Measure how much energy has been used (or generated, or stored). Use them to talk about:
Kilowatt-hours are the units of electricity that you pay your energy supplier for.
Measure how fast energy is used (or generated, or stored) at a single point in time. Use them to talk about:
Kilowatts are a measure of power, or how quickly energy is being transferred from one place to another.
With practice, using the right units will become second nature. In the meantime, try this mnemonic:
A lot of people (even professional electricians!) use kilowatts and kilowatt-hours interchangeably. This is a bad thing for a couple of reasons.
First, it's just universally important to be correct, precise and consistent when communicating about any technological or scientific topic.
Mixing up kilowatts and kilowatt-hours is exactly equivalent to saying “I drove to work, a distance of five miles per hour from my house, at an average speed of 30 miles”. It makes no sense at all, it's deeply confusing, and it hinders accurate communication.
Secondly, it has real-world consequences.
An apocryphal example: recently, a solar farm was proposed with a generation capacity of 500 megawatts (which is reasonable large, but not unheard of), and also some onsite battery storage, the capacity of which was not specified in the proposal.
Unfortunately, the local community misinterpreted the proposal, incorrectly believing that the onsite battery storage capacity would be 500 megawatt-hours, which would make it comfortably the largest such facility in Europe. This misunderstanding created widespread uproar and vicious opposition to the proposal.
The mistake was caused, at least in part, by confusion about the units of measurement. To reuse the previous metaphor, it's the difference between designing a car that is able to travel 500 miles, and a car that is able to travel at 500 miles per hour!
The ultimate loser here, of course, is the climate. If we can't communicate effectively regarding proposals and plans to generate clean energy (or reduce energy demand), then we risk delaying projects that are critical to our decarbonisation efforts.
No. Anyone who understands the difference between the speed of a car and the distance travelled by the car, or between the rate that water comes out of a tap and the volume of water in a bucket, can easily understand the difference between kilowatts and kilowatt-hours.
I think it's because “kilowatt-hours” (kWh) sounds tantalisingly similar to “miles per hour” (mph). So it's tempting to think that, if kilowatt-hours are like miles per hour (a measure of speed) then kilowatts must be like miles (a measure of quantity). But this is exactly backwards! Kilowatts measure the rate of energy transfer, and so they already have the “per hour” part built in. One kilowatt-hour doesn't mean “one kilowatt per hour”, it means one kilowatt for an hour. It's equivalent to saying “one mile per hour, for one hour” - or one mile!
Kilowatts (and kilowatt-hours) are the scale at which this problem usually manifests, because the confusion usually exists in conversations about domestic energy: rooftop solar, home batteries, heat pumps, energy bills etc.
People working with megawatts or gigawatts (in other words, utility-scale energy) usually have sufficient experience or academic background to use the correct units. If you disagree, feel free to substitute “kilo” with any other prefix (or none at all) everywhere above.