Calculator inputs
3 columns large • 2 columns small • 1 column mobileExample data table
These sample scenarios show how inputs change the recommended panel count.
| Period kWh | Sun hours | Losses | Panel W | Estimated panels | Installed DC kW |
|---|---|---|---|---|---|
| 600 | 4.5 | 15% | 450 | 11 | 4.95 |
| 900 | 5.2 | 14% | 550 | 12 | 6.60 |
| 1200 | 4.0 | 18% | 400 | 28 | 11.20 |
Formula used
We estimate the needed DC system size from your energy and solar resource.
Where PSH is peak sun hours per day.
Panels are the DC size divided by one panel’s wattage.
We round up to ensure enough modules.
How to use this calculator
- Choose Period kWh if you have a monthly bill total.
- Or choose Daily kWh if you track daily consumption.
- Enter peak sun hours for your location and season.
- Set losses to reflect heat, wiring, inverter, and dirt.
- Pick your panel wattage and optional spare margin.
- Press Calculate to view panels, size, and area.
- Use Download PDF or Download CSV for records.
Sizing insights
Energy input quality and billing context
Accurate kWh is the foundation of a reliable panel estimate. Use the total kWh from the same billing period length you enter, because a 28‑day cycle will size differently than a 31‑day cycle. If your home has seasonal loads, consider using a higher month to avoid undersizing. For daily tracking, average at least two weeks to reduce noise from weekends and weather.
Peak sun hours and local production realism
Peak sun hours represent equivalent full‑sun energy per day, not daylight length. A site with 5.2 peak sun hours can deliver about 5.2 kWh per installed kW before losses. Values change by month and tilt, so conservative inputs improve year‑round coverage. When unsure, pick a lower sun‑hour value to reflect clouds, haze, and non‑ideal roof orientation.
Losses, performance ratio, and safety margin
System losses bundle inverter conversion, wiring, temperature derate, dirt, mismatch, and shading. Many rooftop systems fall near 10% to 20% losses, but older inverters, hot climates, or partial shading can push higher. This calculator applies losses as a factor on expected energy. Add a spare margin to cover module degradation and to keep production closer to targets over time. For projects with batteries, size panels for average charging demand, then validate with hourly profiles to avoid winter shortfalls and unnecessary overspend during critical months.
Array size, panel wattage, and roof constraints
The required DC size is derived from energy divided by sun hours, days, and the loss factor. Panel count then depends on panel wattage; higher wattage reduces module quantity but not necessarily roof area savings if panel sizes are similar. Use the roof area estimate to verify layout feasibility, allowing setbacks, walkways, and obstacles like vents and skylights.
Inverter target and design checks
The inverter target is estimated using the DC/AC ratio, which helps anticipate clipping and equipment sizing. Ratios near 1.1 to 1.3 are common for grid‑tied systems, but goals vary by tariff, shading, and temperature. Use the expected energy check as a sanity test: if predicted kWh is far from your input, revise sun hours, losses, or margin.
FAQs
1) What peak sun hours should I use?
Use a conservative monthly average for your location. If you only have an annual value, choose the lower season to avoid undersizing. Tilt and shading can reduce effective sun hours.
2) What does “system losses” include?
Losses cover temperature derate, inverter conversion, wiring, soiling, mismatch, and minor shading. Many rooftops land near 10%–20%. Increase the value if your site is very hot or partially shaded.
3) Why is the panel count rounded up?
Modules are purchased as whole units. Rounding up ensures enough DC capacity to meet the target energy after losses. The spare margin adds additional headroom for uncertainty and aging.
4) How should I pick a DC/AC ratio?
Ratios around 1.1–1.3 are common for grid‑tied designs. Higher ratios can increase clipping on bright days but improve low‑light utilization. Check inverter limits and local design rules.
5) Is the roof area estimate exact?
No. It is a planning number based on panel area input. Real layouts require setbacks, row spacing, walkways, and obstacle clearance. Always confirm with a site layout or installer design.
6) Can I use this for off‑grid or battery systems?
Yes for early sizing, but add charging demand, autonomy goals, and winter sun hours. Off‑grid systems typically need more margin and careful hourly modeling for reliability.
Notes and guidance
- Peak sun hours vary by month; use conservative values for year-round designs.
- Losses often land between 10% and 20% for typical rooftops.
- Spare margin covers module aging and real-world uncertainty.
- This tool is for planning; a site survey is recommended for final sizing.