Off Grid Battery Sizing Calculator

Battery Capacity Formula:

\[ \text{Battery Capacity (Ah)} = \frac{\text{Daily Energy Consumption (Wh)} \times \text{Days of Autonomy}}{\text{Battery Voltage (V)} \times \text{Depth of Discharge} \times \text{Efficiency}} \]

Daily Energy Consumption:

watt-hours

Days of Autonomy:

days

Battery Voltage:

volts

Depth of Discharge:

decimal (0-1)

Efficiency:

decimal (0-1)

Required Battery Capacity:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Battery Capacity Formula?

The battery capacity formula calculates the required amp-hour (Ah) capacity for an off-grid system based on daily energy needs, desired days of autonomy, battery voltage, depth of discharge, and system efficiency.

2. How Does the Calculator Work?

The calculator uses the following equation:

\[ \text{Battery Capacity (Ah)} = \frac{\text{Daily Energy Consumption (Wh)} \times \text{Days of Autonomy}}{\text{Battery Voltage (V)} \times \text{Depth of Discharge} \times \text{Efficiency}} \]

Where:

Daily Energy Consumption — Total watt-hours used per day
Days of Autonomy — Number of days system should run without sun
Battery Voltage — System voltage (typically 12V, 24V, or 48V)
Depth of Discharge — Fraction of battery capacity that can be used (0-1)
Efficiency — System efficiency factor (typically 0.8-0.95)

Explanation: The equation accounts for total energy needs, battery characteristics, and system losses to determine the minimum battery bank size.

3. Importance of Proper Battery Sizing

Details: Correct battery sizing ensures reliable power availability, prevents premature battery failure from over-discharge, and optimizes system cost and performance.

4. Using the Calculator

Tips: Enter daily energy consumption in watt-hours, days of autonomy, battery voltage, depth of discharge (typically 0.5 for lead-acid), and system efficiency (typically 0.85). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical depth of discharge for lead-acid batteries?
A: 50% (0.5) is recommended for long life in lead-acid batteries. Lithium batteries can often discharge to 80-90%.

Q2: How many days of autonomy should I plan for?
A: Typically 2-5 days depending on location and criticality of the load. More in cloudy areas, less in sunny regions.

Q3: What affects system efficiency?
A: Inverter efficiency (90-95%), charge controller efficiency (95-98%), wiring losses, and battery charge/discharge efficiency.

Q4: Should I round up the battery capacity?
A: Yes, always round up to the nearest standard battery size and consider adding 10-20% extra capacity for future expansion.

Q5: How does temperature affect battery capacity?
A: Cold temperatures reduce available capacity. For temperatures below 20°C (68°F), you may need to increase capacity by 10-20%.