How Many Solar Panels Do I Need? Size a Home System From Your Bill

How many solar panels do I need to power my house? You can answer that yourself in about 15 minutes with three inputs: your yearly kWh use, how much a kilowatt of panels produces at your address, and the wattage of the panels you’re quoted. Installers use the same math, plus a shade report.

Step 1: Add Up 12 Months of kWh

Find “kWh used” on each bill, or download a year of usage from your utility’s website. Use a full year, since heating and cooling swing usage a lot by season. If you plan to buy an EV or a heat pump, add its expected use.

Step 2: Get Production per kW From PVWatts

NREL’s free PVWatts calculator (pvwatts.nrel.gov) estimates yearly output for a 1kW system at your address, using local weather data, tilt and direction. Enter 1kW, your roof’s direction and tilt, and the default system losses (about 14%), and note the yearly kWh it gives you.

Step 3: How Many Solar Panels Do I Need? The Math

System size (kW) = yearly kWh ÷ PVWatts kWh per kW

Panel count = system size (W) ÷ panel wattage

Example (illustrative)	Value
Yearly use	10,800kWh
PVWatts output, 1kW at the example address	1,350kWh/kW
System size	10,800 ÷ 1,350 = 8.0kW
Panel wattage	400W
Panels needed	8,000 ÷ 400 = 20 panels

Example numbers only. Your location, roof and panel choice change the result.

Step 4: Adjust for Your Utility’s Rules

• Net metering: sizing to about 100% of yearly use is common. Many utilities cap system size near your past use.
• Net billing: exports earn less, so a slightly smaller system that you mostly use yourself, often paired with a battery, can pay back better.

Step 5: Check the Roof

A typical residential panel is roughly 18 to 22 square feet. Fire-code setbacks, vents, shade and roof faces cut down the usable area. Ask installers for a shade report (solar access percentage) and a roof layout before you compare prices.

Common Mistakes

• Sizing from a single month’s bill.
• Forgetting future loads like an EV or heat pump.
• Comparing quotes by panel count instead of kW and estimated yearly kWh.

Worked Examples for Three Different Homes

The same formula works for any home. Below are three illustrative households using EIA’s national average as the middle case. EIA says the average U.S. home used about 10,500 kWh in 2023. The production figures stand in for what PVWatts might return in a cloudier, average and sunny location. Run your own address for real numbers.

Home	Yearly use	PVWatts kWh per kW (example)	System size	400 W panels
Small, gas heat, cloudy area	6,000 kWh	1,100	5.5 kW	14
Average U.S. home	10,500 kWh	1,350	7.8 kW	20
Large, all-electric, sunny area	18,000 kWh	1,600	11.3 kW	29

Panel counts are rounded up, since you can’t install part of a panel. Notice that the sunny home needs fewer panels per kWh used. Location matters as much as consumption.

How Panel Wattage Changes the Count

Higher-wattage panels mean fewer panels for the same system size, but they’re also physically larger. Using the 7.8 kW average-home example:

Panel rating	Panels for 7.8 kW	Approx. roof area at 18–22 sq ft each
350 W	23	414–506 sq ft
400 W	20	360–440 sq ft
430 W	19	342–418 sq ft

Always compare quotes by total kW and estimated yearly kWh. Twenty 400 W panels and nineteen 430 W panels are nearly the same system.

Adding an EV or Heat Pump

Future loads are the most common thing people forget. Here is how to estimate them:

• Electric vehicle: yearly miles ÷ miles per kWh. The DOE’s fueleconomy.gov lists efficiency for each EV, usually as kWh per 100 miles. Example: 12,000 miles at 30 kWh/100 mi = 3,600 kWh a year. At 1,350 kWh per kW, that adds about 2.7 kW, or 7 more 400 W panels. See solar panels to charge a Tesla.
• Heat pump: ask your installer for a load calculation, or compare a year of gas use converted to kWh with the heat pump’s rated efficiency. Results vary a lot with climate and insulation.

Step-by-Step: Reading PVWatts Results

1. Go to pvwatts.nrel.gov and enter your address.
2. On System Info, set DC system size to 1 kW, module type Standard, array type “fixed (roof mount)”, and leave system losses at 14% unless you know otherwise.
3. Set tilt to your roof pitch (a 6/12 roof is about 27°) and azimuth to the direction the roof faces (180° = south, 90° = east, 270° = west).
4. Read AC Annual kWh on the results page. That is your kWh per kW per year.
5. For a house with two roof faces, run each face separately and size each part.

How Much One Panel Produces

Each panel’s yearly output is its rating in kW times your PVWatts figure. Using the same three example locations:

Panel	1,100 kWh/kW	1,350 kWh/kW	1,600 kWh/kW
350 W	385 kWh/yr	473 kWh/yr	560 kWh/yr
400 W	440 kWh/yr	540 kWh/yr	640 kWh/yr
430 W	473 kWh/yr	581 kWh/yr	688 kWh/yr

Divide by 12 for a rough monthly average. Summer months produce well above it and winter months well below, so expect your bill credits to swing by season.

Why Two Installers Quote Different Sizes

• Different production models. Some use PVWatts, others use their own software with on-site shade measurements. Ask each for the estimated first-year kWh.
• Inverter sizing. It’s normal for the panels’ DC rating to be somewhat higher than the inverter’s AC rating, because panels rarely hit their full nameplate output. Compare DC kW to DC kW.
• Utility size caps. Many utilities limit a system to about your past 12 months of use. An installer who knows the local rules may quote smaller.
• Self-use goals. Under net billing, a slightly smaller array plus a battery can beat a big array that exports most of its output at low rates. See net metering vs net billing.

Quick Sizing Checklist

1. 12 months of kWh, plus planned EV or heat pump use.
2. PVWatts kWh per kW for each roof face.
3. Divide to get kW, then by panel wattage to get a count.
4. Check roof space and setbacks.
5. Adjust for your utility’s export rules and size cap.

Roof Pitch Conversion Table

PVWatts asks for tilt in degrees, but roofers talk in pitch (rise over 12 inches of run). The angle is arctan(rise ÷ 12):

Roof pitch	Tilt in degrees
4/12	18.4°
6/12	26.6°
8/12	33.7°
10/12	39.8°
12/12	45.0°

From Panels to Batteries

Panel count covers your yearly energy. Batteries are sized separately, by the kWh you want to use at night or during an outage. For a full example with both, see how many solar panels for 1,000 kWh per month and how many batteries to power a house. Square-footage questions are covered in solar panels for a 2,000 sq ft house.

FAQ

How many solar panels does a 2,000 sq ft house need?

Square footage doesn’t decide it. Your kWh use does. Two houses of the same size can differ by 2x.

How many kWh does one panel make?

Example: a 400W panel at 1,350kWh/kW makes about 540kWh per year.

Does a battery change how many panels I need?

Not for yearly energy. It changes when you use the energy.

How many solar panels do I need for 1,000 kWh a month?

That’s 12,000 kWh a year. At an example 1,350 kWh per kW, you’d need about 8.9 kW, or 23 panels at 400 W.

How many solar panels do I need to go off grid?

More than for a grid-tied system, because you have to cover the worst winter month and charge batteries. Off-grid sizing starts from December production, not the yearly average. See our off-grid solar sizing guide.

Sources: NREL PVWatts Calculator (pvwatts.nrel.gov); NREL Solar Research; DOE Homeowner’s Guide to Going Solar (energy.gov); EIA, average household electricity use (2023); DOE/EPA fueleconomy.gov. Accessed Oct 2026.

Related: Adjustable Solar Panel Mounting Brackets Review, Klein CL390 Clamp Meter Solar Panel Test, BougeRV Arch Pro 200W Flexible Solar Panel Review