Solar Battery Bank Calculator

Multiply daily watt-hours by days of autonomy, then divide by bank voltage and depth of discharge. A system using 3,000 Wh per day, needing 2 days of backup, on a 48V lithium bank at 80% depth of discharge, requires (3,000 × 2) ÷ (48 × 0.80) = 6,000 ÷ 38.4 = 156 Ah — about 6 kWh of usable storage, or two 100Ah batteries. The calculator above returns amp-hours, usable and total kWh, and the number of 100Ah batteries.

Two numbers drive battery bank size: how much energy you use per day and how many days you must run without sun. Depth of discharge then decides how much of the bank is actually usable — you cannot drain any battery to zero without destroying its lifespan. Building depth of discharge into the math is why a "6 kWh" load needs a bank rated well above 6 kWh of nameplate capacity.

This calculator uses:

Each term controls the result:

• Daily Wh — total daily energy the bank must supply. Sum every load's watt-hours, or take daily kWh × 1,000.
• Days of Autonomy — how many consecutive cloudy days the bank carries the load with no solar input. Weekend users pick 1; full-time off-grid homes pick 2–3, more in cloudy climates.
• 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. 80–100% for LiFePO4 lithium, 50% for lead-acid to protect cycle life.

Note how voltage and amp-hours trade off: the same 6 kWh usable bank is 156Ah at 48V but 625Ah at 12V. Energy (kWh) is what matters; amp-hours only make sense alongside a voltage. To turn amp-hours back into energy, use our amp-hours to watt-hours converter, and to check how long a given bank runs a load, the battery runtime calculator.

Example 1 — Cabin, 3,000 Wh/day, 2 days, 48V lithium, 80% DoD: Ah = (3,000 × 2) ÷ (48 × 0.80) = 6,000 ÷ 38.4 = 156.25 Ah. Usable energy is 6 kWh; total installed energy is 156.25 × 48 = 7.5 kWh. That is two 100Ah 48V modules, or a single 200Ah unit for headroom.

Example 2 — Same load on lead-acid at 50% DoD: Ah = 6,000 ÷ (48 × 0.50) = 6,000 ÷ 24 = 250 Ah. Halving the usable depth nearly doubles the required amp-hours — a clear illustration of why lithium banks are smaller and lighter.

Example 3 — Small 12V system, 2,400 Wh/day, 1 day, 50% DoD: Ah = (2,400 × 1) ÷ (12 × 0.50) = 2,400 ÷ 6 = 400 Ah — four 100Ah 12V batteries in parallel. On 48V lithium at 80% the same load needs only 62.5 Ah.

Example 4 — Off-grid home, 10,000 Wh/day, 3 days, 48V lithium, 90% DoD: Ah = (10,000 × 3) ÷ (48 × 0.90) = 30,000 ÷ 43.2 = 694 Ah, about 33.3 kWh installed — a large residential lithium bank sized for three sunless days.

Days of autonomy is the most consequential choice you make, because it multiplies bank size directly. It answers: if the sun disappears, how long must the batteries carry the house? The right number depends on climate and risk tolerance.

In sunny, reliable climates like the Southwest, 1–2 days is common because clear weather returns quickly and a generator covers rare gaps. In cloudy or stormy regions, 3–5 days is safer. Grid-backup systems can use as little as 0.5–1 day because the grid, not the battery, is the primary source. Every extra day adds proportional cost, so many designers cap autonomy at 2–3 days and add a generator or extra solar rather than an enormous battery.

Depth of discharge is the other lever. Draining a LiFePO4 battery to 80–90% each cycle is fine and still yields thousands of cycles. Draining lead-acid past 50% can slash its life from 500 cycles to under 200. Sizing to a conservative DoD protects the investment. Pair this with the battery bank size calculator for autonomy-focused sizing and the off-grid solar calculator to match panels to the bank.

1. Enter daily energy use in watt-hours. Add every appliance's watt-hours, or convert daily kWh × 1,000.
2. Set days of autonomy. 1 for grid-backup or sunny sites, 2–3 for off-grid, 3–5 for cloudy climates.
3. Choose bank voltage. 12V for small setups, 24V for mid-size, 48V for whole-home efficiency.
4. Set depth of discharge. 80–90% for lithium, 50% for lead-acid.
5. Read the results. Primary is amp-hours; secondary gives usable kWh, total installed kWh, and 100Ah battery count.

The amp-hours and voltage this calculator returns must be assembled from individual batteries, and how you wire them determines whether you gain voltage or capacity. Getting this right is essential to hitting the target bank size safely.

Series wiring (positive to negative) adds voltage while amp-hours stay constant. Four 12V 100Ah batteries in series make a 48V 100Ah bank — 4,800 Wh. Parallel wiring (positive to positive) adds amp-hours while voltage stays constant. Four 12V 100Ah batteries in parallel make a 12V 400Ah bank — also 4,800 Wh. Series-parallel combines both: eight 12V 100Ah batteries as 48V 200Ah delivers 9,600 Wh. The stored energy in watt-hours is identical for a given set of batteries; only the voltage-versus-current split changes.

This is why the calculator lets you pick bank voltage: the same usable kWh becomes far fewer amp-hours at 48V than at 12V, allowing thinner cable and smaller breakers. A practical caution — avoid wiring more than three or four battery strings in parallel, because current sharing becomes uneven and a weak string can be overworked. Manufacturers of drop-in lithium batteries publish maximum parallel counts. For large banks, prefer higher voltage over many parallel strings. To convert any bank's amp-hours back into energy, use the amp-hours to watt-hours converter, and to see how long the finished bank runs a specific load, the battery runtime calculator.

A battery bank's real-world capacity and lifespan depend on factors the nameplate does not show, and understanding them helps you set depth of discharge and autonomy honestly. Temperature is the biggest. Batteries are rated near 25°C (77°F); lead-acid loses 20–35% of usable capacity near freezing, and lithium loses some capacity in the cold and must not be charged below 0°C without an internal heater. In hot climates, sustained heat accelerates aging and self-discharge. If your bank lives in an uninsulated shed or a vehicle, plan for reduced winter capacity or add insulation and heating.

Charge rate matters too. A battery can only absorb charge so fast, expressed as a C-rate: 0.5C means a 100Ah battery accepts up to 50A. If your solar array can deliver more current than the bank can absorb, the charge controller limits it and you lose midday harvest. Sizing the array's charging current between roughly 0.1C and 0.5C of the bank keeps charging efficient without stressing the cells.

Finally, depth of discharge trades directly against cycle life. A LiFePO4 bank cycled to 80% might last 3,000–6,000 cycles; the same bank cycled shallower lasts even longer. Lead-acid cycled to 50% may reach 500 cycles but collapses to under 200 if routinely drained deeper. Sizing generously and discharging shallowly is the cheapest way to extend bank life. Pair this with the off-grid solar calculator to match the array to the bank you size here.

• Sizing to usable energy but buying nameplate. A 6 kWh daily need at 80% DoD requires 7.5 kWh installed, not 6 kWh.
• Draining lead-acid past 50%. It technically works but destroys cycle life; always size lead-acid at 50% DoD.
• Forgetting round-trip and inverter losses. Add 10–20% headroom, or lower DoD, so real runtime matches the plan.
• Mixing chemistries or ages. A weak or mismatched battery drags down the entire parallel bank.
• Choosing too many autonomy days. Every extra day adds cost; a generator is often cheaper than a fifth day of battery.

RV and van. Typical banks run 100–400Ah at 12V (1.2–5 kWh). One or two days of autonomy is plenty because you move to sun and can charge from the alternator or shore power.

Off-grid cabin. Weekend cabins often use 200–400Ah at 12V or 24V; full-time cabins move to 48V and 400–800Ah (10–20 kWh) with 2–3 days of autonomy.

Whole-home off-grid. These systems almost always use 48V for efficiency and safety, with banks of 20–40 kWh and 2–3 days of autonomy, backed by a generator for the darkest stretches.

Whatever the scale, size the bank from real daily consumption and a realistic autonomy target, then confirm the solar array can recharge it with the solar panel wattage calculator and protect it with a correctly sized charge controller. One more planning tip: buy the bank you will need in two or three years, not just today. Batteries are easiest to match when purchased together as a single set, because paralleling a brand-new module with a bank that has already aged and lost capacity drags the whole bank down to the weaker cells. Sizing with a little growth headroom now avoids an awkward and inefficient mixed-age expansion later.