill: mostly kWh, sometimes kW

Residential bills mostly charge for energy: cents per kWh consumed. Commercial and industrial customers usually also pay demand charges, billed per kW of their highest draw in the month, because the grid must be built for the peak even if it lasts fifteen minutes. Demand charges, stacked on delivery charges, are why a fast-charging site can lose money while barely selling electricity; a handful of simultaneous 250 kW sessions sets a peak that dominates the bill. That math is closing the only Tesla Superchargers north of the White Mountains on October 25, 2026: almost $15,900 in net charging revenue against more than $32,500 in electric bills.

Scaling up: MW and GW

The prefixes climb by factors of a thousand, and the same power/energy distinction holds at every step. A kilowatt (kW) is household scale: one space heater. A megawatt (MW, 1,000 kW) is commercial or utility scale: a large wind turbine produces a few MW; a utility-scale solar farm runs from tens to hundreds of MW. A gigawatt (GW, 1,000 MW) is fleet scale: a large nuclear reactor produces about one GW, and grid-wide demand is discussed in tens of GW. Energy scales the same way: MWh and GWh. When Ember estimates 459 GWh of battery additions worldwide in 2026, that is stored energy; when a grid operator says demand peaked at 85 GW, that is instantaneous power.

When a number looks wrong at any of these scales, check whether the claim is about a rate or an amount. "The plant produces 500 MWh" is incomplete without a time frame (per day? per year?), while "the plant produces 500 MW" is a complete statement of capacity. "The battery delivered 50 MW for four hours" implies 200 MWh. House style: MW and GW for capacity, MWh and GWh for energy, and both numbers for storage whenever the source provides them.