Battery Bank Size Calculator

To size a battery bank, multiply your daily watt-hours by the days of autonomy you want, then divide by the bank voltage and usable depth of discharge: Ah = (Daily Wh × Days) ÷ (V × DoD). For a cabin using 2,000 Wh per day, wanting 2 days of backup on a 48V lithium bank at 80% depth of discharge: Ah = (2,000 × 2) ÷ (48 × 0.80) = 104 Ah. That rounds up to two 100Ah 48V batteries, or a single 105Ah unit.

Two decisions dominate the result: how many days you must ride through with no charging (autonomy), and how deeply you can safely discharge (chemistry). Lithium's 80–100% usable depth roughly halves the amp-hours you must buy compared with lead-acid at 50%. The calculator above builds both in and also estimates how many 100Ah batteries the bank requires.

The full sizing equation is:

Work through it term by term:

• Daily watt-hours — your total 24-hour energy consumption. Sum every load: fridge, lights, pumps, electronics. If you only know appliance watts and hours, multiply them (a 60W fridge averaging 12 hours of compressor time ≈ 720 Wh/day).
• Days of autonomy — how long the bank must run with no solar or generator input. Off-grid homes typically use 2–5 days to survive cloudy stretches; grid-backup systems often use 1 day.
• Bank voltage — 12V for small systems, 24V for mid-size, 48V for whole-home. Higher voltage means fewer amp-hours for the same energy and thinner cable.
• Depth of discharge (DoD) — the usable fraction. Use 0.80 (80%) for LiFePO4 lithium and 0.50 (50%) for lead-acid to protect cycle life.

The numerator is total energy you must store; the denominator converts that energy into rated amp-hours after reserving headroom for depth of discharge. This is the storage counterpart to the solar battery bank calculator, which also factors in solar array size and charge controllers.

Example 1 — Small off-grid cabin (48V lithium): Daily use 2,000 Wh, 2 days autonomy, 80% DoD. Required Ah = (2,000 × 2) ÷ (48 × 0.80) = 4,000 ÷ 38.4 = 104 Ah. Round up to two 100Ah 48V batteries. Total rated energy is 104 × 48 ≈ 5,000 Wh.

Example 2 — Same cabin on 24V lead-acid, 3 days: Daily use 3,000 Wh, 3 days, 50% DoD. Required Ah = (3,000 × 3) ÷ (24 × 0.50) = 9,000 ÷ 12 = 750 Ah. That is a large flooded bank — for example six 6V 250Ah golf-cart batteries in a series-parallel 24V arrangement. Notice how lead-acid at 50% DoD roughly doubles the amp-hours versus lithium.

Example 3 — RV weekend (12V lithium): Daily use 1,000 Wh, 1 day, 80% DoD. Required Ah = (1,000 × 1) ÷ (12 × 0.80) = 1,000 ÷ 9.6 = 104 Ah. A single 100Ah 12V lithium battery is right at the edge; a 120Ah unit gives comfortable margin.

Example 4 — Whole-home backup (48V lithium, 1 day): Daily use 10,000 Wh, 1 day, 80% DoD. Required Ah = (10,000 × 1) ÷ (48 × 0.80) = 10,000 ÷ 38.4 = 260 Ah, or about three 100Ah 48V batteries (roughly 15 kWh rated). Always round up and add margin for aging and cold.

Autonomy is the number of no-charge days the bank must cover, and it is the single biggest cost lever in an off-grid design. Each extra day multiplies battery cost. The right figure depends on your climate and how critical the loads are: a sunny desert site might run 2 days of autonomy, while a cloudy northern site running medical equipment might need 5.

Depth of discharge is the second lever. Draining a battery deeper stores more usable energy per dollar but shortens its life. Lead-acid punishes deep cycling severely — routine discharges below 50% can cut cycle life from 500 cycles to under 200. LiFePO4 is built for 80–100% cycling with minimal wear, which is the main reason it dominates modern off-grid banks despite a higher upfront price. When you compare chemistries, compare usable watt-hours per dollar over the battery's cycle life, not sticker price. To turn a target autonomy into a full system with panels, use the off-grid solar calculator.

The sizing formula is only as good as its daily watt-hour input, so it is worth measuring rather than guessing. There are three reliable methods.

1. From your utility bill. Divide your monthly kilowatt-hours by 30 and multiply by 1,000. A 900 kWh month is 900 ÷ 30 = 30 kWh per day, or 30,000 Wh — though off-grid designs usually target only the essential loads, not the whole bill.

2. From an appliance audit. List each device, its wattage, and hours of use per day, then multiply and sum. A 60W fridge running a 40% compressor duty cycle averages 60 × 24 × 0.40 = 576 Wh/day; ten 10W LED bulbs for 5 hours add 500 Wh; a laptop at 45W for 4 hours adds 180 Wh. Add inverter idle draw (often 20–40W continuous) and phantom loads.

3. With a meter. A plug-in energy monitor or a DC shunt on the battery gives the most accurate figure by logging real consumption over a few days. This captures duty cycles and surges that spec-sheet math misses.

Whichever method you use, feed the daily total into the calculator above. If you only know appliance amps, convert them first with the amps to watts calculator, then multiply by run hours.

A bank sized perfectly on paper can still disappoint in the field because two factors quietly shrink capacity: cold and age. Plan for both from the start.

Temperature. Battery capacity is rated at about 25°C (77°F). Lead-acid can lose 20–35% of usable capacity near freezing, and even lithium loses some capacity in the cold and must not be charged below 0°C without a heater. If your bank lives in an unheated shed or a winter cabin, add 20–30% capacity to compensate, or insulate and heat the enclosure.

Age and cycles. Every battery fades. Quality LiFePO4 retains about 80% of its capacity after 3,000–6,000 cycles; lead-acid may reach 80% after only 300–500 cycles. A bank sized exactly for today's needs will fall short in a few years, so many designers oversize by 10–20% or plan to add capacity later. Because mismatched batteries drag a bank down to the weakest unit, expansion is easiest when you buy identical batteries and reserve space and cabling for future strings. Confirm the finished, derated bank against your peak load with the battery runtime calculator.

A bigger bank always rides through more cloudy days, but every extra amp-hour costs money, space, and weight — so the real design question is value, not maximum size. The honest metric is dollars per usable kilowatt-hour over the battery's cycle life, not the sticker price of a single battery.

Compare an example 5 kWh of storage. A lead-acid bank limited to 50% depth of discharge must be rated about 10 kWh to deliver it, and may last only 400–500 cycles before fading to 80%. A LiFePO4 bank cycling at 80% needs roughly 6.25 kWh rated and commonly lasts 3,000–6,000 cycles. Even though lithium costs more per rated kWh up front, its usable energy and long life often make it cheaper per delivered kilowatt-hour by the end of its service. This is why the depth-of-discharge input in the calculator above is not a minor tuning knob — it swings both the amp-hours you buy and the lifetime cost of every kilowatt-hour the bank ever delivers. Size the bank on usable energy, then divide total cost by usable kWh times expected cycles to compare options fairly.

1. Enter daily energy use in watt-hours. Total everything that runs in 24 hours. If you have a utility bill, divide monthly kWh by 30 and multiply by 1,000 to get daily Wh.
2. Set days of autonomy. Use 1 for grid backup, 2–3 for sunny off-grid, 3–5 for cloudy climates or critical loads.
3. Choose bank voltage. 12V for small/RV, 24V for mid-size, 48V for whole-home. Higher voltage cuts current and cable cost.
4. Set depth of discharge. 80% for lithium, 50% for lead-acid.
5. Read the required amp-hours and the estimated number of 100Ah batteries. Round up and add 10–20% margin for aging, cold weather, and future loads.

Once you know the required amp-hours, plan the wiring with the series and parallel battery calculator and check how long the finished bank lasts under a specific load with the battery runtime calculator.

• Sizing to rated capacity, not usable. Forgetting depth of discharge undersizes the bank by 20–50%.
• Underestimating daily loads. Phantom loads, inverter idle draw, and refrigeration duty cycles add up — measure with a meter rather than guessing.
• Too little autonomy. One cloudy stretch can leave a 1-day bank flat; most off-grid failures trace to thin autonomy.
• No margin for cold or aging. Capacity drops in the cold and fades with cycles; add 10–20% headroom.
• Mismatched batteries. Mixing chemistries, ages, or capacities in one bank drags performance down to the weakest unit.

A quick reality check: after sizing, run the runtime calculator at your peak load to confirm the bank survives a worst-case evening.

Off-grid homes. The bank is the heart of the system. Size it from a careful energy audit, choose 48V for whole-home loads, and pair it with enough solar to recharge fully on an average day. The solar battery bank calculator ties storage to array size.

Grid-tie with backup. Here you typically need only 1 day of autonomy for essential circuits, because the grid does the heavy lifting. Size the bank to carry the fridge, lights, and internet through a typical outage.

RV and marine. Space and weight are tight, so lithium's higher usable depth and energy density win. A 100–300Ah 12V lithium bank covers most weekend-to-week-long trips; add solar to extend indefinitely.

Telecom and remote monitoring. These sites demand high autonomy (often 3–5 days) because service visits are costly. Oversize the bank and derate for temperature extremes. Whatever the application, finish by converting the rated amp-hours to energy with the Ah to kWh calculator so you can compare quotes on an apples-to-apples kWh basis.