System Size Calculator
Find the right solar system size from your monthly bill or annual kWh usage. Get panel count, roof space, payback period, and 25-year savings by state — free, no signup.
Solar System Size Calculator
Enter 12 months of utility bills or annual kWh, desired offset, and your state. Get recommended system size, panel count, estimated production, and cost range.
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What This Tool Covers
The System Size Calculator takes your actual electricity consumption and translates it into a precise solar system recommendation. Enter your usage data, and it returns the DC system size, panel count, roof area requirement, projected production, and estimated cost range - everything you need before requesting a quote.
Inputs Accepted
- • 12 months of utility bills or annual kWh consumption
- • Desired offset percentage (e.g. 80%, 100%, 120%)
- • State selection for peak sun hours (PSH)
- • System efficiency percentage (derate factor)
- • Panel wattage (e.g. 400W, 420W, 450W)
Outputs Generated
- • Recommended DC system size in kilowatts
- • Number of solar panels required
- • Estimated annual production (kWh)
- • Actual offset percentage achieved
- • Estimated roof area needed (sq ft)
- • Estimated installed cost range ($)
Features
State-Level PSH Data
Uses real peak sun hour averages by state so your system size reflects actual local solar resource, not a generic national figure.
Offset Targeting
Set your desired offset from 50% to 120% and the calculator sizes the system accordingly - useful for net metering strategies that target bill credits.
Roof Area Estimate
Converts panel count and wattage into approximate roof area required, helping identify whether your roof can physically fit the recommended system.
How It Works
Enter Your Annual Consumption
Input 12 months of utility bill data or your annual kWh total. The calculator uses this as the baseline energy demand your solar system needs to offset.
Select Your State and Offset Goal
Choose your state to load the corresponding peak sun hours. Set your target offset percentage - 100% means the system produces as much as you consume annually.
Set Panel and System Parameters
Enter your panel wattage and system efficiency (derate factor). The default efficiency of 80% accounts for inverter losses, temperature derating, and wiring losses.
Review the Recommendation
The calculator returns your recommended DC system size, panel count, expected annual production, roof area, and estimated installed cost range based on current national averages.
Adjust and Compare Scenarios
Change panel wattage, efficiency, or offset target to compare different system configurations side by side before engaging a contractor.
Use Cases
Homeowner Pre-Quote Research
Know what size system to expect before a salesperson visits. If a contractor quotes a system significantly larger or smaller than the calculator suggests, ask why.
Installer Proposal Validation
Installers can quickly sanity-check a proposed system size against consumption data before committing to a full design - useful in early sales conversations.
EV or HVAC Load Addition
Adding an EV charger or heat pump increases consumption. Re-run the calculator with the new projected kWh total to size a system that covers your future load.
Calculation Methodology
Target Annual Production
Target kWh = Annual kWh × Offset %
DC System Size
System kW = Target kWh ÷ (PSH × 365 × Efficiency)
Panel Count
Panels = (System kW × 1000) ÷ Panel Wattage
Roof Area
Area (sq ft) = Panels × 17.5 sq ft per panel (standard 400W panel footprint)
Pro Tips
Use 13 months of bills if available. Including 13 months catches any billing cycle anomalies and gives a more accurate annual baseline than summing 12 potentially overlapping months.
Target 90–95% offset, not 100%. Oversizing for 100% offset often costs more than the marginal savings. Many utilities with NEM also pay less for excess export than the retail rate.
Use 78–80% for system efficiency. The default 80% is realistic for modern string inverter systems. Microinverter or power optimizer systems may achieve 82–85%; older or shaded systems may be closer to 75%.
Account for planned consumption increases. If you're buying an EV or adding HVAC, add the expected annual kWh for those loads before running the calculator so you size for your future bills, not your current ones.
Frequently Asked Questions
What is a good system efficiency to use?
80% (0.80) is the standard derate factor for most residential string inverter systems. It accounts for inverter efficiency (~97%), temperature losses (~5%), soiling (~2%), wiring losses (~2%), and other real-world factors. Microinverter systems typically achieve 82–85%.
Where do I find peak sun hours for my state?
The calculator loads PSH values by state automatically. These are annual daily averages derived from NREL's National Solar Radiation Database. Values range from around 3.5 PSH (Pacific Northwest) to 6.0+ PSH (Arizona, New Mexico).
Why does my installer's quote differ from this calculator?
This calculator uses state-level PSH averages. Your installer uses site-specific shading analysis, actual roof pitch, azimuth orientation, and possibly a different efficiency assumption. Small differences (5–15%) are normal. Large discrepancies warrant a direct question.
Can I size for more than 100% offset?
Yes. Set the offset to 110% or 120% if you plan to add loads (EV, battery, heat pump) or want to bank credits in net metering programs. Note that some utilities cap system size at 100% of annual consumption - check your utility's interconnection rules.
Does this calculator account for shading?
Not directly. Shading losses are reflected partially in the system efficiency input - if your roof has moderate shading, reduce efficiency to 72–75%. For accurate shading analysis, use a dedicated shading tool with a 3D site model.
How accurate is the cost estimate?
The cost range is based on national average installed cost per watt (typically $2.50–$3.50/W for residential before incentives). Regional labor costs, equipment choices, and roof complexity all affect final price. Treat the estimate as a ballpark, not a quote.
What panel size should I enter?
Use the wattage of the panels your installer is proposing. If you don't know yet, 400W is a reasonable default for current residential panels. Higher-wattage panels (420–450W) reduce the panel count but have a similar physical footprint.
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