☀ How Many Solar Panels Do I Need?
Based on your electric usage, location, and roof — not a generic estimate
Size scenarios — adjust for your goals
Next step: With your estimated system size, use the Permit Readiness Wizard to get county-specific permit fees and inspection requirements, or the Payback Calculator to estimate your ROI.
Estimate only. Actual system size recommendation from a licensed installer will differ based on your specific roof dimensions, orientation, local shading analysis, and usage patterns. This calculator assumes south-facing panels at optimal tilt; east/west-facing roofs require 15–25% more capacity for the same offset.
How Solar System Size Is Calculated
Your solar installer uses a straightforward formula to size your system — one you can replicate yourself with this tool. The core inputs are: your annual electricity usage (in kWh), your local peak sun hours per day, and your desired offset percentage (how much of your bill you want to eliminate).
Step 1 — Calculate Annual Usage
Divide your average monthly electric bill by your rate per kWh to get monthly usage, then multiply by 12 for annual usage. Example: $180/month ÷ $0.14/kWh = 1,286 kWh/month × 12 = 15,429 kWh/year. Your utility bill shows this directly — look for "kWh used" on the usage detail page.
Step 2 — Find Your Peak Sun Hours
Peak sun hours (PSH) is not the number of daylight hours — it's a standardized measure of how much solar energy your location receives per day, normalized to 1,000 W/m². Phoenix, AZ averages about 6.5–7.0 PSH. Miami, FL averages about 5.5. Boston, MA averages about 4.0–4.5. The NREL PVWatts tool provides precise PSH data for any U.S. location.
Step 3 — Account for System Losses
A solar system is not 100% efficient. Inverter conversion losses, wiring losses, temperature effects, and soiling typically reduce output by 15–25%. This calculator uses an 80% efficiency factor — a standard assumption. Your installer's proposal may use a more precise figure based on your specific equipment and roof conditions.
Step 4 — Size for Your Offset Goal
Most homeowners target 100% offset — a system sized to produce as much power annually as they consume. If you plan to add an EV or heat pump water heater, sizing to 110–120% builds in headroom. Many installers recommend starting at 100% and adding battery storage rather than oversizing panels, since oversized panels in states with poor export rates (California NEM 3.0) produce excess that earns very little credit.
Permit fees are often tiered by system size — systems over 10kW typically cost more to permit and may require additional documentation (engineering stamp, load calculations). Use our Permit Wizard to see how system size affects your specific county's permit cost.
Panel Wattage and Count
Panel wattage has increased significantly over the past decade. Standard residential panels in 2024–25 are 380–430W. Higher wattage panels produce the same power in less roof space — useful if your usable roof area is limited. Premium TOPCon and HJT panels (430–480W) cost more per watt but require fewer panels and less racking hardware. The "best" panel wattage depends on your roof constraints and budget.
What This Calculator Does Not Account For
- Roof orientation (east/west-facing roofs need 15–25% more capacity)
- Roof tilt angle (optimal is roughly equal to your latitude)
- Specific shading trees, chimneys, or HVAC equipment
- Seasonal usage variation (if your summer bills are much higher than winter)
- Future usage changes (EV purchases, additions to home)
A licensed solar installer's proposal will use satellite imagery, shade analysis software (Aurora, Helioscope, or similar), and your actual 12-month utility data to refine these estimates significantly.
System Size FAQ
It depends on your state's export credit rate. In states with full retail net metering (Florida, Colorado, Ohio, North Carolina), a modestly oversized system earns full retail credit for every exported kWh — slightly oversizing (to 110%) makes sense to buffer for future usage growth. In California under NEM 3.0, oversizing a solar-only system produces excess daytime exports that earn only 3–5¢/kWh, making the economics unfavorable. If you're in a low-export-rate state and want more capacity, battery storage is typically a better investment than additional panels.
A standard 400W panel covers approximately 21–22 square feet (roughly 4 ft × 5.5 ft). A 10-panel, 4kW system needs about 220 sq ft of usable roof area. A 20-panel, 8kW system needs about 440 sq ft. After accounting for IFC fire setbacks (18-inch perimeter clearances and hip/ridge access paths required in most jurisdictions), the actual usable area is typically 60–80% of your total south-facing roof surface. A shaded or obstructed roof may have significantly less usable area.
Not a different permit type — but larger systems typically require additional documentation. Systems over 10kW often require a structural engineering letter (certifying your roof can handle the load), more detailed electrical calculations, and in some jurisdictions, a pre-application meeting with the AHJ. Permit fees are also typically higher for larger systems since most counties calculate fees based on project valuation. Use the Permit Wizard to see size-based fee estimates for your county.
Battery storage is increasingly worth considering, especially in California (where NEM 3.0 makes self-consumption far more valuable than export), in areas prone to power outages, and for homeowners with time-of-use utility rates. The economic case is strongest in California and states with low export credit rates. In states with full retail net metering (Florida, Colorado), the economic case for battery is primarily backup power value — the energy arbitrage benefit is minimal when export and import rates are equal. Battery storage requires a separate permit in most jurisdictions.