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Off-Grid Battery Bank Size Calculator

Work out how much battery capacity (in Ah) an off-grid system needs to cover your daily energy use through a set number of days with no charging, based on your battery chemistry and system voltage.

Battery capacity needed312 Ah

How to measure your inputs

  • Daily load: the same watt-hour figure used for solar or wind array sizing.
  • Autonomy days: how many consecutive low-generation days you want covered without needing to ration power.

Methodology

Standard off-grid formula: capacity (Ah) = (daily load Wh × autonomy days) ÷ (depth of discharge × system voltage). Depth-of-discharge presets reflect commonly cited safe-cycling limits by chemistry, not a fixed universal assumption.

Sizing a battery bank that actually gets you through the night

Why depth of discharge changes the answer so much

A lead-acid battery’s usable capacity is roughly half its rated capacity — discharging it further shortens its lifespan dramatically. LiFePO4 (lithium iron phosphate) batteries tolerate much deeper discharge (80%+) without the same degradation, which is why two batteries with the same rated capacity can need very different bank sizes to deliver the same usable energy.

Autonomy days: the number worth thinking carefully about

This is how many consecutive days with little or no charging (cloudy, windless) the battery bank alone needs to cover. 1-2 days is common for grid-tied backup systems; fully off-grid systems in variable climates often plan for 3+ days to avoid running out of power during a real bad-weather stretch.

From Ah to real battery units

Once you have the required Ah capacity at your system voltage, check it against how batteries are actually sold (individual Ah ratings, often needing several wired together to reach both your voltage and capacity targets) to work out the real battery count and wiring configuration.

Frequently asked questions

What system voltage should I choose — 12V, 24V, or 48V?

Higher voltage systems (24V, 48V) carry the same power at lower current, meaning thinner, cheaper cables and less resistive loss — generally preferred for larger systems. 12V is common for small setups (a single cabin, a small RV) where simplicity matters more than efficiency at scale.

Why does battery chemistry matter for sizing?

Chemistry determines safe depth of discharge, which directly changes how much rated capacity you need to buy to get the usable capacity you actually want — a lead-acid bank needs roughly double the rated Ah of an equivalent LiFePO4 bank for the same usable energy.

How many autonomy days should I plan for?

It depends on your climate’s worst realistic weather stretch and how critical continuous power is — 1-2 days is common where backup generation exists as a fallback; 3+ days is more typical for fully off-grid systems with no other power source.

Does this account for battery aging over time?

No — this gives the capacity needed with a new, healthy battery bank. Capacity degrades gradually with age and cycle count, which is a separate factor to budget for in a system’s expected lifespan, not something a single sizing calculation accounts for.