VoltBasePRO
Electrical Reference
Battery & Solar Guide
8 min read
• Updated 2026-08-15

How to Size a Battery Bank for Solar, RV & Backup Power

Complete step-by-step sizing guide for calculating required battery capacity (Ah and Wh), Depth of Discharge, and temperature derating.

Step-by-Step Procedure

1

Create an Appliance Energy Audit

List all electrical appliances, their power draw in Watts, and their estimated daily run hours. Multiply Watts × Hours for each appliance and sum them to obtain Total Daily Watt-Hours (Wh/day).

2

Account for Inverter & Wiring Losses

Divide total daily energy consumption by inverter efficiency (typically 0.88 to 0.92 for pure sine wave inverters): Eadjusted=Edaily/ηinverterE_{\text{adjusted}} = E_{\text{daily}} / \eta_{\text{inverter}}.

3

Select Battery Chemistry & Depth of Discharge (DoD)

Choose your battery type: Lead-Acid (50% max usable DoD) or Lithium LiFePO4 (85% to 90% usable DoD).

4

Include Days of Autonomy

Decide how many cloudy or off-grid days the system must operate without solar or grid recharging (typically 1 to 3 days for off-grid solar).

5

Calculate Total Nominal Battery Capacity (Ah)

Formula: Capacity (Ah)=Daily Wh×Autonomy DaysSystem Voltage×DoD×η\text{Capacity (Ah)} = \frac{\text{Daily Wh} \times \text{Autonomy Days}}{\text{System Voltage} \times \text{DoD} \times \eta}.

Worked Practical Example

Off-Grid Camper Van Solar Setup

Daily consumption: 12V 1200Wh/day, 1 day of autonomy, using LiFePO4 (85% DoD) and 90% inverter efficiency.

  • •Daily energy needed: 1,200 Wh
  • •Accounting for 90% inverter efficiency: 1,200 / 0.90 = 1,333.3 Wh
  • •Accounting for 85% LiFePO4 DoD: 1,333.3 / 0.85 = 1,568.6 Wh nominal bank size
  • •Dividing by 12.8V nominal voltage: 1,568.6 Wh / 12.8 V = 122.5 Ah
  • •Recommendation: Install two 12V 100Ah LiFePO4 batteries in parallel (200Ah total, 2,560 Wh nominal).
✓ Target Battery Bank: 200 Ah @ 12.8V LiFePO4

Common Mistakes to Avoid

  • ✕Assuming 100% of a lead-acid battery's rated Ah is usable (leads to rapid battery destruction).
  • ✕Ignoring the self-consumption and idle power draw of the inverter itself (which draws 15W to 40W 24/7).
  • ✕Using undersized DC cables between battery and inverter, causing premature low-voltage cutoffs.

Frequently Asked Questions

Should I build a 12V, 24V, or 48V battery bank?

Systems under 1,500W load work well at 12V. For loads between 1,500W and 3,000W, a 24V system cuts cable current and size in half. For loads over 3,000W, a 48V system is standard.

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