Free Solar Tool
Solar Panel Calculator
Wondering how many solar panels you need? Enter your monthly usage or electric bill, your region's sun hours, panel wattage, and target offset to estimate your system size in kW, the number of panels, and the roof area required.
Turn System Sizing Into a Signed Solar Proposal
Calculate How Many Solar Panels You Need
Adjust the inputs to match your home. The system size, panel count, and roof area update instantly.
The average US home uses about 900 kWh per month — check your utility bill for your exact figure.
Your estimated solar system
System size
7.1 kW
Panels needed
18
Roof area needed
315 sq ft
Monthly usage
900 kWh
Sun hours
5.3/day
Offset target
100%
Est. annual production
10,950 kWh
These figures assume an 80% system performance ratio and standard regional sun hours. Real output depends on roof orientation, tilt, shading, and equipment. Always get a detailed design from a licensed solar installer before buying.
Ballpark estimate only
These figures are rough estimates for planning purposes only. Actual results will vary based on your specific situation, local market conditions, and contractor pricing. Do not use these estimates for budgeting, contracts, or financial decisions without first obtaining written quotes from licensed professionals.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on typical averages and should be used for general planning purposes only. They are not a substitute for a professional assessment or written contractor quote. Actual costs, savings, and results will vary significantly based on your specific circumstances, local market conditions, equipment choices, and contractor pricing. Always confirm any estimate with a licensed contractor, financial advisor, or qualified professional before making purchasing or financial decisions.
How the Panel Count Is Calculated
Start with your electricity usage
Enter your monthly kWh directly from your utility bill, or enter your monthly dollar amount and we divide it by your region's average rate to estimate kWh. We then apply your desired offset percentage to find how much of that usage you want solar to cover.
Divide by sun hours and system losses
Daily usage is divided by your region's peak sun hours and an 80% performance ratio — which accounts for inverter, wiring, soiling, and temperature losses — to find the system size in kilowatts needed to hit your target.
Convert kilowatts into panels
The system size in watts is divided by your chosen panel wattage (400W by default) and rounded up to a whole panel. Higher-wattage panels reduce the count, which can matter on a smaller roof.
Estimate roof area
Each standard panel takes roughly 17.5 square feet, so the panel count is multiplied by that figure to estimate the usable roof area required. A real site survey adjusts for vents, setbacks, and shading.
What Affects How Many Panels You Need
Your electricity usage
The biggest driver. A home with electric heat, a pool, or EV charging uses far more than the 900 kWh average and needs a proportionally larger system. Start from at least 12 months of bills for accuracy.
Regional sun hours
A panel in the sunny Southwest produces far more than the same panel in the Pacific Northwest. Fewer peak sun hours means you need more panels to generate the same energy.
Panel wattage
Higher-output panels deliver the same system size with fewer units, which helps when roof space is tight. Most current residential panels fall between 380W and 450W.
Roof orientation and shading
South-facing, unshaded roof sections produce the most. East/west exposures and shade from trees or chimneys lower output, so a real design may call for more panels than this estimate.
Desired offset
Covering 100% of usage takes more panels than a partial offset. The right target depends on roof space, budget, and your local net-metering rules for exported power.
System losses
Inverter efficiency, wiring resistance, dust, and high temperatures all reduce real output below the panels' rated capacity. The 80% performance ratio used here reflects typical real-world losses.
Are You a Solar Installer? Turn an Estimate Into a Signed Proposal On-Site
A panel count is just the start — closing the job means a clear proposal the homeowner can say yes to. SubcontractorHub's solar proposal tools let your reps build a full system design, production estimate, savings, and monthly financing payment on a tablet at the kitchen table, then collect a signature on the spot.
Financing options from GoodLeap, Sunlight Financial, and Service Finance are embedded right in the proposal with approval in minutes. Solar contractors using SubcontractorHub close more deals because the customer sees the system, the savings, and the monthly payment all in one place.
See How It WorksBuild full solar proposals on a tablet at the home
Show system size, production, and savings clearly
Present monthly financing payments from partners
Digital signature collected on the spot
Signed job flows directly to operations — zero re-entry
Common Questions About Sizing Solar Panels
How many solar panels do I need to power my house?
Most US homes need somewhere between 15 and 25 panels to offset their full electricity usage, but the exact number depends on how much power you use, how much sun your area gets, the wattage of each panel, and the offset percentage you want. The calculation works from your monthly kWh usage divided by your region's peak sun hours and a system performance factor, then divided by panel wattage. Enter your numbers above to get a tailored estimate.
How is solar system size calculated from my electric bill?
First, your monthly kWh usage is converted to a daily figure. That daily usage is divided by your region's peak sun hours and an 80% performance ratio (to account for inverter, wiring, and temperature losses) to find the kilowatts of solar capacity you need. If you only know your bill in dollars, divide it by your average electricity rate to estimate kWh first. This calculator handles both paths for you.
What size solar panel should I use — 400W or something else?
Most residential panels installed today are in the 380–450 watt range, with 400W a common default. Higher-wattage panels mean you need fewer of them to hit the same system size, which can matter on a small roof. This calculator defaults to 400W but lets you adjust the panel wattage to match the equipment your installer offers.
How much roof space do I need for solar panels?
A standard 400W residential panel occupies roughly 17–18 square feet, so a 20-panel system needs about 350 square feet of usable, unshaded roof. South-facing sections with minimal obstructions are ideal. This calculator estimates the rough roof area for your panel count, but a site survey accounts for vents, chimneys, setbacks, and shading.
Should I offset 100% of my electricity use?
Many homeowners target a 90–100% offset, but the right number depends on your goals, roof space, budget, and local net-metering rules. Offsetting 100% maximizes bill savings where net metering is favorable; a smaller system may have a faster payback where export rates are low. Use the offset slider above to see how system size and panel count change with your target.
How accurate is this solar panel calculator?
It is a ballpark planning estimate. The calculator uses standard assumptions — regional peak sun hours and an 80% performance ratio — that work well for early planning, but your actual results depend on roof orientation, tilt, shading, local weather, and the specific equipment installed. Treat the output as a starting point and get a detailed design and quote from a licensed solar installer before committing.
Are You a Solar Installer Looking to Close More Jobs?
SubcontractorHub helps solar contractors design systems, present financing, and close deals on-site — with a platform built for the way solar sales actually works.
Book a Free DemoAll calculations are ballpark estimates based on standard assumptions and should be verified by the user with a licensed solar installer.