Get an instant panel count, system size, and 25-year savings estimate based on your location, energy usage, and panel wattage. No sign-up required.
Accurately sizing your solar panel array is the foundation of every successful installation. Too few panels and the system underperforms; too many and the customer overpays on both equipment and financing.
Our Solar Panel Size Calculator uses your monthly electricity usage, local peak sun hours (based on your state), and selected panel wattage to determine exactly how many panels you need — along with total system wattage, estimated annual production, and projected 25-year savings.
Whether you're doing a quick pre-sales estimate or finalizing a proposal, this tool gives you a professional-grade answer in seconds.
Location-Based Estimates
Uses real peak sun hour data for all 50 U.S. states to ensure accurate energy yield calculations.
Panel Count & System Size
Instantly calculates number of panels, total kW, and estimated daily/annual production.
Savings Projection
Projects 25-year electricity savings factoring in utility rate escalation and panel degradation.
Initial Site Assessment
Run a quick panel count during your first customer conversation using just their utility bill and location.
Sales Proposals
Generate a professional system size recommendation to anchor your proposal before detailed shading analysis.
Roof Space Verification
Cross-check whether the roof has enough area to fit the calculated panel count at your chosen wattage.
Enter Your Monthly kWh Usage
Find your average monthly electricity consumption on your utility bill. Use a 12-month average for the most accurate sizing.
Select Your State
Choose your state from the dropdown. The calculator automatically applies the correct peak sun hours for your region using NREL data.
Choose Panel Wattage
Select the wattage of the panels you plan to install (300W–700W range). Higher wattage means fewer panels needed.
Set System Efficiency
Default is 80% (accounting for inverter losses, wiring, temperature, and soiling). Adjust if your design has specific derating factors.
Review Your Results
Instantly see panel count, system size in kW, estimated annual production, and 25-year savings projection.
Number of Panels
e.g. 18
Minimum panel count to offset monthly usage. Round up to the nearest full panel.
System Size (kW)
e.g. 7.2 kW
Total DC capacity. Used for interconnection applications and financing.
Annual Production (kWh)
e.g. 9,800
Expected yearly output based on peak sun hours and system efficiency.
25-Year Savings
e.g. $42,000
Cumulative savings with 2.9% rate escalation and 0.5%/yr panel degradation.
Payback Period
e.g. 7.8 yrs
Time to recover system cost. National average is 7–9 years.
These calculations use NREL peak sun hour data, PVWatts system derating assumptions, and standard solar performance modeling used by installers and engineering tools worldwide.
This determines how many panels are required to offset monthly electricity usage. The 30.44 value represents the average number of days per month. Efficiency reflects total system losses including inverter loss, wiring, temperature, dust, and shading.
Converts total panel output into system capacity in kilowatts. This is the standard system size metric used in solar proposals, permitting, and utility interconnection applications.
Estimates yearly electricity generation based on system size and solar resource. Peak sun hour values come from NREL solar radiation data for fixed-tilt systems at optimal orientation.
Savings are calculated annually assuming 2.9% utility rate escalation and 0.5% panel performance degradation per year. This reflects long-term financial modeling used in solar ROI analysis.
Peak sun hour data is sourced from the National Solar Radiation Database (NSRDB). The default 80% system efficiency follows NREL PVWatts standard derating methodology used across industry modeling tools.
National average estimates. Use the calculator above for location-specific results.
| Monthly Bill | Est. kWh/Month | System Size | Panel Count (400W) | Est. 25-Yr Savings |
|---|---|---|---|---|
| $75 | 600 kWh | 4.0 kW | 10 panels | $18,000 |
| $100 | 900 kWh | 6.0 kW | 15 panels | $27,000 |
| $150 | 1,200 kWh | 8.0 kW | 20 panels | $36,000 |
| $200 | 1,500 kWh | 10.0 kW | 25 panels | $45,000 |
| $300 | 2,200 kWh | 14.7 kW | 37 panels | $66,000 |
| $400 | 3,000 kWh | 20.0 kW | 50 panels | $90,000 |
Based on national average of 4.5 peak sun hours/day and 80% system efficiency. Actual results vary by location.
Use 12-Month Average kWh
Never size off a single month's bill. Summer and winter usage can vary 40–60%. Pull the full 12-month history from the utility account portal.
Account for Future Load Growth
If the customer plans to add an EV charger or pool pump, add 15–20% to their current usage before sizing.
Verify Roof Space Availability
A 400W panel is approximately 21 sq ft. Multiply panel count × 21 to estimate minimum roof area needed.
Don't Over-Size Net Metering
In states with 1:1 net metering, sizing above 100% offset may yield no additional financial benefit. Size to actual offset.
The calculator is accurate to within 5–10% for most U.S. locations when you use your actual 12-month average kWh consumption. It uses verified NREL peak sun hour data and standard PVWatts derating. For a bankable final design, always follow up with a full site assessment and shade analysis.
A peak sun hour is one hour of sunlight at an irradiance of 1,000 W/m². It's not total daylight hours — it's the equivalent number of hours per day at full intensity. Arizona averages 6.5 peak sun hours/day; Seattle averages 3.8. This directly impacts how much energy your panels produce.
In most cases, yes — but check your utility's net metering policy first. Some utilities offer reduced credit for production above your usage. If net metering is 1:1, size to 100%. If credits are reduced above a threshold, size to that threshold instead.
80% (0.80) is the industry standard derating factor used by NREL's PVWatts. It accounts for inverter efficiency (~96%), wiring losses (~2%), temperature derating (~3%), soiling (~2%), and mismatch (~2%). If your system has high-quality components and minimal shading, 82–85% may be appropriate.
Yes — higher wattage panels produce more power per unit, so fewer are needed. A 400W panel system requires fewer panels than a 350W system for the same energy output. However, higher-wattage panels typically cost more per unit, so the total system cost may be similar.
Yes, with caveats. For commercial projects, also factor in demand charges, time-of-use rates, and tilt angle optimization. The calculator's savings projections are based on residential utility rates and may understate commercial savings in high-demand scenarios.
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