How Many Solar Panels Do I Need? Solar System Size Calculator

Work out how many solar panels your home needs from your electric bill or your 12-month kWh total. You get a panel count, system size in kW, expected yearly output and the roof area the panels will take.

Your state

Fills in typical rates and sunshine for your state. You can change any value.

Average monthly bill

Used only if yearly kWh below is 0.

Electricity rate

Your all-in price per kWh: total energy charges divided by kWh used.

Yearly electricity use

Best option: the 12-month kWh total from your utility. Leave at 0 to estimate from your bill.

Yearly output per kW

kWh a 1 kW system makes in a year where you live (NREL PVWatts is the best free source).

How much of your use to cover

Under reduced export credits, 70–90% often pays back faster than 100%.

Panel wattage

Most home panels sold in 2026 are about 400–450 W.

Your result

Panels needed 20 panels

System size: 8 kW
Yearly use (kWh): 10,549 kWh
Expected yearly output: 10,800 kWh
Roof area for panels: 420 sq ft

To cover the share of your use you chose, plan on about 20 panels (8 kW), producing about 10,800 kWh a year and taking roughly 420 sq ft of roof.

Educational estimate only. Not financial, tax or legal advice.

Yearly use vs expected solar output

View as table

kWh per year	kWh
Your use	10,549 kWh
Solar output	10,800 kWh

Compare: cover 80%, 100% or 120% of your use

Result	A 80% offset	B 100% offset	C 120% (room for an EV)	Difference (B − A)
Panels	16	20	24	+4
System size	6.4 kW	8 kW	9.6 kW	+1.6 kW
Yearly output	8,640 kWh	10,800 kWh	12,960 kWh	+2,160 kWh
Roof area	336 sq ft	420 sq ft	504 sq ft	+84 sq ft

Results update as you type. Without JavaScript you see the worked example below.

How to use it

Pick your state to fill a typical electricity rate and yearly output per kW.
Enter your yearly electricity use in kWh, or leave it at 0 and enter your average monthly bill and rate.
Set how much of your use to cover, such as 80% under low export credits or 120% if an EV is coming.
Enter the panel wattage from your quote, then read the panel count, kW and roof area.

On this page

How many solar panels do I need? Divide the kWh you want to cover by what 1 kW of panels makes where you live, then by the panel wattage. With the defaults, a home using about 10,549 kWh a year needs 20 panels of 400 W, an 8 kW system that makes about 10,800 kWh a year on roughly 420 sq ft of roof.

How to use the solar panel calculator

Your state. Fills a typical Electricity rate and Yearly output per kW. Change either if you have better numbers.
Yearly electricity use. The best input: the 12-month kWh total from your utility. Leave it at 0 to estimate from your bill.
Average monthly bill and Electricity rate. Used only when yearly kWh is 0. The tool divides the bill by the rate and multiplies by 12.
Yearly output per kW. kWh a 1 kW system makes in a year at your location. The default is 1,350. For your roof, run PVWatts (opens in a new tab).
How much of your use to cover. 100% by default. Lower it where exports earn little; raise it for a planned EV or heat pump.
Panel wattage. From your quote; 400 W by default.

The results include a bar chart comparing your yearly use with the expected solar output, plus a scenario table that reruns your inputs at 80%, 100% and 120% coverage (the last labeled "room for an EV") so you can see how panel count, kW, output and roof area change.

How the calculation works

Yearly use (kWh)  = your 12-month kWh, or monthly bill ÷ rate × 12
Target kWh        = yearly use × coverage %
kW needed         = target kWh ÷ yearly output per kW
Panels            = round up (kW needed × 1,000 ÷ panel watts)
System size (kW)  = panels × panel watts ÷ 1,000
Expected output   = system size × yearly output per kW
Roof area         = panels × 21 sq ft

Assumptions that match the code:

Round up to whole panels, so the system size and output are usually a little above the target.
Yearly output per kW is already net of losses (wiring, heat, inverter), as a PVWatts AC result is.
About 21 sq ft per panel, the footprint of a typical 400 W module, with no allowance for setbacks or gaps.
No fixed charge in the bill method. Subtract it from the bill first if you know it.

Worked example

Default inputs: $160 monthly bill, 18.2¢ per kWh, yearly use left at 0, 1,350 kWh per kW, 100% coverage, 400 W panels.

Yearly use: $160 ÷ $0.182 × 12 = 10,549 kWh.
kW needed: 10,549 ÷ 1,350 = 7.81 kW, so 7,814 W ÷ 400 W = 19.5, rounded up to 20 panels.
System size: 8 kW; expected yearly output: 10,800 kWh; roof area: 420 sq ft.

The default scenario table shows 16 panels (6.4 kW, 8,640 kWh, 336 sq ft) at 80%, 20 panels at 100%, and 24 panels (9.6 kW, 12,960 kWh, 504 sq ft) at 120%.

Second example, Houston-area home: 13,000 kWh a year from the utility account, 1,421 kWh per kW (PVWatts for Houston), 90% coverage, 430 W panels.

Target: 11,700 kWh, which needs 8.23 kW, or 19.1 panels, rounded up to 20 panels.
System size: 8.6 kW; expected output: 12,221 kWh a year (94% of use); roof area: about 420 sq ft.

Same use, different sunshine

Panels needed for 10,000 kWh a year with 400 W panels, using PVWatts yields for each city (standard settings: 20° tilt, south-facing, 14% losses):

Location	kWh per kW per year	kW needed	400 W panels
Phoenix	1,755	5.70	15
Los Angeles	1,677	5.96	15
Denver	1,595	6.27	16
Tampa	1,568	6.38	16
Charlotte	1,446	6.92	18
Houston	1,421	7.04	18
Atlanta	1,402	7.13	18

What your result means

The panel count is a starting point for comparing quotes, not a design. If an installer proposes far more panels than this shows for your real usage, ask why. If they propose fewer, ask what share of your use it covers.

Next, check that the panels fit with the roof calculator, and price the system in the solar panel cost calculator. Our guide to how much solar panels cost explains what drives the price. The median US home system installed in 2025 was 7.7 kW (LBNL, 2026 (opens in a new tab)).

Rules that change the answer

Export credits. Under full retail net metering, such as Florida's PSC Rule 25-6.065 for investor-owned utilities, sizing to 100% makes sense. Under California's Net Billing Tariff (opens in a new tab), extra exports earn far less than retail, so a smaller system or a battery often works better. Arizona's APS and TEP also pay a lower export rate, as our Arizona solar guide explains.
Texas. There is no statewide net metering; in deregulated areas your retail provider decides what exports earn. See solar panels in Texas.
Utility size caps. Many utilities limit system size to your past usage or a kW cap. Ask before ordering more than 100%.
Roof and orientation. DOE notes that south-facing roofs with a 15 to 40 degree slope generally perform best (DOE (opens in a new tab)). East or west roofs make less per kW.

Common mistakes

Using one summer bill instead of 12 months of usage.
Forgetting a planned EV, pool pump or heat pump.
Using the national default yield instead of your own PVWatts figure.
Sizing to 120% where exports earn a fraction of retail.
Assuming the roof area shown includes setbacks and walkways.

Limits of this calculator

This is an educational sizing estimate, not a system design. It does not model shading, roof orientation, inverter sizing, utility size caps or seasonal mismatch between use and output. A site survey and a production estimate for your exact roof should confirm the final size.

Frequently asked questions

How many solar panels does an average house need?

With the defaults, a home using about 10,549 kWh a year needs 20 panels of 400 W, an 8 kW system. Your number can be quite different: the same 10,000 kWh needs 15 panels in Phoenix but 18 in Houston, because each kW of panels makes more power where the sun is stronger.

Should I size solar to cover 100% of my use?

Only if exports earn close to the retail rate. Where exports are credited at a fraction of retail, such as California's Net Billing Tariff, extra panels mostly produce low-value exports, so covering 70 to 90 percent often pays back faster. Under full retail net metering, as Florida's investor-owned utilities offer, 100 percent is reasonable.

Is it better to use my yearly kWh or my monthly bill?

Use the 12-month kWh total from your utility account whenever you can. The bill method divides dollars by your rate, so fixed charges and tiered or time-of-use pricing can throw it off. Most utilities show 12 months of usage online or on the bill's usage chart.

What panel wattage should I enter?

Use the wattage on your quote. Most home panels sold in 2026 are roughly 400 to 450 watts, which is why 400 W is the default. Higher-wattage panels mean fewer panels and less roof for the same kW, but the kW you need does not change.

Where do I find the yearly output per kW for my house?

Run PVWatts, the free tool from the National Laboratory of the Rockies (formerly NREL), for your address and roof angle, then divide its yearly kWh by the system size. PVWatts with standard settings gives about 1,421 kWh per kW in Houston and 1,755 in Phoenix.

Should I add panels for a future electric car or heat pump?

If you expect one within a few years, sizing up now is usually cheaper than adding panels later, because permitting and labor are partly fixed costs. The 120 percent scenario in the tool shows the effect. An EV driven 12,000 miles a year adds about 4,000 kWh, which the EV solar calculator converts to panels.

Why does the roof area seem small?

The tool counts about 21 square feet per panel, the panel footprint only. Fire-code setbacks, vents, chimneys and shading take more space, so check whether the panels physically fit with the roof calculator.

Sources

Checked against these sources. Dates show when each was last reviewed.

PVWatts Calculator (opens in a new tab), National Laboratory of the Rockies (formerly NREL) (as ofOctober 2026)
Homeowner's Guide to Going Solar (opens in a new tab), U.S. Department of Energy (as ofOctober 2026)
U.S. Distributed Solar and Storage: 2026 Data Update (opens in a new tab), Lawrence Berkeley National Laboratory (as ofOctober 2026)
Net Billing Tariff (opens in a new tab), California Public Utilities Commission (as ofOctober 2026)
Rule 25-6.065: Interconnection and Net Metering of Customer-Owned Renewable Generation (opens in a new tab), Florida Administrative Code (via Cornell Law LII) (as ofOctober 2026)
Electric Power Monthly, Table 5.6.B (opens in a new tab), U.S. Energy Information Administration (as ofOctober 2026)

Last updated: October 18, 2026How we build our calculators

On this page

25D credit (Residential Clean Energy Credit): A 30% federal tax credit homeowners could claim for solar and batteries they paid for. It ended for expenditures made after December 31, 2025; the IRS counts the expense when installation is completed.
48E credit: A federal business tax credit for clean electricity property. Companies that own leased or PPA systems on homes may claim it, subject to construction deadlines and sourcing rules. The homeowner does not claim it.
Avoided cost: What it would cost a utility to produce or buy the same power elsewhere. Some states pay rooftop exports at an avoided-cost rate, which is far below retail.
Dealer fee: A charge a solar lender takes from the loan, usually built into the amount you borrow so the advertised interest rate looks low. The CFPB has warned these fees can add tens of percent to the price.
Degradation: The slow loss of a panel’s output over time. NREL research puts the median around 0.5% per year, and most panel warranties guarantee roughly 80–90% of original output after 25 years.
Escalator: A yearly percentage increase written into a lease or PPA, often 0–3%. A 2.9% escalator raises the payment by about 100% over 25 years.
FEOC rules: Federal “foreign entity of concern” sourcing limits added in 2025 that a business must meet to claim 48E credits on projects starting construction in 2026 or later.
Interconnection: Your utility’s approval to connect a solar system to the grid. Most utilities require an application before installation and a final permission to operate (PTO) before you turn it on.
Inverter: The device that turns the direct current (DC) from panels or batteries into the alternating current (AC) your home uses. Types include string inverters, microinverters and hybrid inverters that also manage a battery.
Islanding (backup mode): When a solar-plus-battery system disconnects from the grid during an outage and keeps powering your home. Ordinary grid-tied solar without a battery shuts off in an outage for line-worker safety.
kW (kilowatt): A unit of power: how fast electricity is made or used at one moment. Solar system size is quoted in kW of panel capacity (DC). A 7 kW system has panels rated 7,000 watts in total.
kWh (kilowatt-hour): A unit of energy: using 1 kW for one hour. Your electric bill charges you per kWh, and solar output and battery capacity are measured in kWh.
Microinverter: A small inverter mounted under each panel. Shade on one panel affects only that panel, and each panel can be monitored separately. Usually costs a little more than one central string inverter.
NABCEP certification: A voluntary professional certification for solar installers from the North American Board of Certified Energy Practitioners. It is a useful signal, but you should also check state contractor licensing.
NEM 3.0: The common name for California’s Net Billing Tariff, which applies to new rooftop solar customers of PG&E, SCE and SDG&E who applied on or after April 15, 2023. Exports earn hourly values far below retail rates.
Net billing: A billing rule where exported solar earns a credit set by a separate, usually lower, export rate rather than the full retail price. California’s Net Billing Tariff (often called NEM 3.0) is the best-known example.
Net metering: A billing rule where each kWh your panels send to the grid earns a credit equal (or close) to the retail price you pay. Credits offset power you buy later, often rolling over month to month.
Offset: The share of your yearly electricity use that solar is designed to produce. A 100% offset aims to match a year of use; under low export credits a smaller offset can pay back faster.
Payback period: How many years of bill savings it takes to recover what you paid for a system. A shorter payback means a better investment, all else equal.
Peak sun hours: Daily sunlight expressed as the number of hours at full-strength sun (1,000 watts per square meter). Five peak sun hours means the day’s sunlight equals five hours of full sun.
PPA (power purchase agreement): A contract where a company owns the system and you pay for the electricity it produces at a set price per kWh, often rising each year.
PTO (permission to operate): The utility’s final go-ahead to switch on a grid-connected system. Running a system before PTO can violate your utility agreement.
Round-trip efficiency: The share of energy you get back out of a battery compared with what went in. About 90% is typical for home lithium batteries.
Solar lease: A contract where a company owns the panels on your roof and you pay a fixed monthly amount, often with a yearly increase (escalator), usually for 20–25 years.
Specific yield: Yearly energy a solar system produces per kW of capacity, in kWh/kW. It depends on sunshine, tilt, direction and shade. Sunny Southwest roofs often reach 1,600–1,800; cloudier northern roofs 1,100–1,300.
Time-of-use (TOU) rate: An electricity plan where the price per kWh changes by time of day, with the highest prices in late afternoon and evening. Batteries save money by covering those peak hours.
True-up: The annual settlement on a net metering or net billing account. Charges and credits from the past 12 months are totaled, and you pay any balance owed. Leftover credits may be paid out at a low rate or lost.
Usable capacity: The share of a battery’s energy you can actually draw. Spec sheets list it in kWh; compare usable, not total, capacity between batteries.

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