Solar System Size Calculator
Free solar system size calculator. Enter your monthly bill, state, and panel wattage to get system kW, panel count, annual production, 25-year savings, and CO₂ offset. No signup.
Solar System Size Calculator
Enter your monthly bill or annual kWh, select your state, and set your desired offset. Get recommended system size, panel count, estimated production, and cost range.
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What This Tool Covers
This solar system size calculator determines the right system size for a home or business based on electricity usage, local solar resource, and desired bill offset. Enter a monthly bill or annual kWh and get a recommended system size in kilowatts, panel count, and estimated cost range in seconds.
Inputs Required
- • State (for peak sun hours)
- • Monthly utility bill ($) or annual kWh
- • Current utility rate ($/kWh)
- • Desired offset percentage (50–100%)
- • System efficiency / derate factor
- • Panel wattage (W)
Outputs Provided
- • Recommended system size (kW DC)
- • Number of panels needed
- • Estimated annual production (kWh)
- • Roof area required (sq ft)
- • Estimated system cost range ($)
- • Annual savings estimate
Features
Bill or kWh Input
Start from a monthly dollar bill or annual kWh - the tool converts either to a sizing baseline using your local utility rate.
Offset Percentage Control
Set your target offset from 50% to 100%. See how system size changes when you target 80% versus 100% offset - useful for budget-constrained customers.
Roof Area Estimate
Calculates the roof area required for the recommended panel count so you can check feasibility before visiting the site or pulling satellite imagery.
How It Works
Enter Your Location and Usage
Select your state to load local peak sun hour data, then enter either your monthly utility bill in dollars or your annual kWh consumption from a utility statement.
Set Your Utility Rate and Offset Goal
Enter your current utility rate per kWh and choose what percentage of your electricity use you want solar to cover. Most customers target 90–100%.
Specify System Efficiency and Panel Wattage
Enter the system derate factor (typically 0.80–0.85 for losses from wiring, inverter, and temperature) and your panel wattage to calculate exact panel count.
Get Your System Recommendation
The tool outputs the recommended DC system size in kilowatts, number of panels, annual production, required roof area, and an estimated installed cost range based on current national pricing benchmarks.
Use Cases
First-Contact Lead Qualification
Run a quick size estimate during an initial customer call to set expectations on system size and cost before a formal site visit or design is done.
Budget Scenario Planning
Show customers the tradeoff between 70% and 100% offset - a smaller system costs less upfront while still delivering meaningful savings, useful when a customer has a fixed budget.
Roof Feasibility Check
Use the roof area output to quickly check whether a customer's roof has enough usable space for the recommended system before scheduling a site survey.
Calculation Methodology
The sizing method follows the standard solar industry approach used by NREL and most design software platforms.
Annual kWh from Bill
Annual kWh = (Monthly Bill ÷ Utility Rate) × 12
Converts a monthly dollar bill to annual energy consumption using the entered utility rate per kWh.
Required System Size
kW DC = (Annual kWh × Offset%) ÷ (Peak Sun Hours × 365 × Efficiency)
Divides the target annual production by the available solar resource and system efficiency to determine the DC nameplate capacity needed.
Panel Count
Panels = (kW DC × 1000) ÷ Panel Wattage
Converts kilowatts to watts, then divides by individual panel wattage. Round up to the nearest whole panel.
Roof Area Required
Area (sq ft) = Panels × ~17.5 sq ft per panel
Uses an average panel footprint of approximately 17.5 square feet (65″ × 39″) for standard 400W residential panels. Adjust for different form factors.
Pro Tips
Size for the past 12 months, not the peak month. Customers often show you their July bill when usage is highest. Ask for the full 12-month average. Oversizing for peak summer usage creates excess production in low-use months with no financial benefit in states without robust net metering.
Account for planned EV or HVAC changes. If a customer plans to add an EV or upgrade to heat pumps, their future consumption will be 20–40% higher. Size the system for future load rather than today's bill to avoid a system that's too small within two years.
Use 0.80 as your default derate factor. This accounts for inverter efficiency (~97%), wiring losses (~2%), temperature derating (~5%), soiling (~2%), and mismatch (~2%). A derate of 0.80 is conservative enough to match real-world performance in most climates.
In states with reduced net metering, target 80–90% offset. In states like California (NEM 3.0) or Nevada where export compensation is low, oversizing leads to excess production that earns minimal credit. A slightly undersized system that self-consumes nearly all its output often delivers better economics.
Frequently Asked Questions
How many solar panels do I need for a 2,000 sq ft home?
It depends on energy use, not home size. A 2,000 sq ft home using 1,000 kWh/month in a state with 5 peak sun hours typically needs a 7–9 kW system, or roughly 17–22 panels at 400W each. Use this calculator with your actual utility bill for a precise answer.
What is a good system efficiency or derate factor?
A derate factor of 0.80 is standard for most residential systems. High-performance systems with premium inverters and minimal wiring losses may achieve 0.85. Older systems or those with shading issues may be closer to 0.75.
What is the difference between kW DC and kW AC?
kW DC is the nameplate capacity of the solar panels. kW AC is the actual power output after inverter conversion losses. A 10 kW DC system with a 0.97 inverter efficiency produces approximately 9.7 kW AC. System sizing uses DC capacity as the primary metric.
Should I target 100% offset?
Not always. In states with strong net metering (like many northeastern states), 100% offset makes sense. In states where excess exports earn minimal compensation, targeting 80–90% offset with higher self-consumption often delivers better financial returns.
How accurate is the cost estimate?
The cost range uses national average installed cost benchmarks (typically $2.50–$3.50 per watt for residential systems in 2026). Actual costs vary by installer, equipment brand, roof complexity, and local labor rates. Treat this as a planning range, not a final quote.
What if my roof doesn't have enough space?
If the required roof area exceeds available usable space, consider higher-wattage panels (430–450W models need fewer panels for the same system size), ground mounts, or carport installations. You can also reduce the target offset percentage to fit the available roof area.
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