5 — How to Calculate Battery Capacity?

When building an off-grid power system, one of the first confusing steps is understanding how much battery capacity you actually need. It sounds complicated at first, but the calculation becomes simple once you understand which battery type you’re dealing with.

Before anything else, the battery choice matters a lot. Different batteries behave very differently in terms of safty, prformance, price, weight, and even how much instant current they can deliver.

In my vineyard cabin, I personally use a LiFePO4 (Lithium Iron Phosphate) battery. Based on all my research, it’s the most advanced and stable technology today.

The name LiFePO4 actually comes from the elements inside: Lithium + Iron (Fe) + Phosphate (PO4). Some people write it differently, but the core idea is always the same.

Older battery types like Gel batteries have much shorter cycle life (charge/discharge lifespan), while AGM or so-called “dry batteries” perform poorly and degrade very quickly. Not something I'd reccomend for off-grid use.

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🔋 Battery Capacity Formula

The usable capacity depends on your energy consumption. The battery label always shows its voltage (V) and amp-hour (Ah).

Formula:

Capacity (Watt) = Voltage × Ampere

Example: A 12V 100Ah LiFePO4 battery:

12 × 100 = 1200 watts

But this is only the theoretical capacity. In real life, you can’t use 100% safely. For LiFePO4, safe usage is about 80–90%.

So a 1200W LiFePO4 battery realistically gives: ~1000W usable.

Gel batteries are even more limited. They can only handle about 50% usable capacity.

Meaning a 1200W gel battery is actually: ~600W usable.

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📊 Battery Comparison Tables

1) General Usable Capacity Comparison

2) Technology & Performance Comparison

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⚡ Why You Should Not Use the Full Capacity

Fully draining a battery again and again dramatically shortens its life. Just like a phone battery, but even more sensitive.

LiFePO4: avoid going below 20% regularly

Gel: avoid going below 50% at all costs

Keeping the battery in its “healthy zone” increases lifespan by sometimes 3–5× more.

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✔ Simple Rule for Choosing Battery Size

For LiFePO4:

Daily consumption × 1.2 (20% safety margin)

For Gel Batteries:

Daily consumption × 2 (because you can use only half safely)

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🔍 Example Calculation

Let’s say your daily energy use is:

Lights: 70W
Fridge: 700W
Router + devices: 60W
Total: 830W

For LiFePO4:

830 × 1.2 ≈ 1000W → 12V 100Ah LiFePO4

For Gel Battery:

830 × 2 ≈ 1660W → 12V 150Ah Gel

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⭐ Final Summary

LiFePO4 batteries provide the safest, longest-lasting and most efficient solution for off-grid systems. Gel batteries work but must be handled much more carefully. AGM/dry batteries degrade too fast and usually end up costing more in the long run.

Always size your battery based on your real energy consumption, add a proper safety margin, and avoid deep discharges for the best lifetime.