Battery Capacity Calculator (Off-Grid)

How This Tool Works

The Battery Capacity Calculator sizes your battery bank for off-grid solar or backup power systems. Unlike the simple "daily usage × autonomy days" formula, this calculator accounts for depth of discharge (DoD), inverter efficiency, and system losses — all of which significantly increase the nominal capacity you need.

For a 25 kWh/day household wanting 2 days of autonomy with LFP batteries at 80% DoD, you need roughly 70 kWh of nominal battery capacity — not 50 kWh. The 40% difference comes from the DoD buffer (you can only use 80% of the battery), inverter conversion losses (10%), and wiring losses (5%). Undersizing any of these factors leaves you without power during cloudy stretches.

The calculator also estimates cost by battery chemistry. LFP (lithium iron phosphate) is recommended for all new installations — it's safer, lasts 4× longer than lead-acid, and allows 80% DoD vs 50% for lead-acid. The upfront cost is 2× higher but the cost per delivered kWh over the battery's life is half.

1. Daily energy usage — total daily kWh consumption. Off-grid homes typically run 10-30 kWh/day because they're more efficient by necessity.
2. Autonomy days — how many cloudy days the battery must cover. 2 is standard, 3 is conservative, 1 is acceptable if you have a generator.
3. System voltage — 48V for whole-home (recommended), 24V for small systems, 12V for RVs/cabins.
4. Depth of discharge — LFP: 80-90%. Lead-acid: 50%. NMC: 70-80%. Lower DoD extends battery life.
5. Inverter efficiency — 85-95%. Hybrid inverters are ~90%.
6. System losses — wiring, BOS, temperature. 3-7% typical.

The "nominal capacity" is what you need to buy. The "usable capacity" (what you actually get) is lower due to DoD and losses. This calculator shows both.

When to Use This Calculator

The DoD multiplier

Depth of discharge (DoD) is the percentage of battery capacity you actually use. LFP batteries allow 80% DoD — you can use 80% of the rated capacity. Lead-acid allows only 50%. To get 40 kWh of usable energy from LFP at 80% DoD, you need 50 kWh nominal (40 ÷ 0.80). From lead-acid at 50% DoD, you need 80 kWh nominal (40 ÷ 0.50) — 60% more battery for the same usable energy.

The loss cascade

Energy passes through the battery (charge/discharge efficiency ~95%), then the inverter (DC→AC, ~90%), then wiring (~5% loss). Combined: 0.95 × 0.90 × 0.95 = 81% round-trip efficiency. For every 100 kWh you put in, you get 81 kWh out. The calculator accounts for all of these.

LFP vs lead-acid lifecycle economics

LFP: $600/kWh, 6,000 cycles, 80% DoD. Over 6,000 cycles at 80% DoD: 6,000 × 0.80 = 4,800 kWh delivered per kWh of capacity. Cost per delivered kWh: $600 / 4,800 = $0.125/kWh. Lead-acid: $250/kWh, 1,500 cycles, 50% DoD. Over 1,500 cycles: 1,500 × 0.50 = 750 kWh delivered. Cost per delivered kWh: $250 / 750 = $0.333/kWh. LFP is 2.7× cheaper over its lifespan despite costing 2.4× more upfront.

Why 48V is the standard

Higher voltage means lower current for the same power, which means thinner wire and lower voltage drop. A 5,000W load at 48V draws 104A; at 12V it draws 417A. The 12V system needs 4× thicker wire and loses 16× more energy to resistance. 48V is the sweet spot for whole-home off-grid — high enough for efficiency, low enough to be safe to touch.

When to add a generator

If your autonomy requirement exceeds 3 days, the battery bank becomes prohibitively expensive. A $4,000 generator that runs 20 hours/year (during multi-day storms) is cheaper than adding 30 kWh of battery capacity ($18,000). Most off-grid homes use a hybrid approach: 2 days battery + generator backup for extended outages.

Frequently Asked Questions

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Further Reading

Deep-dive articles and guides related to this calculator.