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Solar Panel Size Calculator
Find your recommended solar system size in kilowatts, how many panels you need, and how much roof area it takes — based on your monthly electricity usage and local peak sun hours.
Educational calculators — always consult a licensed professional before making financial decisions.
Find this on your utility bill — the U.S. average is about 900 kWh/month.
This is your region's average peak sun hours per day, not total daylight hours.
Most of the continental United States.
Higher-wattage panels need fewer panels for the same system size.
Most common residential panel wattage today.
Recommended system size
8.22 kW
Based on 10,800 kWh/year usage at Average (~4.5 hrs/day) sun exposure
Sizing assumptions
This sizing estimate assumes an 80% system derate factor (covering inverter, wiring, soiling, and temperature losses) and does not account for roof shading, orientation, or local permitting restrictions. A site assessment from a local installer will give the most accurate final system size.
Estimates reflect national averages for educational purposes only. Equipment prices and labor rates vary by region. Always get 2–3 quotes before proceeding.
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What you'll need
- ·Your average monthly electricity usage in kWh (from a recent utility bill)
- ·An idea of your region's peak sun hours (low, average, or high)
- ·The panel wattage you're comparing (350W, 400W, or 450W)
What you'll get
- ✓Recommended system size — Sized in kilowatts (kW) from your usage and sun hours
- ✓Panel count — How many panels of your chosen wattage you'll need
- ✓Roof area needed — Approximate square footage required
- ✓Estimated install cost — National average cost range before incentives
Example system sizes by usage
These examples assume average peak sun hours (4.5 hrs/day) and an 80% system derate factor covering inverter, wiring, soiling, and temperature losses. Your actual recommended size depends on your usage, regional sun exposure, and panel wattage — use the calculator above for a figure specific to your home.
How it works
Enter your usage
Tell us your average monthly electricity usage in kWh from a recent utility bill.
Select your sun hours
Choose your region's peak sun hours and the panel wattage you're comparing.
Get your recommended size
See your recommended system size (kW), panel count, roof area needed, and estimated cost.
Recommended System Size by Monthly Usage
| Monthly Usage | Annual Usage | Recommended System Size |
|---|---|---|
| 600 kWh | 7,200 kWh | ~5.48 kW |
| 900 kWh | 10,800 kWh | ~8.22 kW |
| 1,200 kWh | 14,400 kWh | ~10.96 kW |
| 1,800 kWh | 21,600 kWh | ~16.44 kW |
Estimates assume average (4.5 hr) peak sun hours and an 80% system derate factor. A site assessment from a local installer will give the most accurate final system size.
How to size a solar system
The standard formula is system size (kW) = annual usage (kWh) ÷ (peak sun hours × 365 × ~0.8). The 0.8 is a "derate" that accounts for real-world losses (inverter, wiring, heat, soiling, and shading). For example, a home using 10,800 kWh/year in a 5-sun-hour region needs roughly a 7.4 kW system.
- Annual usage. Add up 12 months of kWh from your utility bills — the most important input.
- Peak sun hours. ~4 in the Northeast/Pacific NW, ~5 in the Midwest/mid-Atlantic, ~5.5–6+ in the Southwest.
- Panel wattage. At 400W per panel, a 7.4 kW system is about 18–19 panels.
Panel count & roof area
Each residential panel is roughly 18 sq ft, so plan on about that much unshaded, well-oriented roof per panel plus setbacks required by fire code. A 7–8 kW system (18–20 panels) typically needs ~350–400 sq ft of usable roof. South-facing is ideal; east/west works with a modest production penalty; heavy shade from trees or chimneys can rule out sections entirely.
Should you oversize?
- Plan for new loads. Adding an EV, heat pump, or pool later can raise usage 20–40% — size for it now if the roof and budget allow.
- Mind utility caps. Many utilities cap system size at ~100–120% of your historical usage for net metering, so overbuilding can be uneconomic.
- Battery vs. bigger array. Under net-billing rules, a battery to store your own power can beat oversizing for export.
Once you have a target size, price it with the Solar Panel Installation Cost Calculator.
Frequently asked questions
How do I calculate the solar system size I need?+
Take your annual electricity usage in kWh and divide it by your area's peak sun hours per day, multiplied by 365 days and a system derate factor (typically 0.80 to account for inverter, wiring, and soiling losses). The result is your recommended system size in kilowatts (kW). This calculator does that math for you automatically.
How many solar panels do I need for my house?+
Once you know your system size in watts, divide it by the wattage of the panel you're considering. For example, a 7,200-watt system using 400W panels needs 18 panels, while the same system using 450W panels needs only 16. Higher-wattage panels reduce the total panel count for the same output.
How much roof space do solar panels need?+
A standard residential solar panel takes up roughly 18 square feet, including spacing for mounting hardware. A typical 20-panel system needs about 360 square feet of usable, unshaded roof area. Panel count and roof area both scale with your system size and chosen panel wattage.
Does shading affect how many panels I need?+
Yes. Shading from trees, chimneys, or neighboring structures can significantly reduce a panel's output, sometimes requiring a larger system or microinverters/power optimizers to compensate. This calculator provides a baseline estimate and does not account for site-specific shading — a local installer can assess your roof for actual sun exposure.
Should I size my system for 100% of my usage?+
Many homeowners size their system to offset 80-100% of their annual usage, since exceeding 100% often provides diminishing financial returns depending on your utility's net metering policies. Some choose a smaller system to reduce upfront cost, while others size up to cover future usage like an EV or home addition. Your ideal size depends on your budget, roof space, and local utility rules.
Want to try different numbers?
Back to the calculator ↑Solar Panel Size Calculator is built and maintained by the RealCostIQ editorial team. Cost ranges and rates are checked against published industry data and contractor quotes, and revised when the underlying figures move. Read our data methodology or more about who builds this. Every calculation runs in your browser — no account, and none of your inputs are stored.
Cost ranges and rates here are checked against contractor quotes and published industry data. If a number still looks off, email Support@RealCostIQ.com and we'll review and fix it.