Solar System Sizing: The Formula and a Worked Example

Quick answer

Solar System Sizing is the process of choosing how many kilowatts (kW) of solar panels a property needs, by dividing the electricity it uses in a year by what one kW of panels produces there.

The result is then trimmed to fit the roof, the budget and the utility's rules on how much a home system may export or how large it may be.

Quick facts

The key facts about solar system sizing, with sources:

Formula: Annual kWh used ÷ annual kWh per kW at your site = kW needed
US average home use (2022): 10,791 kWh a year, about 899 kWh a month ¹
Production per kW, Charleston, SC: About 1,454 kWh/year (PVWatts run) ²
Production per kW, Atlanta, GA: About 1,402 kWh/year (PVWatts run) ²
NLR's average modelled home system: 7.15 kW DC, within a 3–11 kW range ⁴
Panel count: kW ÷ panel wattage in kW, rounded up

Key takeaways

Size starts with your own 12 months of kWh, not your house's square footage.
Divide yearly kWh by what one kW makes on your roof. That gives the kW you need.
In our Southeast models, one kW facing south makes about 1,400 to 1,450 kWh a year.
An average-use home lands near 7.4 kW in Charleston and 7.7 kW in Atlanta.
Then trim to fit: roof space, budget, utility size caps and what exports are worth.

What sizing means in plain terms

Sizing answers one question: how many panels should go on this home? The answer is given in kilowatts, or kW, of panel capacity. A kilowatt (kW) is 1,000 watts. Ten 440 W panels make 4.4 kW.

The goal is to match what the panels make in a year with what you want them to cover. That might be all of your use, or only part of it. Your use is counted in kilowatt-hours on your bills.

There is no "standard" size. NLR uses an average of 7.15 kW DC for its analyses, with a range of 3 to 11 kW ⁴. DOE's low-income weatherization program says its typical installs are about 5 kW ⁵. Your number depends on your bills, your roof and your utility.

How the formula works

Sizing uses three numbers. First, your yearly use in kWh. Second, how many kWh one kW of panels makes on your roof in a year. Third, the wattage of the panel you will use.

kW needed = yearly kWh ÷ kWh per kW. Then panels = kW ÷ panel kW, rounded up.

The middle number is where most quotes differ. It depends on sun, roof direction, tilt, shade and losses. A free model such as PVWatts gives it for any address ³. Some utilities publish a rule of thumb.

Berkeley Electric Cooperative in South Carolina tells members to divide annual kWh by 1,800, or average monthly kWh by 150 ⁷. That divisor is Berkeley's figure for its own area, not a national constant.

Worked example: an average-use home in two cities

Inputs: the EIA's 2022 US average of 10,791 kWh a year ¹, and production per kW from our PVWatts version 8 runs on 7 October 2026 (7 kW DC, standard module, roof mount, south-facing, 20° tilt, 14% losses) ². Panel: a 440 W module.

Step	Charleston, SC	Atlanta, GA
Annual use	10,791 kWh	10,791 kWh
÷ kWh per kW per year	÷ 1,454	÷ 1,402
= system size	about 7.4 kW	about 7.7 kW
÷ 0.44 kW per panel	16.9 → 17 panels	17.5 → 18 panels

Use your own 12 months of bills, not the national average. A roof facing east or west needs roughly 15% more kW for the same kWh, based on our Charleston runs (8,883 kWh east and 8,869 kWh west, against 10,179 kWh south).

Types of sizing target

Target	When it fits	Watch out for
Offset 100% of yearly use	Utility credits exports near retail	Summer surplus may earn little if exports are credited below retail
Offset daytime use only	Exports are credited well below retail	Smaller system, smaller savings, but more of each kWh used on site
Fill the roof	Planning for an EV or electric heating	Utility size caps; oversupply you cannot use or sell well
Size to a battery plan	You want backup or to shift solar to the evening	Battery losses mean more panel output is needed

Where size shows up on your quote

Quotes list "system size" in kW DC. That is the panel count times the panel wattage. Many also list an AC size, which is the inverter rating. The gap between them is the DC-to-AC ratio.

Look for "estimated annual production" in kWh, and an "offset" percentage. Offset is production divided by your yearly use. If a quote shows 100% offset, check which 12 months of use it was based on.

Your utility application lists the size too. It must match the permit drawings and the installed system.

Benefits of the right size, and the limits

A well-sized system

Covers the share of your bill you chose, without paying for surplus you cannot use.
Stays within your utility's size cap.
Leaves room on the roof to add panels later, if planned.
Gives a clear yardstick for checking output each year.

What sizing cannot fix

Your use will change as your household changes.
Weather varies, so no size hits 100% offset every year.
Export credit rates can change after you sign.
A small or shaded roof may cap size below your target.

Common sizing errors on quotes

Sizing from one summer bill instead of a full year.
Using a generic production figure instead of one for your roof's direction and shade.
Ignoring the inverter: a high DC-to-AC ratio is normal, but clipping losses should be in the estimate.
Adding a battery without resizing; storage losses mean more panel output is needed to fill it.
Using square footage as a guide. Floor area does not tell you how much power a home uses.

How size drives cost, lifespan and warranty

We do not quote dollar prices here. Size is the biggest single driver of the total price. See cost by system size for current figures.

Total kW sets most of the price. Quotes are often priced per watt, so a 7 kW system costs roughly 7,000 times the price per watt.
Price per watt can fall as size grows. Some costs, such as permits and design, are fixed per job.
Extra work. A larger system may need a main panel upgrade. Check with the panel upgrade check.
Lifespan. DOE guidance says major parts typically carry 25-year warranties and the system should produce for at least 30 years ⁵. Size for the use you expect over that time.

The process: sizing in six steps

You can do the math yourself. A licensed installer does the design and the installation.

Add up 12 months of kWh from your bills or the utility's usage download.
Adjust for known changes: an EV, a heat pump, a pool, or efficiency upgrades you plan first.
Get kWh per kW for your roof from PVWatts or the system size calculator.
Divide to get kW, then convert to panels with the panel count calculator.
Check the result against roof space from the site assessment.
Check your utility's size cap and export credit rate, then trim the size if needed.

Checking the size still fits, year after year

Sizing is not a one-time job. Each year, compare your use and your solar output. If your use grew, perhaps from an EV, you may want to add panels. Our guide to adding panels explains what is involved.

Panels also lose a little output each year. One current data sheet warrants no more than 0.33% loss a year after the first year ⁶. Over many years, that slowly lowers your offset.

Red flags in a sizing proposal

Ask questions if a quote shows any of these:

No list of the 12 months of use behind the size.
An offset well over 100% with no reason, such as a planned EV.
A size above your utility's cap.
Production per kW far above a PVWatts run for your address.
A battery added with no change to the panel count or production estimate.

Rules that cap system size

Utilities and state laws set the limits. Most caps are written into a net metering or interconnection tariff, in kW. Some are AC, some DC, and some are tied to your past use.

In South Carolina, Duke Energy Carolinas' Rider RSC applies to residential systems up to 20 kW AC ⁸. Berkeley Electric's Renewable Surplus Rider also sets a 20 kW AC limit ⁷.

Georgia Power's residential summary limits residential renewable facilities to 10 kW ⁹. Virginia law allows residential net metering up to 25 kW on investor-owned utilities ¹⁰.

The DOE advises asking your utility about eligible system size, rates and how bill credits work before you buy ⁴.

Sizing in SC, GA and VA

Duke Energy Carolinas' Solar Choice rider in South Carolina nets exports within each time-of-use period before crediting any excess at $0.0419 per kWh (Rider RSC, effective 1 January 2026) ⁸.

That pushes the best size toward what the home uses, not the largest system the roof holds. Berkeley Electric members can use its divide-by-1,800 rule ⁷.

In Georgia and Virginia, check your utility's current credit rate before sizing past your use.

System sizing compared with related terms

Term	What it is	Unit
System sizing	Choosing how many kW to install	kW
Solar production	What the system makes over time	kWh
Panel wattage	Power rating of one panel	W
Peak sun hours	Daily sunlight used in quick sizing math	hours/day
Solar design	Placing the panels and choosing equipment	—

Common misconceptions about sizing

Myth Bigger homes need bigger systems.: Reality Use, not floor area, sets size. A small home with electric heat can use more than a large one with gas.
Myth You should always offset 100%.: Reality Where exports are credited well below retail, a smaller system can pay back faster.
Myth The roof decides the size.: Reality The roof sets the maximum. Your use and utility rules set the target.
Myth One kW makes the same everywhere.: Reality In our runs, one south-facing kW made about 1,454 kWh in Charleston and 1,402 kWh in Atlanta.

When sizing matters most to you

Use these rules:

If your utility pays well below retail for exports, size closer to your daytime use.
If you plan an EV or heat pump within a few years, size for it now if the roof and cap allow.
If two quotes differ in size, compare their kWh per kW and the use figures behind them.
If you want a battery, size it with the battery sizing calculator and adjust the panels.
Next step: get your own number from our sizing calculator, then see average system size by state.

Questions about solar system sizing

How many solar panels do I need for a 2,000 sq ft house?

Floor area is a weak guide; use your yearly kWh instead. An average-use home, at 10,791 kWh a year, needs about 7.4 kW in Charleston, which is 17 panels at 440 W. A home with electric heat or an EV may need far more, and a very efficient one less.

What size solar system do I need for 1,000 kWh a month?

That is 12,000 kWh a year. At about 1,454 kWh per kW, from our Charleston south-facing model, divide to get roughly 8.3 kW. At Atlanta's 1,402 kWh per kW, it is about 8.6 kW. That works out to 19 or 20 panels at 440 W.

Should I oversize my solar system?

Only if you expect more use soon, such as an EV, and your utility credits exports fairly. Where excess is credited well below retail, extra panels pay back slowly. Also check the utility's size cap first, since a system above it may not qualify for credits at all.

Does adding a battery change system size?

It can. A battery returns only part of what it stores, so charging it from solar means setting aside more panel output than the battery delivers. If you want the battery to cover evening use every day, the panels must make enough for both daytime use and charging.

What is the average home solar system size?

NLR uses an average of 7.15 kW DC for its analyses, with a range of 3 to 11 kW. The DOE weatherization program says its typical installs are about 5 kW. Your right size depends on your own use and roof, so treat averages only as a rough check.

How do I find my yearly electricity use?

Add up the kWh from your last 12 monthly bills. Many utilities also let you download a year of usage from your online account. Use a full year so that both summer cooling and winter heating are counted. If you moved in recently, ask the utility for the home's past use.

Can I add more panels later?

Often, if the inverter has spare room and your utility's size cap allows it. The addition goes through the permit office and the utility again. New panels may not match the old ones exactly. Leaving roof space at the start makes adding later easier.

What is the 1,800 rule for solar sizing?

It is a rule of thumb from Berkeley Electric Cooperative in South Carolina.

It tells members to divide annual kWh by 1,800 to get system size in kW, or average monthly kWh by 150.

It is that utility's figure for its own area, not a national constant, so check it against a model for your roof.

Does roof direction change the size I need?

Yes. A roof facing away from south makes less per kW, so you need more kW for the same kWh. In our Charleston model, east- and west-facing arrays made about 13% less than south-facing, so they would need roughly 15% more kW to match.

Sources

U.S. EIA, FAQ: How much electricity does an American home use? (2022 data), retrieved October 8, 2026.
PVWatts V8 API runs by HyreSolar, 7 October 2026 (7 kW, standard module, roof mount, tilt 20°, azimuth 180°/90°/270°, losses 14%, NSRDB PSM V3 TMY), retrieved October 7, 2026.
NLR (formerly NREL), PVWatts V8 API documentation, retrieved October 8, 2026.
US DOE Solar Energy Technologies Office, Homeowner’s Guide to Going Solar, retrieved October 8, 2026.
US DOE Weatherization Assistance Program, Solar Frequently Asked Questions, retrieved October 8, 2026.
Qcells, Q.TRON BLACK (Q.TRON BLK S-G3R.12+/BFG 435–450) data sheet, 2025-08 Rev04, retrieved October 8, 2026.
Berkeley Electric Cooperative, Renewable energy (Renewable Surplus Rider), retrieved October 8, 2026.
Duke Energy Carolinas (SC), Rider RSC Residential Solar Choice (effective 1 January 2026), retrieved October 8, 2026.
Georgia Power, Behind-the-Meter Interconnection Summary for Residential Customers (rev. 15 Aug 2025), retrieved October 8, 2026.
Code of Virginia §56-594 (net energy metering), retrieved October 8, 2026.

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: Usage from the EIA FAQ (2022 data, re-read 8 Oct 2026); production per kW from PVWatts V8 runs on the stated date with inputs listed; the example divides those numbers.

Average system sizes from DOE pages. Berkeley and Duke terms from sc-local/facts.js and the source documents; GA and VA caps from the Georgia Power summary and Va. Code §56-594.

Suggest a correction. We fix errors and say what changed.