7 mistakes to avoid when sizing PV and battery banks off-grid

The 7 Sizing Pitfalls

1. No load profile: Averages hide peaks. List devices, watts, hours/day, and note motor surges.
2. Autonomy picked at random: Choose days of autonomy from weather/outage history, access to backup, and criticality of loads.
3. Assuming nameplate is usable: Respect DoD limits, BMS reserves, round‑trip losses, and temperature derates.
4. Ignoring power (kW) while sizing energy (kWh): Check inverter continuous/surge ratings and battery c‑rate against peak loads.
5. Winter PV underestimated: Size array for worst‑month irradiance, tilt, shading, snow, and wiring/temperature effects.
6. Forgetting BOS losses and voltage drop: Include wiring, controller, and conversion losses; keep DC voltage drop within accepted limits.
7. No replacement plan: Batteries and inverters have different lifetimes. Budget replacements and logistics.

Reproducible Design Math

1) Daily Energy and Autonomy

E_daily (kWh) = Σ (Watts_i × Hours_i) ÷ 1000

E_autonomy = E_daily × Days_autonomy

2) Required Battery Bank (kWh)

Account for usable DoD, round‑trip efficiency, and temperature:

E_batt_req = E_autonomy ÷ (DoD_usable × RTE × Temp_factor)

• DoD_usable (e.g., 0.8 for 80% usable per warranty)
• RTE round‑trip efficiency of battery+inverter (0.85–0.95 typical)
• Temp_factor 0.85–0.95 if cold reduces usable energy or power

3) Power Check (kW)

Ensure P_continuous and P_surge of inverter and battery c‑rate meet peak loads and motor starts.

4) PV Array for Worst Month

P_PV_req (kW) = E_daily ÷ (PSH_worst × System_derate)

• PSH_worst = peak‑sun‑hours of the worst month at your tilt/azimuth
• System_derate (0.65–0.8 typical, includes temperature, wiring, dust, controller)

5) Charge Rate and Controller Sizing

Target charge rate 0.1–0.3C depending on chemistry and goals. Verify MPPT input window (Vmp/Voc at temperature) and controller current limits.

Worked Example (Illustrative)

Adjust to your site and datasheets.

Battery: E_autonomy = 3.26 × 3 = 9.78 kWh → E_batt_req = 9.78 ÷ (0.8×0.9×0.9) ≈ 15.1 kWh. Round up for growth and days with no sun.

PV array: P_PV_req = 3.26 ÷ (3.0 × 0.75) ≈ 1.45 kW. Add margin for snow/dust and to support charging while serving loads (e.g., 1.8–2.2 kW).

Power check: Inverter should support pump surge; consider soft‑start or pressure tank to reduce starts.

Safety and Standards

Use listed equipment and follow manufacturer instructions. Relevant references include NEC Articles 690/710 (PV and stand‑alone systems), UL 1741 (inverters), UL 9540/9540A (ESS), IEC 62109 (inverter safety). Local Authorities Having Jurisdiction (AHJs) may add placement and fire‑safety rules; always verify locally.

Owner Checklist

• 12 months of measured/estimated loads by season; motor surge data.
• Outage/weather history to choose days of autonomy.
• Site survey: tilt/azimuth, shading, snow, worst‑month PSH.
• Equipment datasheets: usable DoD, c‑rate, RTE, temperature limits.
• Wiring plan and voltage drop calculations; controller/inverter ratings.
• Replacement plan and lifecycle cost (batteries vs inverters).

Where to Verify

• IEA/IEA PVPS reports on off‑grid solar and storage.
• NREL tools (SAM) and design guides for stand‑alone systems.
• NEC 690/710, UL 1741, UL 9540/9540A, IEC 62109 and local AHJ guidance.
• Manufacturer datasheets for power, DoD, temperature, and warranty throughput.

Plain‑Language Disclaimer

This article is educational and not legal, engineering, or financial advice. Codes, tariffs, and product specifications change. Work with qualified professionals and follow your local requirements.