Battery Capacity Calculator

Q: How do I calculate battery capacity in Ah?

Ah = (kWh × days × 1000) ÷ (Voltage × DoD% ÷ 100 × Efficiency% ÷ 100). For example, 5 kWh/day, 2 days autonomy, 48V, 80% DoD, 90% efficiency: Ah = (5 × 2 × 1000) ÷ (48 × 0.8 × 0.9) = 10000 ÷ 34.56 = 289 Ah.

Q: What depth of discharge should I use?

Lead-acid batteries: 50% DoD maximum to protect longevity. Lithium (LiFePO4): 80–90% DoD safely. AGM/gel: 50–60%. Using higher DoD than recommended rapidly degrades battery life.

Q: What is battery autonomy?

Autonomy is the number of days the battery bank can power your loads without recharging (no sun, no generator). 1–2 days is typical for solar backup, 3–5 days for full off-grid systems in cloudy climates.

Find the battery bank size (Ah) needed for your solar, off-grid, or backup power system. Enter your daily energy use, backup days, battery voltage, depth of discharge, and system efficiency.

Calculate Required Battery Capacity

Daily Energy Use (kWh/day)

Autonomy (days without recharge)

Battery Voltage (V)

Depth of Discharge (%)

System Efficiency (%)

Battery Bank Sizing

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Required Capacity (Ah)

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Usable Energy (kWh)

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Total Bank (kWh)

Formula

Ah = (kWh/day × days × 1000) ÷ (Voltage × (DoD ÷ 100) × (Efficiency ÷ 100))

The DoD factor ensures you don't fully deplete the battery (extending lifespan). The efficiency factor accounts for round-trip losses in charging and discharging the battery bank.

DoD Recommendations by Battery Type

Lithium Iron Phosphate (LiFePO4): 80–90% DoD. Excellent cycle life even at high DoD. Best choice for solar and EV applications.
Sealed Lead-Acid (AGM/Gel): 50–60% DoD maximum. Deeper discharge accelerates sulfation and plate damage.
Flooded Lead-Acid: 50% DoD maximum. Cheaper upfront but requires regular maintenance (water top-up).
NiMH/NiCd: 70–80% DoD. Good deep cycle capability but largely superseded by lithium.

System Voltage Guide

12V: Small systems, caravans, boats under 1 kWh. High DC current for moderate loads.
24V: Medium systems 1–3 kWh. Halves the DC current vs 12V — smaller cables.
48V: Recommended for home solar systems. Quarters the current vs 12V — practical for 3–20+ kWh banks.
Higher voltages (96V, 120V+): Large commercial or EV battery systems.

Frequently Asked Questions

How do I calculate battery capacity in Ah?

Ah = (kWh × days × 1000) ÷ (V × DoD × Efficiency). For a 48V system needing 5 kWh/day for 2 days, with 80% DoD and 90% efficiency: Ah = (5 × 2 × 1000) ÷ (48 × 0.8 × 0.9) = 10,000 ÷ 34.56 = 289 Ah. You'd typically buy 300Ah or 2× 150Ah batteries in series/parallel.

What depth of discharge should I use?

For LiFePO4 lithium: use 80–90% DoD — these batteries are designed for it. For lead-acid (flooded, AGM, gel): limit to 50% DoD. Exceeding the recommended DoD significantly reduces battery cycle life. If budget is tight, it's better to buy correctly sized lead-acid than to overwork undersized lithium.

What is battery autonomy?

Autonomy is how many consecutive days your battery bank can power your loads without any recharge from solar or grid. 1 day suits urban solar backup where grid is reliable. 2–3 days covers most cloudy-weather scenarios. 5–7 days is used for remote off-grid systems in climates with extended cloudy periods.

How many batteries do I need?

Divide the required Ah by the Ah rating of a single battery. Then check whether batteries should be in series (to reach voltage) or parallel (to reach Ah). For example, 4× 12V 100Ah batteries can be wired as: 4 in series = 48V 100Ah bank, 4 in parallel = 12V 400Ah bank, or 2S2P = 24V 200Ah bank.