Quick answer
Kilowatt A kilowatt (kW) is a unit of power equal to 1,000 watts: it measures how fast electricity is being produced or used at a given moment.
Solar system sizes, inverter outputs and battery power ratings are quoted in kW. Energy over time, the thing on your bill, is the kilowatt-hour (kWh): one kilowatt sustained for one hour.
Quick facts
The key facts about kilowatt, with sources:
Key takeaways
- A kilowatt is 1,000 watts. It is a rate, like speed.
- A kilowatt-hour is the amount, like distance. Your bill counts kWh.
- A solar quote lists size in kW DC (panels) and often kW AC (inverter).
- A battery has two numbers: kW for what runs at once, kWh for how long.
- Utilities cap home systems in kW, often at 10–25 kW in SC, GA and VA.
Why the "per hour" matters
EIA puts it plainly: one kWh is one kilowatt generated for one hour. Run a 1 kW heater for three hours and you use 3 kWh. Run a 3 kW heater for one hour and you also use 3 kWh, but it drew three times the power while it ran.
Solar adds a twist. A watt rating on a panel is a ceiling reached only in strong light, so a 6 kW array is not producing 6 kW most of the day.
The number of kWh it makes depends on local peak sun hours. That is why two identical 6 kW systems in Columbia and Richmond produce different totals.
Sources: [1]
kW vs kWh at a glance
| Kilowatt (kW) | Kilowatt-hour (kWh) | |
|---|---|---|
| What it measures | Power: how fast | Energy: how much |
| Analogy | Speed of a car | Miles driven |
| Solar panels | System size, e.g. 6 kW DC | Yearly production, e.g. 8,765 kWh 7 |
| Home battery | How many appliances at once | How long they can run |
| Utility bill | Demand charges on some rates | Energy charges on every residential rate |
How kilowatts add up in a home system
Kilowatts are just watts in bundles of 1,000. To get a system’s size, multiply the panel count by each panel’s watts, then divide by 1,000. Sixteen 425 W panels give 6,800 W, or 6.8 kW.
That number is the DC side, before the inverter. The inverter changes DC power into the AC power your home uses. Each inverter has its own kW limit. With microinverters, you add up each unit’s output. Sixteen Enphase IQ8M units at 325 VA each come to 5.2 kVA of AC output.
So one system can have two kW numbers. The DC number is the panels. The AC number is the most the inverters can send to your home at once.
The kinds of kW on a solar quote
- kW DC: panel count × panel watts ÷ 1,000. Sixteen 425 W panels make 6.8 kW DC.
- kW AC: the inverter's output limit. It is often lower than kW DC on purpose; Powerwall 3 accepts up to 20 kW of solar at STC but delivers 11.5 kW AC continuous 3.
- Peak kW: a short burst a battery or inverter can give for seconds, for example when a motor starts.
- Price per watt: installers divide the system price by DC watts. Our cost-per-watt guide explains how to compare quotes on that basis.
- Interconnection limits: many utilities cap residential systems in kW AC. Duke Energy Carolinas' SC solar rider, for instance, applies to systems up to 20 kW AC 6.
Example: the two numbers on one battery
Tesla's Powerwall 3 data sheet lists both units side by side. The arithmetic below uses only those two figures.
| Spec | Value | What it answers |
|---|---|---|
| Maximum continuous discharge power | 11.5 kW AC 3 | Can it run the oven, AC and dryer together? |
| Nominal battery energy | 13.5 kWh AC 3 | For how long? |
| At full 11.5 kW | 13.5 ÷ 11.5 ≈ 1.2 hours | A big load drains it fast |
| At a steady 1 kW | 13.5 ÷ 1 = 13.5 hours | Essentials stretch it overnight |
A battery with plenty of kWh but too few kW will trip when a large motor starts; one with lots of kW but few kWh runs everything briefly. Both matter, and the battery sizing calculator checks each one. Real run time is lower once inverter losses and reserve settings are counted.
Example: what one kilowatt of panels makes in a year
NREL’s PVWatts tool modelled the same 6 kW array (south-facing, 25° tilt, 14.08% losses) in three cities. We divided each result by 6.
Each extra kW of panels adds about this many kWh a year on a clear, south-facing roof. Shade and steep or east-facing roofs lower it.
Where you meet kilowatts, megawatts and gigawatts
Homes live in kilowatts. You will see kW on the installer quote, the permit plans, the inverter and battery data sheets, and the utility’s connection forms. Some business bills also show peak kW as a demand figure.
Power plants and solar farms are counted in megawatts (1 MW = 1,000 kW) and national capacity in gigawatts (1 GW = 1,000 MW).
For context, NREL’s analysis uses an average home system of 7.15 kW DC. A 5 MW community solar farm has the size of several hundred home systems. The community solar vs rooftop guide compares the two.
What a bigger or smaller kW size gets you
More kW
- More yearly kWh, if the roof gets the sun.
- Often a lower price per watt, since fixed costs are spread out.
- Room for a future EV or heat pump.
Fewer kW
- Lower total price and less roof used.
- Less surplus sent to the grid at a low export rate.
- Fits under utility size caps more easily.
What the kW number cannot tell you
A kW size says nothing about how much energy you will get. That depends on sun, shade, roof angle and heat. A shaded 8 kW array can make less than an unshaded 6 kW one.
It also says nothing about savings. Savings depend on the kWh you use at home, the kWh you export and how your utility credits them. See net metering vs net billing for how export credits differ.
How kilowatts drive cost
System price grows with kW, but not in a straight line. Permits, design, utility paperwork and labor to get on the roof cost about the same for a small system as a mid-size one. So the cost per kW often falls as size rises.
Other cost drivers: panel and inverter type, battery kW and kWh, roof type, the number of roof faces, wiring distance and any electrical panel upgrade. We do not quote prices here. The cost by system size guide shows what changes at 5, 8 and 10 kW.
Lifespan is set by the parts, not the kW. Qcells warrants at least 90.58% of rated power after 25 years 4. Tesla lists a 10-year warranty on Powerwall 3 3.
How an installer picks your kW size
- Add up 12 months of kWh from your bills.
- Divide by the kWh per kW your roof makes, from a tool such as PVWatts 10.
- Check the result against roof space, shade and the utility’s kW cap.
- Choose panels and inverters, then list kW DC and kW AC on the plans.
- Submit the permit and the utility connection form; both name the kW size.
- After inspection, the utility gives permission to operate. See permission to operate.
Keeping a system near its rated kW
Upkeep for kW means checking that peak output on clear days stays near past years. Your monitoring app shows a daily power curve in kW. On a cool, clear spring day, the peak should be close to the inverter’s AC limit or a steady share of the DC size.
Heat lowers peak kW. The DOE says hot cells lose voltage faster than they gain current. Spring peaks are often higher than summer peaks for that reason. A slow drop over years is normal. A sudden drop calls for a service visit. Our monitoring service page explains what to watch.
Warning signs and when to call a pro
- Peak kW on clear days falls well below the same month last year.
- The power curve has a flat top every day at a level lower than expected.
- A battery trips or shuts off when a large appliance starts. Its kW limit may be too low for that load.
- A breaker trips during sunny afternoons.
- Any of these: call your installer or a licensed electrician. Do not open equipment yourself.
Rules that use the kilowatt
Unit. NIST SP 330 sets the SI prefix kilo as 1,000, so a kilowatt is exactly 1,000 watts.
Utility size caps. Duke Energy Carolinas’ SC Rider RSC, effective 1 January 2026, covers systems up to 20 kW AC. Georgia Power limits residential renewable systems to 10 kW. Code of Virginia §56-594 allows residential net metering up to 25 kW on investor-owned utilities.
Equipment. Inverters that connect to the grid are tested to UL 1741 and IEEE 1547-2018, as the Powerwall 3 and Enphase IQ8 sheets list 3 5. The local building department applies its adopted National Electrical Code to the kW of panels and the size of your service.
Mistakes we see in quotes and forums
- Myth "My 8 kW system makes 8 kWh a day."
- Reality It makes roughly kW × peak sun hours × system losses. In Columbia, SC, PVWatts models about 4 kWh per kW per day across the year 7.
- Myth "kW and kWh are interchangeable on a battery."
- Reality They answer different questions: power (what runs at once) and energy (how long).
- Myth "Bigger kW always means a bigger bill offset."
- Reality Only if the roof gets the sun. A shaded 8 kW array can make less than an unshaded 6 kW one.
- Myth "kW DC and kW AC should be equal."
- Reality DC is usually higher. The gap is the DC-to-AC ratio, set on purpose.
Kilowatt limits in SC, GA and VA
In South Carolina, Duke Energy Carolinas’ Rider RSC applies up to 20 kW AC 6, and Dominion Energy SC’s Solar Choice rider also stops at 20 kW AC 13.
A 6 kW array near Columbia makes about 1,461 kWh per kW a year 7. Georgia Power caps residential systems at 10 kW 14. Virginia allows up to 25 kW on investor-owned utilities 15.
When the kilowatt matters to you
- Comparing quotes: check kW DC, kW AC and price per watt together.
- Sizing: divide your yearly kWh by your local kWh per kW.
- Buying a battery: list what must run at once and check its kW.
- Signing up with the utility: make sure the size fits its kW cap.
Size it yourself
- System size calculatorHow many kW your yearly kWh calls for
- Panel count calculatorTurn a kW target into a number of panels
- Typical system size by stateHow your plan compares
Questions about kilowatt
What is the difference between kW and kWh?
kW measures power, the rate at which electricity flows right now. kWh measures energy, the amount used or made over time. A 2 kW load running for 3 hours uses 6 kWh. Your bill counts kWh, while system and battery sizes are in kW.
How many kWh does a 1 kW solar system produce?
It depends on sunlight. PVWatts models a south-facing array tilted 25° near Columbia, SC, at about 1,461 kWh per kW per year, or roughly 4 kWh per kW per day on average. Atlanta comes out near 1,414 and Richmond near 1,361.
Is 1 kW equal to 1,000 watts?
Yes. A kilowatt is exactly 1,000 watts, and a megawatt is 1,000 kilowatts. The prefix kilo means 1,000 in the SI system that NIST publishes. So a 425 W panel is 0.425 kW.
What does kW DC mean on a solar quote?
It is the combined lab rating of the panels: panel count times panel watts, divided by 1,000. The inverter's kW AC output is a separate, usually lower, number. Ask for both, since utility caps often use kW AC.
How many kW of solar does a house need?
Divide your yearly kWh by what one kW makes where you live. An average US home uses about 10,791 kWh a year; near Columbia, SC, that works out to about 7.4 kW. Your own 12 months of bills are the input that matters.
What size battery in kW do I need?
Enough kW to run everything you want on at the same time. Add the watts of those loads, and allow for motors that surge at start-up. Powerwall 3 lists 11.5 kW continuous. Then check kWh for how long it must last.
Why is my inverter rated in fewer kW than my panels?
Because panels rarely reach their full rating on a roof. A smaller inverter costs less and still catches most of the energy. On the brightest hours, a little power is cut off, which is called clipping. Installers choose the ratio on purpose.
Is a 10 kW system too big for a home?
It depends on your use and your utility. A home with an EV or electric heat may use enough kWh to justify it. But Georgia Power caps residential systems at 10 kW, and export credits may be low, so size to your own use.
Sources
- US EIA, Electricity explained: Measuring electricity, retrieved .
- NIST, SP 330 (The International System of Units), Section 2, retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- Qcells, Q.TRON BLK M-G2+ series data sheet (420–440 W, 2026-02 Rev01 NA), retrieved .
- Enphase Energy, IQ8M and IQ8A Microinverters data sheet (DSH-00243-3.0-EN-2024-12-10), retrieved .
- Duke Energy Carolinas (SC), Rider RSC Residential Solar Choice (effective 1 January 2026), retrieved .
- NREL (National Laboratory of the Rockies), PVWatts v8 API, NSRDB PSM V3 TMY; 6 kW, 25° tilt, south, 14.08% losses, Columbia SC (34.0, −81.03), retrieved .
- NREL PVWatts v8 API, same inputs, Atlanta GA (33.75, −84.39), retrieved .
- NREL PVWatts v8 API, same inputs, Richmond VA (37.54, −77.44), retrieved .
- US DOE Solar Energy Technologies Office, Homeowner’s Guide to Going Solar, retrieved .
- US DOE Solar Energy Technologies Office, Solar Performance and Efficiency, retrieved .
- IEEE, 1547-2018 Standard for Interconnection and Interoperability of Distributed Energy Resources, retrieved .
- Dominion Energy South Carolina, Residential Solar Choice rider (Rider to Rate 5), retrieved .
- Georgia Power, Behind-the-Meter Interconnection Summary for Residential Customers (rev. 15 Aug 2025), retrieved .
- Code of Virginia §56-594 (net energy metering), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Unit relationships from EIA and NIST. Battery, panel and inverter figures from the manufacturer data sheets (re-read 2026-10-08); the 20 kW AC limit from the Duke rider held in sc-local/facts.js. Production per kW from PVWatts v8 runs made on 7 Oct 2026 (ac_annual ÷ 6 kW).
Suggest a correction. We fix errors and say what changed.