Battery Sizing Calculator

Blog post description.

Wiratama

11/16/20251 min read

1. Definition

A Battery Sizing Calculator determines the required battery capacity (Ah and Wh) needed to power an electrical load for a specified duration. This is commonly used in solar PV systems, backup power systems (UPS), telecom towers, DC loads, and off-grid installations.
The calculator considers load power, runtime, battery voltage, depth of discharge (DoD), and system efficiency.

2. Background Theory

Battery sizing is based on the fundamental relationship between energy, power, and capacity:

Energy Required (Wh):

Where:

  • P = Load power (Watts)

  • t = Required runtime (hours)

Corrected for System Efficiency:

Actual battery energy needed is higher due to losses:

Where η\etaη = System efficiency (0–1).

Ampere-Hour (Ah) Requirement:

Battery capacity is determined by voltage and allowable depth of discharge:

Where:

  • V = Battery voltage

  • DoD = Maximum allowed discharge fraction (e.g., 0.8 for 80%)

Lithium batteries typically use 80–90% DoD, while lead-acid uses 50% or less, making lithium require smaller Ah for the same usable energy.

3. How the Calculator Works

The calculator follows these steps:

  1. User inputs:

    • Load power (W)

    • Runtime needed (hours)

    • Battery voltage (V)

    • Depth of discharge (%)

    • System efficiency (%)

  2. Calculates total energy demand:
    E=P×t

  3. Adjusts for system losses:
    Eeff=E/η

  4. Computes required battery capacity in Ah:
    Ah=Eeff/(V×DoD)

  5. Computes total battery energy in Wh:
    Wh=Ah×V

  6. Displays:

    • Required energy

    • Energy after efficiency correction

    • Required battery capacity (Ah)

    • Resulting battery bank energy (Wh)

This allows the user to quickly estimate battery size for off-grid or backup applications without going through manual engineering calculations.