Lead-Acid Battery Calculator

Lead-acid batteries power everything from backup systems to off-grid setups, but sizing them correctly means accounting for battery type, depth of discharge, and real-world efficiency losses. The Lead-Acid Battery Calculator lets you either calculate runtime (how long a battery will last) or calculate battery size (what capacity you need). Enter your load, system voltage, battery type (AGM, Flooded, or Gel), and minimum remaining charge to get your required battery capacity, along with load current, usable capacity, and efficiency factor.

V
hours
Ah

Only needed for runtime calculation

%

Recommended 50% for lead-acid battery longevity

Results

Required Battery Capacity

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Load Current

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Usable Capacity

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Efficiency Factor

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Frequently Asked Questions

How do I calculate the battery current for my application?

Battery current (in amps) is calculated by dividing the power consumption (in watts) by the system voltage. For example, a 100W load on a 12V system draws 8.33 amps (100W ÷ 12V = 8.33A).

How do I calculate the size of battery I need?

Battery size is determined by multiplying the load current by the runtime duration, then dividing by the usable capacity percentage. Environmental factors and battery age may require additional capacity.

What is the recommended discharge level for lead-acid batteries?

Lead-acid batteries should not be discharged below 50% of their capacity for optimal longevity. Deep discharges significantly reduce battery life and performance.

How do temperature extremes affect battery performance?

Temperatures below 0°F or above 85°F reduce battery efficiency by approximately 20%. Cold temperatures slow chemical reactions, while high temperatures can damage the battery and reduce capacity.

Why does battery age affect capacity calculations?

Batteries lose capacity over time due to sulfation and plate degradation. A battery older than 6 months may have reduced capacity, requiring oversizing to maintain reliable performance.

What's the difference between AGM, Gel, and Flooded lead-acid batteries?

AGM (Absorbed Glass Mat) batteries are sealed and maintenance-free with good discharge rates. Gel batteries are also sealed but have slower discharge rates. Flooded batteries require maintenance but are typically the most economical option.

How do I convert watts to amps for battery calculations?

To convert watts to amps, divide the wattage by the system voltage. For DC systems: Amps = Watts ÷ Volts. For AC loads through an inverter, add 10-15% for inverter losses before calculating.