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 Sizing Calculator runs the same formula instantly.

Best for: facility engineers and EPCs screening rooftop or carport PV before inverter/MPPT quotes.

Not ideal for: camping kits, portable power stations, or stamped interconnection designs (see Commercial Solar Interconnection Requirements).

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)

Module count ≈ (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
η (derate) Typically 0.75–0.85; see Solar System Losses & Derating

CTA: Enter the three inputs in the Solar Panel Sizing Calculator on the Solar calculator hub.

Step-by-step (commercial) #

Conclusion: Lock energy and PSH before shopping panel wattage.

  1. Define the energy target — offset kWh/day (or monthly ÷ 30), not nameplate wish lists.
  2. Pick planning PSH — use annual or worst-month PSH depending on whether bill offset or winter autonomy matters.
  3. Choose η honestly — soiling and hot roofs often pull below 0.80.
  4. Compute array kW — formula above or the sizing tool.
  5. Estimate modules — optional nameplate W → count for BOM screens.
  6. Next BOS — production, MPPT, inverter (links below).

Worked example (numbers) #

Given: Warehouse wants 40 kWh/day daytime offset, PSH 5.0, η 0.80, 450 W modules.

Array kW = 40 ÷ (5.0 × 0.80) = 10.0 kW
Modules ≈ 10,000 ÷ 450 ≈ 23 panels

PSH Array kW (same 40 kWh, η=0.80)
4.0 12.5 kW
5.0 10.0 kW
5.5 9.1 kW

Then estimate annual yield in the PV Energy Production Calculator (array kW × PSH × days × (1 − loss)).

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 #

  1. Using daylight hours as PSH — daylight can be 12 h while PSH is ~4–6.
  2. η = 1.0 because the inverter is 98% — see losses guide.
  3. Sizing from module count first — start from kWh.
  4. Ignoring winter resource when autonomy matters.
  5. Skipping production check after array kW.

Next steps #

  1. Run Solar Panel Sizing Calculator.
  2. Confirm derate: Solar System Losses & Derating.
  3. Annual kWh: PV Energy Production Calculator.
  4. Controllers: How to Size a Solar Charge Controller → MPPT Sizing.
  5. Hub path: Solar calculator.

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 do I size a commercial solar array?

Use Array kW = Daily kWh ÷ (PSH × η). Example: 40 ÷ (5 × 0.80) = 10 kW. Then estimate module count from nameplate watts.

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.

How is this different from the sizing calculator?

This page explains the method. The Solar Panel Sizing Calculator computes the same formula with live inputs 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 MPPT/inverter and review interconnection. Start on the Solar calculator hub.