Calculator inputs
Enter your energy use, sunlight, and sizing preferences. The estimator computes panels, capacity, roof area, and annual output.
Example data table
Sample scenarios show how sunlight, losses, and panel wattage affect required panel count.
| Scenario | Daily kWh | Sun hours | Derate | Panel W | Seasonal | Panels | kWdc |
|---|---|---|---|---|---|---|---|
| Small home | 10 | 5.5 | 0.80 | 550 | 1.10 | 5 | 2.75 |
| Family home | 18 | 4.5 | 0.78 | 500 | 1.15 | 12 | 6.00 |
| Small office | 35 | 4.0 | 0.75 | 550 | 1.20 | 26 | 14.30 |
Formula used
The estimator computes the PV array size required to meet a target daily energy demand:
- Effective multiplier = Derate × Inverter efficiency × (1 − Shading loss)
- Target daily energy = (Daily kWh × (1 + Margin) × Seasonal factor) + Battery overhead
- Array size (kWdc) = Target daily energy ÷ (Peak sun hours × Effective multiplier)
- Panel count = ceil((Array size × 1000) ÷ Panel wattage)
- Roof area needed = Panel count × Panel area
Battery overhead increases the target to cover autonomy and storage losses using DoD and efficiency. Annual energy is estimated from the final kWdc, sun hours, and effective multiplier.
How to use this calculator
- Enter your average daily energy use in kWh/day.
- Set peak sun hours for your location and season.
- Choose your panel wattage and realistic loss factors.
- Add a seasonal factor to size for low-sun months.
- Optionally add a future margin for growth or uncertainty.
- If planning backup, enable battery sizing and set autonomy.
- Press Submit to see results above the form.
- Use the CSV/PDF buttons to save and share scenarios.
Daily load and sizing targets
Panel count starts with daily energy use in kWh/day. This tool converts that demand into a generation target by applying a growth margin and a seasonal factor for low-sun months. For example, 12 kWh/day with a 10% margin and 1.10 seasonal factor becomes 14.52 kWh/day before any storage overhead is considered.
Sun hours and production realism
Peak sun hours represent equivalent full-power sunlight hours per day. Because sun hours vary by location and season, a small change can meaningfully shift the array size. If peak sun hours drop from 5.0 to 4.0, the required DC capacity rises by roughly 25% for the same target, which often adds several panels and more roof area.
Loss factors and effective multiplier
Real systems produce less than nameplate. The estimator combines a derate factor, inverter efficiency, and shading loss into one effective multiplier: Derate × Inverter × (1 − Shading). A typical setup might use 0.80 × 0.96 × 0.95 = 0.730. That value directly scales energy output, so conservative loss assumptions are safer during early planning.
Roof fit and layout constraints
Area checks reduce surprises later. The calculator multiplies panel count by panel area to estimate required roof space, then compares it to usable roof area. Usable area should exclude setbacks, vents, and shaded zones. If roof fit is “No,” consider higher-watt modules, improved shading management, or reduced margin after validating actual consumption patterns.
Inverter sizing and scenario comparison
After rounding to whole panels, the tool estimates inverter size using the DC/AC ratio. Ratios around 1.10–1.25 commonly balance clipping and cost. Use the example table as a baseline, then run multiple scenarios: change sun hours for winter, adjust shading for tree growth, and export CSV or PDF to compare options side-by-side with installers or stakeholders.
FAQs
What inputs matter most for panel count?
Daily kWh, peak sun hours, and the effective multiplier drive results. Panel wattage then converts required kWdc into a whole-panel quantity.
How should I choose peak sun hours?
Use a credible local average and test a lower value for winter. If unsure, compare 4.0, 5.0, and 6.0 hours to see sensitivity.
Why does shading change results so much?
Shading reduces energy across the day, not just at noon. Even 10–20% shading loss can add multiple panels because it lowers the effective multiplier.
Does a battery increase panel requirements?
Yes. Storage adds overhead to cover autonomy plus DoD and efficiency limits. The tool increases the energy target to compensate for these losses.
Is the inverter size output a final specification?
It is a planning estimate based on DC/AC ratio. Final inverter selection depends on module stringing, temperature, code rules, and site constraints.
How accurate are the annual kWh and CO₂ numbers?
They are directional estimates using sun hours and a generic emissions factor. Use local production modeling and grid factors for compliance reporting.