| Energy | Sun hours | Derate | Safety | Panel | Panels | Array | Area incl. spacing | Inverter |
|---|---|---|---|---|---|---|---|---|
| 900 kWh/month | 5.00 | 75% | 10% | 550 W | 16 | 8.80 kWdc | 42.32 m² | 7.65 kWac |
- kWh/day conversion: daily = kWh, monthly = kWh / 30.437, annual = kWh / 365.
- PV array size: PV_kWdc = (kWh_day / (PSH × derate)) × (1 + safety).
- Panel count: Panels = ceil((PV_kWdc × 1000) / panel_watt).
- Area: Area = panels × panel_area, then apply spacing allowance.
- Inverter recommendation: Inverter_kWac ≈ max(peak_load, PV_kWdc / DCAC_ratio).
- Optional string limit (cold voltage): Voc_cold = Voc × (1 + |coeff| × (25 − Tmin)), Series_max = floor(Vmax / Voc_cold).
- Choose an energy basis and enter the energy value in kWh.
- Enter peak sun hours for your project location and season.
- Set derate and safety margin to reflect real conditions.
- Enter panel wattage, area, and spacing allowance for layout.
- Optionally add peak AC load and stringing inputs for checks.
- Press Calculate to view results above the form.
- Use the CSV or PDF buttons to export your calculation record.
Design Inputs That Matter
For construction estimates, start with reliable energy data. Use utility bills, tenant schedules, and equipment lists. A practical baseline is 750–1,200 kWh/month for small offices and 1,500–3,000 kWh/month for mixed-use spaces, then refine with measured submeter totals where available. If demand charges matter, also record peak kW and operating hours, because PV may offset energy without fully reducing peak demand.
Interpreting Peak Sun Hours
Peak sun hours represent daily equivalent full-sun energy. A site may average 4.0–6.0 h/day annually, but seasonal variation can exceed 20%. For conservative sizing, use the lower seasonal value when the load is highest, or apply a safety margin to compensate for short winter days and haze. Tilt, azimuth, and nearby reflective surfaces can shift the effective value, so confirm assumptions for unusual orientations.
Choosing Derate and Safety Margins
The derate factor bundles losses from temperature, soiling, wiring, mismatch, and inverter efficiency. Typical planning values range 70–85%. Use 75% for dusty sites or high temperatures, and 80–85% for cleaner, well-maintained arrays. Soiling alone can be 2–10% depending on rainfall and cleaning intervals. Add 5–15% safety when future loads, operating hours, or tenant fit-outs are uncertain.
Layout, Area, and Construction Constraints
Area is not only module footprint. Add spacing for access lanes, parapet setbacks, tilt row gaps, and maintenance zones. A 10–25% allowance is common on flat roofs. Always confirm structural capacity, wind uplift detailing, drainage paths, and fire access requirements before finalizing the layout. For ballasted systems, check dead load limits; for penetrations, confirm waterproofing details and routing of conduits to minimize rework.
Example Data Used in Practice
Example sizing inputs for a preliminary submittal:
- Energy: 900 kWh/month, PSH: 5.0 h/day, Derate: 75%
- Safety: 10%, Panel: 550 W, Area: 2.3 m², Spacing: 15%
- Computed outcome: about 9 panels, 4.95 kWdc, and ~23.8 m² including spacing.
- With DC/AC ratio 1.15, inverter guidance is roughly 4.30 kWac or higher if peak load requires.
1) What derate factor should I choose?
Use 70–75% for dusty or hot sites and minimal cleaning. Use 80–85% for cleaner sites, good airflow, and planned maintenance. If unsure, start at 75% and adjust after review.
2) Where do peak sun hours come from?
Peak sun hours can be taken from local solar resource summaries, developer datasets, or measured irradiance. For planning, use an annual average, then check the lowest-season value if winter production matters.
3) Does this include battery storage sizing?
No. This sizes the PV array for energy production. Storage requires separate inputs for autonomy hours, depth of discharge, round-trip efficiency, and backup load selection.
4) How does shading affect the result?
Shading reduces effective sun hours and increases mismatch losses. If shading is expected, lower peak sun hours or derate to reflect obstruction, then confirm with a shade study before procurement.
5) Why use a DC/AC ratio?
A DC/AC ratio helps match the PV array to inverter capacity. Moderate oversizing improves energy capture in low-light periods, while excessive oversizing may increase clipping and heat stress.
6) How do I estimate roof area correctly?
Use module footprint plus spacing allowance, then subtract setbacks, skylights, HVAC, and access routes. Validate with roof drawings and a site walk to confirm obstructions and safe maintenance paths.
7) Is this suitable for permits and final design?
It is for preliminary sizing and budgeting. Final design should use manufacturer datasheets, structural checks, detailed electrical stringing, and code-compliant layouts verified by a qualified professional.