How Many Solar Panels Do I Need?

Enter your monthly kWh usage and location — get your panel count instantly.

Your energy usage

Monthly kWh usage?

kWh/mo

Peak sun hours?

hrs/day

Panel wattage?

System losses?

Panel footprint?

You need approximately

24 solar panels

System size9.6 kW

Annual production13,560 kWh

Your annual usage13,200 kWh

Roof space needed432 sq ft

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How to Use This Calculator

Enter your monthly kWh usage

The most important input is your monthly kWh usage — not your dollar bill amount. Find this on your utility bill under "kWh used" or "energy consumed." The US average is about 875 kWh/month, but your actual usage depends on home size, climate, and appliances. If you have electric heating, an EV, or a pool, your usage could be 2–4× the average.

Use the scenario buttons

Not sure where to start? Click Apartment (500 kWh/mo), Small house (750 kWh/mo), Medium house (1,100 kWh/mo), or Large house (1,800 kWh/mo) to pre-fill realistic values for your home type. Adjust the monthly kWh to match your actual bill.

Set your peak sun hours

Peak sun hours vary dramatically by location. They represent the equivalent hours of full-intensity sunlight per day averaged over the year — not total daylight. Phoenix averages 6.5, Miami 5.5, Denver 5.0, Chicago 4.0, Seattle 3.5. Enter your local value or use our Peak Sun Hours Calculator to look up your city.

Choose panel wattage

Modern residential panels range from 350W (value tier) to 450W+ (premium). Higher-wattage panels reduce the total count but cost more per unit. The 400W default is the 2026 sweet spot — widely available, competitively priced, and a good balance of efficiency and cost.

Expand More options

Use system losses to model your specific installation. A south-facing roof with no shading can achieve 90%+ efficiency. A roof with partial shading or an older string inverter might drop to 78–82%. The panel footprint option adjusts the roof area estimate for different panel dimensions.

The Formula

Panel count calculation uses this standard solar sizing formula:

Annual kWh needed = Monthly kWh × 12 Daily kWh needed = Annual kWh ÷ 365 System kW = Daily kWh ÷ (Peak sun hours × System efficiency) Panel count = ceiling(System kW × 1000 ÷ Panel wattage) Roof area = Panel count × Panel footprint (sq ft)

The ceiling function means we always round up — you can't have half a panel. So if the math gives 14.3 panels, you get 15 panels, which means your system slightly oversizes to ensure full coverage.

Try changing the peak sun hours field to see how location dominates the panel count. Going from 3.5 hours (Seattle) to 6.5 hours (Phoenix) nearly cuts the panel count in half — location matters more than almost any other factor.

Example

The Chen family — Chicago, IL

The Chens use 1,200 kWh/month in their 3-bedroom home. Chicago averages 4.0 peak sun hours per day. They're considering 400W panels with standard 86% system efficiency.

Monthly kWh usage1,200 kWh

Annual kWh usage14,400 kWh

Peak sun hours4.0 hrs/day

Panel wattage400W

System efficiency86%

Result

Panels needed29 panels

System size11.6 kW

Annual production~14,590 kWh

Roof space needed~522 sq ft

If the same family lived in Phoenix (6.5 PSH), they'd only need 18 panels for the same consumption. That's 11 fewer panels, saving roughly $4,000–$6,000 on installation. Location is the most powerful variable in solar sizing.

FAQ

How many solar panels does the average US home need? ▼

The average US home uses about 875 kWh/month (10,500 kWh/year). With average sun exposure (4.5 peak sun hours) and 400W panels at 86% efficiency, that works out to 20–22 panels and an 8–9 kW system. But averages rarely describe any individual home — enter your actual monthly kWh from your utility bill for a precise number.

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

For 1,000 kWh/month with 400W panels and 4.5 peak sun hours at 86% efficiency, you need about 22 panels (8.8 kW system). In sunnier areas like Phoenix (6.5 PSH), you'd only need 15 panels. In Seattle (3.5 PSH), you'd need 27 panels. The panel count varies by up to 80% based on location alone.

Can I put solar panels on a flat roof? ▼

Yes — flat roofs are actually great for solar because you can install panels on adjustable racking at the optimal angle for your latitude. The tradeoff is that tilted racks require more spacing between rows to avoid self-shading, so flat roofs may fit fewer panels per square foot than a south-facing pitched roof. The roof area estimate in this calculator assumes a pitched roof — add 20–30% to the sq ft if your roof is flat.

What happens if I don't have enough roof space for all the panels I need? ▼

If roof space limits your panel count, you have three options: (1) use higher-wattage panels (450–500W) to fit more capacity in less space, (2) reduce your electricity consumption before sizing (the cheapest kWh is one you don't use), or (3) accept partial offset — size for 70–80% of your usage and reduce your bill rather than eliminate it. Use our Solar Roof Calculator to find the maximum system your roof can accommodate.

Do I need more panels in winter? ▼

This calculator sizes your system based on annual average sun hours, which already accounts for the seasonal variation. If your goal is to cover 100% in winter (the worst solar month), you'd need to significantly oversize — producing 200–300% more in summer than you can use. Most homeowners size for annual average coverage and use net metering credits earned in summer to offset winter grid usage. This is the standard approach recommended by NREL.