How Many Solar Panels Do I Need? (Commercial Array Formula)
Quick answer: How many solar panels for a commercial array? #
Array kW = Daily kWh ÷ (Peak sun hours × System efficiency η)
Panels ≈ (Array kW × 1000) ÷ Module watts
Example: 40 kWh/day, PSH 5.0, η 0.80, 400 W modules → 40 ÷ (5 × 0.80) = 10 kW ≈ 25 panels.
This Guide owns the commercial method. Instant calc: Solar Panel Calculator on the Solar calculators hub.
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.
What is the commercial 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 calculators hub → Applications.
How to size 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).
Example 1: 40 kWh/day warehouse #
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 |
Example 2 (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)).
Example 3: Building square feet is a roof cap—not the energy formula #
GSC-style ask: “how many panels for a commercial building by square feet?”
Answer: Floor or roof area does not replace daily kWh. Use bills (or load tools) for energy; use area only to cap what fits.
Screen: A 20,000 ft² roof that fits ~8 kW of modules cannot deliver a 10 kW energy target at your PSH/η—accept partial offset, add another plane/carport, or revise the energy goal. Do not invent kWh from square feet alone.
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 |
Why 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.
- Converting building square feet directly into panel count without daily kWh.
Next step: Run the solar panel calculator #
- 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 calculators hub → 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.
FAQ #
How many solar panels do I need?
Use Array kW = Daily kWh ÷ (PSH × η), then panels ≈ (Array kW × 1000) ÷ module W. Example: 40 kWh/day, 5.0 PSH, η 0.80 → 10 kW ≈ 25 × 400 W. Instant path: Solar Panel Calculator.
How to figure out how many solar panels I need?
Start from energy, not roof area: convert the bill to daily kWh (monthly ÷ 30), pick site peak sun hours, apply η (often 0.75–0.85), then divide into module watts. Same worked ladder as above.
How to calculate how many solar panels you need?
panels ≈ Daily kWh ÷ (PSH × η) × 1000 ÷ module W. Example: 30 kWh/day ÷ (5 × 0.80) = 7.5 kW ≈ 19 × 400 W. Commercial walkthrough stays on this page; homes can use the same calculator presets.
Can I size commercial panels from building square feet?
No—square feet only caps what fits. Convert utility kWh to daily energy, apply PSH and η, then check whether the roof can hold that array kW.
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—use it for commercial solar panel calculator searches.
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 calculators 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.