System Size Calculator
Estimate solar DC capacity, panel count and approximate roof area from electricity usage and editable assumptions. Review illustrative gross energy value and simple payback.
Solar System Size Calculator
Enter annual or representative monthly kWh usage, an energy offset target and editable generation assumptions. Estimate DC capacity, panel count, annual generation and installed cost from your entered price.
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What This Tool Covers
Estimate DC capacity from annual usage, a target energy offset and editable generation assumptions. The tool rounds up to whole panels and gives an approximate module area and cost based on your entered price. Its financial outputs are illustrative gross energy value, not a site-specific bill-savings forecast.
Inputs Accepted
- • Annual kWh usage, or monthly kWh usage annualised by the tool
- • Desired offset percentage (e.g. 80%, 100%, 120%)
- • Illustrative state presets or your own peak sun hours (PSH)
- • System efficiency percentage (derate factor)
- • Panel wattage (e.g. 400W, 420W, 450W)
Outputs Generated
- • Estimated DC system size in kilowatts
- • Number of solar panels required
- • Estimated annual production (kWh)
- • Target annual energy offset used in sizing
- • Estimated roof area needed (sq ft)
- • Estimated installed cost from your entered $/W
Features
Editable Resource Presets
State presets are illustrative inputs. Replace peak sun hours and electricity rates with site-specific resource data and your current tariff.
Offset Targeting
Compare annual energy targets from 50% to 120%. An energy offset is not a guaranteed bill offset; check export permissions and tariff treatment separately.
Roof Area Estimate
Multiplies panel count by an assumed 17.5 sq ft footprint. This excludes access, setbacks and roof obstacles; confirm fit through a layout using actual module dimensions.
How It Works
Enter Your Annual Consumption
Enter annual kWh usage or representative monthly usage. The monthly mode multiplies usage by 12; it does not analyse 12 separate bills or seasonal consumption.
Select Your State and Offset Goal
Choose a state to load illustrative presets, then replace them with project-specific inputs. A 100% target matches estimated annual generation to annual usage; it does not match production to demand at every hour.
Set Panel and System Parameters
Enter your panel wattage and system efficiency (derate factor). The default generation factor is 78%, an illustrative assumption covering aggregate losses. Replace it with a justified project value.
Review the Recommendation
Review estimated DC capacity, panel count, annual generation and approximate module area. Cost uses your entered $/W; the tool does not retrieve current national prices.
Adjust and Compare Scenarios
Change panel wattage, efficiency, or offset target to compare successive system scenarios 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
Worked example: At 10,000 kWh/year, 100% offset, 5 peak sun hours/day and an 80% generation factor, estimated capacity is 6.85 kW DC. With 400 W modules, rounding up gives 18 panels and 7.2 kW DC; modeled annual generation is 10,512 kWh. These are assumed inputs, not a site forecast.
For a site assessment, review the DOE consumer guidance on electricity needs and roof suitability and use a location-based model such as PVWatts to investigate production assumptions. This calculator does not call PVWatts or retrieve weather data. Learn how specific yield relates capacity to annual generation and why export compensation affects energy value.
Target Annual Production
Target kWh = Annual kWh × (Offset % ÷ 100)
DC System Size
System kW = Target kWh ÷ (PSH × 365 × Efficiency)
Panel Count
Panels = round up [(System kW × 1000) ÷ Panel Wattage]
Roof Area
Area (sq ft) = Panels × 17.5 sq ft per panel (illustrative footprint; verify actual dimensions)
Pro Tips
Check the billing dates. Use a full year without overlapping or missing periods, and document unusual consumption before adjusting the baseline.
Compare energy targets. Evaluate targets against the customer’s load, project cost, tariff and export conditions; there is no universal best offset percentage.
Document loss assumptions. The default 78% is illustrative. Use a project-specific factor and avoid counting the same loss twice.
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?
The 78% default is an illustrative generation factor. Use a justified project loss estimate; inverter architecture alone does not establish the correct percentage.
Where do I find peak sun hours for my state?
The state presets are illustrative and have not been verified as current NREL averages. Obtain site-specific annual solar-resource data appropriate to the array orientation and update the input.
Why does my installer's quote differ from this calculator?
The calculator uses simplified annual assumptions. A site design can account for roof geometry, shading, equipment limits, electrical requirements and tariff conditions. Ask which assumptions explain the difference.
Can I size for more than 100% offset?
The tool allows an annual energy target above current usage. This does not establish utility permission, export eligibility or financial value. Check expected future loads and connection conditions before choosing a target.
Does this calculator account for shading?
It does not model shading directly. Use a site assessment to justify any aggregate loss adjustment rather than applying a fixed percentage to every shaded roof.
How accurate is the cost estimate?
Cost equals rounded DC capacity multiplied by your entered installed price per watt. It is not a retrieved market average or price range. Use an itemised quote and confirm which costs it includes.
What panel size should I enter?
Enter the rated wattage of the proposed module. Higher wattage does not guarantee the same dimensions; check the actual footprint separately when evaluating roof fit.
Related Tools
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