kW vs. kWh: what's the difference?

The one-sentence version

A kilowatt (kW) is how fast energy is moving right now. A kilowatt-hour (kWh) is how much energy moved in total. Speed versus distance.

If a car travels 60 mph, that's a rate. If it drives for two hours, it covers 120 miles, and that's an amount. Same relationship: run 1 kW for 1 hour and you've used 1 kWh. Run a 5 kW oven for 30 minutes and you've used 2.5 kWh.

The reason this trips people up constantly is that solar systems are sold in kW and electricity is billed in kWh. Those are two different units describing two different things, and quotes rarely explain which one you should actually be comparing.

Your system is rated in kW. Your bill is in kWh.

When someone says you're getting a "10 kW system," that's the nameplate: the maximum the panels can produce in ideal conditions, full sun hitting them straight on, at a standard test temperature. It's the size of the faucet, not the water in the bucket.

Your utility, meanwhile, charges you for kWh, the bucket. So does most solar pricing: whether you're comparing quotes on dollars per watt or looking at a monthly payment, the kWh your system actually delivers is what decides the money. That's why two systems with the same kW rating can cost you very different amounts over time. For the wider picture on pricing, see how much solar costs.

So why doesn't a 10 kW system make 87,600 kWh?

Fair question, because that's the arithmetic if it ran flat out forever. There are 8,760 hours in a year, and 10 kW × 8,760 hours = 87,600 kWh. Real systems land closer to 14,000. Here's where the rest goes.

The sun isn't out all day, or overhead when it is. Panels only hit their rated output when sunlight is strong and hitting them at a good angle. The industry accounts for this with peak sun hours: the equivalent number of hours per day your roof gets full-strength sun. Most of Texas runs about 4.5 to 5.5 peak sun hours a day. Morning, evening, clouds, and winter angles are already baked into that number.

So the honest starting math looks like this:

• 10 kW × 5 peak sun hours × 365 days = about 18,000 kWh
• Then subtract system losses of roughly 15 to 20%: inverter conversion, wiring resistance, dust and pollen on the glass, and heat
• Which lands you around 14,000 kWh for the year

That's the trip from 87,600 to 14,000, and none of it is a disappointment. It's just the difference between a rate and a real year.

Why heat, of all things, costs you output

This one surprises people in Texas especially. Solar panels get lessefficient as they heat up. A 100°F roof in August produces somewhat less per panel than the same sun would deliver on a cool day. More sun is still better overall, but it's the reason a nameplate rating measured at a standard test temperature isn't what your roof does in July.

Why two identical systems produce different amounts

Two homes can both install 10 kW and see meaningfully different annual production. What moves the number:

• Orientation.South-facing is the benchmark. East and west give up real production, which is why we sometimes tell people a particular roof face won't work.
• Shading.A tree or a neighbor's roofline clipping your array for two hours a day is a permanent tax on output.
• Tilt and location. Roof pitch and where you live both change how much usable sun the panels see.

This is exactly why estimated annual kWh is the number that matters, not kW. A bigger system on a worse roof can deliver less than a smaller one on a better roof, and the kW rating alone will never tell you that.

Not every production estimate is equally conservative

Which leads to something worth understanding before you read any proposal. Every annual production estimate rests on assumptions: how many peak sun hours to credit your location, how much to subtract for system losses, and how carefully shading was modeled. Those are judgment calls, and reasonable people land in different places on them.

The effect is straightforward. A more optimistic set of assumptions, especially a smaller loss factor, produces a larger annual kWh figure and a better-looking proposal. Nothing about the equipment changed. Only the math on the page did.

The catch is that you don't live with the proposal, you live with the system. If an estimate leaned optimistic, the gap tends to surface later as production that trails what you were shown, long after the paperwork is signed.

We build our estimates on the conservative side on purpose. We apply real loss factors rather than best-case ones, we model shading from an actual assessment of your roof instead of assuming it away, and we would rather quote a number your system comfortably beats than one it spends twenty years chasing. Underpromising and overdelivering makes for a less exciting sales presentation. It also makes for a homeowner who is still happy in year eight.

Whoever you're talking to, it's fair to ask what loss factor they applied and whether the shading came from a real site assessment. Anyone confident in their numbers will happily walk you through them.

One more place this matters: your battery

Batteries carry both ratings, and they answer different questions. The kWh rating is capacity, how long it can carry your home. The kW rating is power, how much it can deliver at any one moment, which is what determines whether it can start your air conditioner rather than just run your lights.

You need enough of both, which is the whole subject of how many kWh of battery it takes to back up a house.