Calculator
Formula used
First, average daily energy use is estimated as: Daily kWh = Monthly kWh ÷ 30.
Next, a performance ratio accounts for real-world reductions: PR = (1 − Losses%) × Shading × Soiling × Temperature × Inverter.
Required system size is then: kW = (Daily kWh ÷ (Sun Hours × PR)) × Offset, and a design margin increases this target.
Finally, the panel count is: Panels = ceil(kW_with_margin ÷ (Panel_W ÷ 1000)), and area is Panels × Panel area.
How to use this calculator
- Enter your monthly kWh from recent utility bills.
- Choose a target offset based on your energy goals.
- Set peak sun hours using your city’s typical average.
- Adjust losses and derates to reflect your site conditions.
- Provide roof usable area and panel footprint for space checks.
- Press Calculate to view panel count and sizing guidance.
- Download CSV or PDF to save results for quotes.
Energy use baseline
Panel sizing begins with measured consumption. Gather months of kWh to capture summer cooling and winter heating. Average the months, then adjust for planned changes like an electric vehicle or heat pump. The calculator converts monthly kWh into daily demand, so spikes can inflate results. A practical target offset is 70–100% for most homes, while higher offsets suit future load growth or export goals. If you net meter, align the offset with your utility true‑up rules.
Sun hours realism
Peak sun hours describe the daily equivalent of full‑intensity sunlight at your site. They vary with latitude, haze, monsoon cloud cover, and roof orientation. Use a conservative value when unsure, because low sun hours require more array capacity to meet the same energy target. If you have morning or afternoon shade, lower the shading factor to avoid optimistic production assumptions. A shade survey improves accuracy.
Performance ratio factors
Nameplate watts do not equal delivered energy. The performance ratio combines system losses with site derates: shading, soiling, temperature impact, and inverter efficiency. Shading is often the largest surprise; even partial obstruction can cut string output. Soiling matters in dusty regions and improves with cleaning or steeper tilt. Temperature derate reflects hot rooftop operation. Review these inputs with an installer for realistic expectations. High‑efficiency inverters and shorter conductor runs can reduce losses further.
Roof space feasibility
Counted panels must physically fit. Enter usable roof area after excluding setbacks, ridgelines, vents, skylights, and walk paths. Panel footprint varies by model, but two square meters is common. If required area exceeds space, consider higher‑wattage modules, relocating panels to another roof plane, adding a carport array, or lowering the offset target.
Turning results into action
Use outputs to frame decisions, not as a final design. Required kW with margin adds headroom for module aging, weather variability, and future demand. Compare estimated panel count to the roof‑based maximum to spot constraints early. If space is tight, prioritize efficiency upgrades and load shifting first. Export CSV or PDF for consistent quotes and ask installers to validate assumptions on site. Confirm service capacity and breaker space during early project planning.
FAQs
How accurate is the panel count?
It is a planning estimate based on your inputs. Installer layouts, roof tilt, azimuth, local weather, and equipment choices can change production and the final count.
What value should I use for peak sun hours?
Use a conservative local average from solar resource data or installer reports. If unsure, start lower; it prevents undersizing and makes the quote review easier.
Why include shading and soiling factors?
Both reduce real energy output. Shading can cut string performance, while dust reduces irradiance. Modeling them helps avoid optimistic production estimates.
What design margin is reasonable?
Many homeowners use 5–10% to cover variability, module aging, and minor load growth. Increase it if you expect significant future electricity demand.
What if the roof area is insufficient?
Consider higher‑wattage panels, using another roof plane, adding a carport/pergola array, or lowering the offset target. Efficiency upgrades can also reduce required capacity.
Does this include battery storage sizing?
No. This tool focuses on array sizing and panel count. Battery capacity depends on outage needs, night usage, and desired backup duration.
Example data table
| Scenario | Monthly kWh | Sun hours | Panel W | Target offset | Estimated panels | Array area (m²) |
|---|---|---|---|---|---|---|
| Small home | 600 | 5.0 | 400 | 80% | 10 | 21.0 |
| Typical home | 900 | 5.0 | 450 | 90% | 13 | 27.3 |
| High usage | 1500 | 4.5 | 550 | 100% | 22 | 46.2 |
Examples assume moderate losses and typical derate factors. Your site conditions may vary.