battery capacity calculator

Why Battery Capacity Matters

A battery that is too small will discharge too quickly, age faster, and may leave critical devices without power. A battery that is too large can waste budget and space. A good battery size balances reliability, battery lifespan, and cost.

Whether you are sizing a home backup system, a camper battery, or a small solar installation, understanding battery capacity helps you avoid guesswork. This calculator gives you a practical estimate in both amp-hours (Ah) and watt-hours (Wh), which are the two most common planning units.

Understanding Ah and Wh

Amp-hours (Ah)

Amp-hours describe electric charge capacity. If a battery is rated at 100Ah, it can theoretically deliver 100 amps for 1 hour, or 10 amps for 10 hours (under ideal conditions).

Watt-hours (Wh)

Watt-hours represent total energy. Because energy depends on voltage, Wh often gives a better apples-to-apples comparison between systems.

• Wh = Ah × V
• Ah = Wh ÷ V

For example, a 12V 100Ah battery stores about 1200Wh of nominal energy.

Core Formula Used by the Calculator

For battery sizing (required capacity), the tool uses this approach:

• Load Energy = Load Power × Runtime
• Battery Energy Needed = Load Energy ÷ Efficiency
• Base Ah = Battery Energy Needed ÷ Battery Voltage
• Adjusted Ah = Base Ah ÷ Depth of Discharge
• Recommended Ah = Adjusted Ah × (1 + Safety Margin)

This method accounts for conversion losses and the fact that many batteries should not be drained to 0% for healthy cycle life.

How to Use This Battery Capacity Calculator

Step-by-step for sizing a new battery

• Enter your average load power in watts (W).
• Enter the battery system voltage (typically 12V, 24V, or 48V).
• Enter desired runtime in hours.
• Set realistic efficiency (90% is a common starting point).
• Set your planned maximum depth of discharge.
• Add a safety margin for aging, temperature, and unexpected loads.

Step-by-step for runtime estimation

• Enter known battery capacity in Ah and voltage.
• Enter expected load power in watts.
• Use realistic DoD and efficiency values.
• Calculate estimated runtime in hours and minutes.

Example: Quick Sizing Scenario

Suppose your devices draw 200W and you need 6 hours of backup at 12V:

• Load energy = 200 × 6 = 1200Wh
• Assume 90% efficiency: 1200 ÷ 0.90 = 1333Wh
• Base capacity in Ah: 1333 ÷ 12 = 111Ah
• At 80% DoD: 111 ÷ 0.80 = 139Ah
• With 20% safety margin: 139 × 1.20 ≈ 167Ah

So a practical recommendation would be around a 12V 170Ah battery bank (or larger standard size available in your market).

Battery Chemistry and Depth of Discharge

Lead-acid (flooded, AGM, gel)

Lead-acid batteries usually benefit from shallower cycling. Many systems use 50% DoD for longer life, though some setups may use up to around 70% depending on design goals.

Lithium iron phosphate (LiFePO4)

LiFePO4 batteries commonly allow deeper cycling, often 80% to 95% usable DoD. This can reduce total battery size required for the same runtime.

Practical Tips for Better Accuracy

• Measure real-world load power with a watt meter whenever possible.
• Plan for surge loads from compressors, pumps, or motors.
• Account for cold temperatures, which can reduce effective capacity.
• Include future expansion if you plan to add appliances later.
• Remember batteries lose capacity over time; margins improve long-term reliability.

Final Thoughts

A battery capacity calculator is one of the simplest ways to improve system design decisions. By combining load, runtime, efficiency, and discharge limits, you get a realistic starting point instead of relying on rough guesses.

Use this page to compare configurations quickly, then round up to standard battery sizes and consult manufacturer datasheets for final selection.