Calculator Inputs
Example Data Table
| Scenario | Usage | PSH | Derate | Panel | Estimated panels |
|---|---|---|---|---|---|
| Home, moderate use | 900 kWh/month | 5.0 | 0.80 | 550 W | 14 |
| Apartment, efficient | 450 kWh/month | 4.5 | 0.78 | 450 W | 10 |
| Capacity-based install | 10.0 kW target | 5.0 | 0.80 | 600 W | 17 |
Formula Used
Load profiling and billing baselines
Start with measured consumption, not guesses. Many households see 600–1,200 kWh per month, while efficient apartments may stay near 350–600 kWh. This calculator converts monthly values using 30.4 days, so a 900 kWh bill becomes about 29.6 kWh per day. If you expect growth from an electric water heater or EV charging, add 10–30% headroom to avoid undersizing and frequent grid imports. For best results, use at least three billing cycles and exclude unusual months caused by vacations, outages, or construction work.
Peak sun hours and energy-to-power sizing
Peak sun hours (PSH) translate sunlight into an equivalent number of full‑power hours. Typical annual averages range from 3.5 to 6.5 hours per day depending on climate and seasonality. The required DC array size is Daily kWh ÷ (PSH × Derate). For example, 29.6 kWh/day, 5.0 PSH, and 0.80 derate needs about 7.4 kW DC before rounding. If PSH is uncertain, model low and high scenarios separately.
Derate factor and loss budgeting
Derate captures real‑world losses: temperature, inverter conversion, wiring, mismatch, dust, and downtime. Many residential systems land between 0.75 and 0.85, with cleaner sites and premium components trending higher. Use 0.80 as a conservative midpoint. If your roof runs hot or dust is persistent, reduce derate slightly and compare panel count differences; a small derate change can add multiple modules at larger system sizes.
Panel selection, packing density, and roof fit
Panel wattage determines how many modules meet the required kW. Higher‑watt panels reduce count, but may be larger and heavier. The roof check multiplies panel count by panel area, then adds a spacing allowance for tilt gaps, fire setbacks, and maintenance access. With 14 panels at 2.60 m² and 10% spacing, required usable area is roughly 40.0 m².
Production estimates and design verification
After rounding, the calculator estimates daily production as Array kW × PSH × Derate, then annualizes by 365. Treat this as a planning value, not a performance guarantee. Verify shading, azimuth, tilt, and local interconnection limits during detailed design. If you enter inverter size, the DC/AC ratio helps spot clipping risk; many designs target about 1.10–1.30. Document assumptions in your proposal so clients clearly understand performance variability across seasons.
FAQs
What peak sun hours value should I enter?
Use a long‑term average for your site. If you only know a range, run two cases (low and high) to bracket results. Local solar maps, installers, or historic PV data can refine PSH for your area.
How do I choose a derate factor?
Derate represents losses from heat, wiring, inverter conversion, dust, mismatch, and downtime. 0.80 is a practical default. Use 0.75 for harsh heat or heavy soiling, and 0.85 for clean sites with high‑quality components.
Should I size from energy usage or target capacity?
Energy sizing aims to cover a specific load, using PSH and derate to translate kWh into kW. Capacity sizing is useful when budgets, interconnection limits, or available roof space set the maximum DC size.
How reliable is the roof area estimate?
It is a planning check, not a layout drawing. It multiplies module area by a spacing allowance for access paths, tilt gaps, and setbacks. Complex roofs, vents, and shading may reduce usable area, so confirm with a site layout.
What does the DC/AC ratio mean here?
It is the DC array size divided by inverter AC rating. Higher ratios can increase annual yield but may cause midday clipping on bright days. Many residential designs fall around 1.10–1.30, depending on climate and tariffs.
Why does the calculator round panel count up?
Panels are whole units, and rounding up ensures the installed array meets or slightly exceeds the required kW. This reduces the chance of undersizing when conditions are worse than average, and it better supports future load growth.
How to Use This Calculator
- Select a sizing method: energy usage or target capacity.
- Enter your usage (daily or monthly) or a target system size.
- Set peak sun hours and derate to match local conditions.
- Enter panel wattage and optional future growth headroom.
- Optionally add panel area, spacing, and roof area for a fit check.
- Click Calculate to view results above the form.
- Use the export buttons to download CSV or PDF.