How to Size a Commercial Solar Array: Formula & Example
Introduction #
How to size a commercial solar array: estimate daily energy need (kWh/day), divide by peak sun hours times system efficiency (derate), and convert the resulting array kW into module count. This page walks the method for warehouses and campuses; the Solar Panel Calculator runs the same formula instantly (including how many panels).
Best for: facility engineers and EPCs screening rooftop or carport PV before inverter/MPPT quotes.
Not ideal for: camping kits, portable power stations, cabin shopping kits, or stamped interconnection designs (see Commercial Solar Interconnection Requirements). Off-grid cabins/RVs start at Off Grid Solar Calculator—not this commercial method page.
The sizing formula #
Conclusion: Array kW is driven by energy need ÷ resource ÷ derate—not by “how many panels fit on the roof” alone.
Array kW = Daily kWh ÷ (Peak sun hours × System efficiency η)
How many panels ≈ (Array kW × 1000) ÷ Module watts
| Input | Where it comes from |
|---|---|
| Daily kWh | Bills, meters, or Off Grid Solar Load / Factory Load screens |
| Peak sun hours | Site maps or Peak Sun Hours Calculator (state/zone table + PVWatts) |
| η (derate) | Typically 0.75–0.85; see Solar System Losses & Derating |
CTA: Enter the three inputs in the Solar Panel Calculator on the Solar calculator hub → Applications.
Step-by-step (commercial) #
Conclusion: Lock energy and PSH before shopping panel wattage.
- Define the energy target — offset kWh/day (or monthly ÷ 30), not nameplate wish lists.
- Pick planning PSH — use annual or worst-month PSH depending on whether bill offset or winter autonomy matters.
- Choose η honestly — soiling and hot roofs often pull below 0.80.
- Compute array kW — formula above or the sizing tool.
- Estimate modules — optional nameplate W → count for BOM screens (default screen often 400 W).
- Geometry check — Solar Panel Tilt Angle Calculator if tilt is free.
- Next BOS — production, inverter, MPPT (links below).
Worked example (numbers) #
Given: Warehouse wants 40 kWh/day daytime offset, PSH 5.0, η 0.80, 400 W modules.
Array kW = 40 ÷ (5.0 × 0.80) = 10.0 kW
Modules ≈ 10,000 ÷ 400 = 25 panels
| PSH | Array kW (same 40 kWh, η=0.80) | ~Panels @ 400 W |
|---|---|---|
| 4.0 | 12.5 kW | 32 |
| 5.0 | 10.0 kW | 25 |
| 5.5 | 9.1 kW | 23 |
Second screen (80 kWh/day warehouse): at PSH 4.5 and η 0.80 → 80 ÷ (4.5 × 0.80) ≈ 22.2 kW ≈ 56 × 400 W panels. Re-run in the calculator with your site PSH.
Then estimate annual yield in the Solar Production Calculator (array kW × PSH × days × (1 − loss)).
Daily kWh → array ladder (5.0 PSH, η 0.80, 400 W) #
| Daily kWh | Array kW | ~Panels |
|---|---|---|
| 30 | 7.5 | 19 |
| 40 | 10.0 | 25 |
| 60 | 15.0 | 38 |
| 100 | 25.0 | 63 |
Roof area is not the formula #
Conclusion: Available roof area caps the design; it does not replace the energy formula.
If the roof only fits ~8 kW but the load needs 10 kW at your PSH/η, you either accept partial offset, improve η/PSH assumptions with better data, or add another plane/carport—not “force” 10 kW into 8 kW of space.
Common mistakes #
- Using daylight hours as PSH — daylight can be 12 h while PSH is ~4–6.
- η = 1.0 because the inverter is 98% — see losses guide.
- Sizing from module count first — start from kWh.
- Ignoring winter resource when autonomy matters.
- Skipping production / inverter check after array kW.
Next steps #
- Run Solar Panel Calculator.
- Confirm derate: Solar System Losses & Derating.
- Annual kWh: Solar Production Calculator.
- Inverter AC: Inverter Size Calculator → kW to kVA.
- Controllers: How to Size a Solar Charge Controller → MPPT Sizing.
- Hub path: Solar calculator → Applications.
- Interconnection: Commercial Solar Interconnection Requirements.
Assumptions and disclaimer #
Screens assume steady daily energy and a single PSH. Shading, clipping, and tariff structures need site studies. Planning estimate only—confirm with OEM design tools before procurement. Last updated: 2026-08-08.
FAQ #
How do I size a commercial solar array?
Use Array kW = Daily kWh ÷ (PSH × η). Example: 40 ÷ (5 × 0.80) = 10 kW ≈ 25 × 400 W panels. Then estimate annual yield and inverter AC.
What peak sun hours should I use?
Location peak sun hours (equivalent full-sun hours at 1,000 W/m²)—not clock daylight. Screen with the Peak Sun Hours Calculator (climate band + US state/zone table), then confirm with PVWatts.
How is this different from the sizing calculator?
This page explains the method. The Solar Panel Calculator computes the same formula with live inputs, a kWh→panels ladder, and comparison tables.
What derate should I use?
Often 0.75–0.85 for commercial screens. Details: Solar System Losses & Derating.
What is the next step after array kW?
Estimate annual kWh, then size inverter/MPPT and review interconnection. Start on the Solar calculator hub Applications section.
Is this for cabin or RV kits?
No—commercial roof/campus method. For cabin/RV energy roll-up use the Off Grid Solar Calculator (cabin/RV presets), then panel sizing—do not start from shopping kits.