What Size Solar and Battery System Does My Factory/Warehouse Need?

 

The right system size depends on your goals and your operating profile: whether you want to reduce costs only or aim for full energy independence, your factory/warehouse operating hours, and whether you have machinery with high inductive (motor) loads. Below is a step-by-step sizing method plus practical examples so you can estimate required PV and battery capacity quickly.

 


1) Decide the goal (choose one or combine)

  • Cost reduction / peak shaving: sized to lower grid demand and shift energy use to cheaper periods.
  • Partial backup: cover critical loads for specified hours during outages.
  • Full energy independence: cover total site energy needs for defined autonomy (days/hours).
    Choose your target first — everything else follows.

 


2) Gather the data (you must have these)

  • Electricity bills (kWh per month, peak demand kW) — at least 12 months if possible.
  • Operating hours (hours/day and days/week).
  • List of critical loads and their power (kW) — include motors and starting currents.
  • Peak load (kW) and average load (kW).
  • Site solar resource (peak sun hours per day — typical for SA: 4–6; use 5 as a good baseline unless you have site data).

 


3) Battery sizing method (step-by-step formula)

Definitions / assumptions (conservative baseline):

  • Depth of Discharge (DoD usable) for LiFePO4 ≈ 80% → 0.80.
  • Round-trip efficiency (battery + inverter) ≈ 90% → 0.90.
  • Usable system fraction = DoD × round-trip efficiency = 0.80 × 0.90 = 0.72.

Formula:
Required Battery Capacity (kWh) = (Load (kW) × Autonomy (hours)) ÷ Usable fraction

 


Worked Example A — Small warehouse

Assumptions: average load = 20 kW, you want 8 hours of backup for overnight operations.

  1. Energy needed = 20 kW × 8 h = 160 kWh.
  2. Usable fraction = 0.80 × 0.90 = 0.72.
  3. Battery capacity = 160 ÷ 0.72 = 222.222... kWh.
  4. Round up to practical size → ≈225 kWh battery bank.

(So you would specify approximately 225 kWh of nominal battery capacity.)

 


Worked Example B — Medium factory

Assumptions: average load = 200 kW, you want 12 hours of autonomy.

  1. Energy needed = 200 kW × 12 h = 2400 kWh.
  2. Usable fraction = 0.72 (as above).
  3. Battery capacity = 2400 ÷ 0.72 = 3333.333... kWh.
  4. Round up to practical size → ≈3,334 kWh battery bank.

(So you would specify roughly 3.33 MWh of battery capacity.)

 


4) PV array sizing — cover daily consumption

If your goal is to supply daily energy from PV, calculate PV size in kWp:

Assumptions: peak sun hours = 5 hours/day; system losses (incl. inverter, soiling) → efficiency factor 0.85.
Formula: Required PV (kWp) = Daily energy need (kWh/day) ÷ (PeakSunHours × SystemEfficiency)

Example A (small warehouse)

  • Daily energy = 20 kW × 24 h = 480 kWh/day.
  • PV kWp = 480 ÷ (5 × 0.85) = 480 ÷ 4.25 = 112.941... kWp → ≈113 kWp.

Example B (medium factory)

  • Daily energy = 200 kW × 24 h = 4800 kWh/day.
  • PV kWp = 4800 ÷ 4.25 = 1129.411... kWp → ≈1,130 kWp.

Note: If your site operates fewer than 24 hours/day, use operating-hours × average load to compute daily kWh.

 


5) Inverter sizing & motor/inductive loads

  • Continuous inverter rating: should meet or exceed your peak continuous load. Add margin (recommended ×1.10 to ×1.25) for safety and future growth.
    Example: if peak = 200 kW, inverter sizing ≥ 200 × 1.25 = 250 kW.
  • Motor start / inrush: motors and compressors can require 2–6× start current. Choose inverters or soft-starters with appropriate surge capability, or size a separate inverter(s) for motor circuits.
  • Harmonics and power factor: industrial loads often require power factor correction and robust inverters that tolerate non-linear loads.

 


6) Other practical considerations

  • Critical-load segregation: isolate true-critical loads (lighting, controls, essential production lines) from non-essential loads to reduce battery size.
  • Peak shaving vs full backup: often a smaller battery sized for peak shaving delivers faster ROI than full autonomy.
  • Scalability: design the system modularly so you can add PV or batteries as demand grows.
  • Thermal management: batteries and inverters need suitable ambient temperatures and ventilation. High heat reduces life.
  • Monitoring & BMS: integrate BMS telemetry and site monitoring to track performance, alarms, and ROI.
  • Regulatory & safety: comply with SANS/SABS and local electrical regulations and obtain necessary permits.
  • Warranties & lifecycle costs: size with lifecycle economics in mind — battery warranty cycles, replacements, and maintenance.

 


7) Quick checklist to finalize a design

  • Obtain 12 months of utility bills and a one-line electrical diagram.
  • Log or estimate hourly load profile (kW) for at least one week.
  • Decide required autonomy (hours) and which loads are critical.
  • Calculate battery kWh using the formula above.
  • Size PV (kWp) based on daily kWh and local sun hours.
  • Specify inverter kW and surge capability for inductive loads.
  • Validate with a system integrator or LBSA installer for equipment match and commissioning.

 


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