Work out the battery capacity you need from your daily energy use, how many days you want to ride out, and your battery chemistry.
Capacity is the start. The designer turns it into a real bank — series/parallel layout, charge and discharge limits, the matching inverter, solar and every cable — sized to standard.
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Start from the energy you use in a day, multiply by the days of autonomy you want without charging, then divide by the usable depth of discharge for your chemistry — because you can't safely use 100% of a battery. LiFePO4 gives about 80% usable; lead-acid (AGM/gel/flooded) about 50%, so it needs roughly a 60% bigger nameplate for the same usable energy.
Dividing the nominal energy by the system voltage gives amp-hours. Higher system voltages need fewer amp-hours for the same energy, which is why bigger builds move to 24 V or 48 V.
Typical off-grid builds plan 2–4 days to cover cloudy spells without charging. More days means a bigger, costlier bank — solar and a charger usually do the day-to-day topping up.
You can only use part of a battery without shortening its life. LiFePO4 is ~80% usable; lead-acid ~50%. The nameplate capacity has to be bigger than your usable need by that factor.
Small builds stay 12 V; as energy grows, 24 V and 48 V cut the current (and the cable cost) for the same power. The designer recommends the voltage for your loads.
Wattonomy applies these standards in its calculations. It is not certified, sponsored or endorsed by ABYC, ISO, NFPA or Victron. Last reviewed June 2026 — see the methodology.
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