Calculator
Example data table
| Scenario | Monthly kWh | Sun hours | PR | Panel W | Target offset | Panels | Array (kW DC) | Est. kWh/month |
|---|---|---|---|---|---|---|---|---|
| Starter home | 600 | 5.2 | 0.80 | 450 | 100% | 11 | 4.95 | 626 |
| Growing family | 900 | 5.0 | 0.78 | 550 | 110% | 16 | 8.80 | 1,043 |
| High usage | 1400 | 4.6 | 0.76 | 600 | 100% | 22 | 13.20 | 1,403 |
Formula used
This calculator sizes the required DC array from your energy target using:
Roof fit is estimated from panel area and a spacing factor: AreaNeeded = Panels × (Length × Width) × (1 + Spacing%/100).
How to use this calculator
- Pick a usage method and enter your energy value.
- Set an offset target for how much you want covered.
- Enter peak sun hours and a realistic performance ratio.
- Choose your panel wattage and roof space details.
- Press Calculate panels to see results above.
- Export CSV or PDF to share with an installer.
Why panel count matters for residential sizing
Panel count turns household demand into a rooftop design. Instead of guessing, you translate usage into a DC array size and then into whole modules. A difference of two panels can change string sizing, conduit runs, and racking rows. This calculator also adds a roof check so the energy-driven design is compared against available space before you speak with an installer. Test module wattages to see how count and footprint change.
Inputs that drive the energy target
Your energy target starts with either monthly kWh from a bill or a daily average. The offset percentage scales that baseline, so 90% reduces the target while 110% aims to cover seasonal spikes. Internally the tool converts monthly use to daily use using 12/365, then applies the offset. This keeps the sizing consistent even when you switch between input methods. Enter higher usage if you plan new appliances or an electric vehicle soon.
Sun hours and performance ratio assumptions
Peak sun hours represent the equivalent full-sun production window for your location, not daylight length. Performance ratio captures real-world losses from temperature, wiring, module mismatch, soiling, and conversion. Typical planning values sit near 0.75 to 0.85. Because system kW equals TargetDaily divided by (SunHours times PR), small changes here can add or remove several panels on the same home. If shading is significant, reduce PR or sun hours to reflect seasonal effects.
Roof-space feasibility and layout buffers
Roof feasibility is checked using panel length and width multiplied by the panel count, then expanded by a spacing factor. The spacing factor models setbacks, row gaps, tilt clearances, walkways, and mounting hardware. If the calculated utilization exceeds 100%, the design likely needs higher-wattage modules, a lower offset, improved efficiency, or a different roof plane selection. Also consider local fire setbacks and ridge clearance rules when estimating usable area.
Interpreting outputs and planning next steps
Outputs are intended for fast planning: panel count, DC array size, estimated daily, monthly, and annual production, plus a suggested inverter size using a DC/AC ratio. Use the exports to share assumptions with suppliers and to compare scenarios. For best accuracy, confirm shading, azimuth, tilt, and local interconnection rules, then adjust sun hours and PR accordingly. Run a few scenarios to balance budget, space, and self-consumption.
FAQs
What value should I use for performance ratio?
Start with 0.80 for a typical rooftop system. Lower it to 0.75 if temperatures are high or shading is noticeable. Raise it toward 0.85 for cool, unshaded roofs with strong equipment and good maintenance.
How do I choose peak sun hours?
Use an annual average for your city from reputable solar maps, then adjust for roof tilt and orientation. If you expect winter shading or frequent haze, enter a slightly lower value to stay conservative.
Why does panel count change when I adjust offset?
Offset scales the energy target. A higher target increases the required DC size, and the calculator rounds up to whole panels. Even small changes can push the total past the next panel threshold.
What does spacing factor include?
It adds allowance for code setbacks, walkways, row gaps, clamps, and tilt clearance. If your roof has many obstructions or multiple roof planes, increase spacing to better reflect real layout constraints.
Is the inverter size exact?
No. It is a planning suggestion based on a DC/AC ratio. Installers may choose a different size depending on string voltage, shading, export limits, and whether you prefer clipping or more headroom.
How accurate are the production estimates?
They are directional estimates using sun hours and performance ratio. Accurate modeling needs shade analysis, tilt, azimuth, temperature data, and local weather. Use these numbers to compare scenarios, then validate with a site survey.