Solar battery bank: How to size and build one

Updated 2026-10-25

A solar battery bank stores the energy your panels make during the day, so you can use it at night, on cloudy days and when you need more power than the panels are producing at that moment. In a DIY solar system, it sits between the charge controller and the inverter. To size one, use this formula: battery capacity (Wh) = daily use (Wh) × days of autonomy ÷ usable depth of discharge.

This guide explains each part of that formula, how to choose between lithium and lead-acid batteries, which system voltage to use, and how to wire batteries together safely. A worked example and answers to common questions come at the end.

The sizing formula

Two lines size a battery bank:

• Battery capacity (Wh) = daily use (Wh) × days of autonomy ÷ usable depth of discharge
• Battery capacity (Ah) = battery capacity (Wh) ÷ system voltage (V)

What each term means:

• Daily use is the energy you use in a day, in watt-hours (Wh). Step 1 shows how to find it.
• Days of autonomy is how many days the bank should keep you running with little or no sun (Step 2).
• Usable depth of discharge is how much of a battery’s rated capacity you can use regularly without shortening its life, written as a decimal. It depends on the battery type (Step 3).
• System voltage is the voltage of the whole battery bank, usually 12 V, 24 V or 48 V (Step 4). Batteries are often sold by amp-hours (Ah), so the second line converts watt-hours to amp-hours at your system voltage.

Step 1: Find your daily use

List every device the battery bank will power, then multiply each device’s watts by the hours a day it runs and add up the results. For appliances that cycle on and off, like fridges, use the average daily draw instead of the peak. Add 10 to 20% for things you forgot.

Our guide to planning and building a DIY solar system has a full load list example. It comes to about 2,000 Wh a day for lights, a laptop, an efficient fridge and a water pump, the figure used in the worked example below.

Step 2: Choose days of autonomy

Days of autonomy decide how long the bank can carry you through bad weather before the batteries run low:

• 1 day: enough for a weekend cabin or a backup circuit that’s used mainly in good weather, or for a system with a generator to fall back on.
• 2 to 3 days: a common choice for regular off-grid use in most climates.
• 3 days or more: for full-time off-grid living, especially in places with long cloudy spells or short winter days.

Each extra day of autonomy adds a full day’s use to the bank, so this one choice can double or triple its size. A backup generator for the occasional long cloudy stretch is often cheaper than a battery bank big enough to cover it.

The panels also have to recharge the bank after a cloudy spell. If your array only just covers your daily use, the batteries will be slow to recover, so consider extra panel capacity alongside extra days of autonomy. Our guide to how much power a solar panel produces shows how to estimate your array’s daily output.

Step 3: Pick a battery chemistry

Most DIY battery banks use one of three battery types, and they differ in how much of their capacity you can use, how long they last and how much care they need.

Cycle life depends heavily on how deeply and how often a battery is discharged, and on its temperature, so check the datasheet for the battery you’re considering.

LiFePO4 is the usual choice for new DIY banks. It costs more upfront, but you can use most of its capacity, it lasts many more cycles, weighs much less and needs no maintenance. Its main limit is charging in freezing weather: most LiFePO4 batteries mustn’t be charged below 0°C (32°F), so a cold-climate bank needs a heated battery, a heated space, or a battery management system (BMS) or charge controller that blocks charging in the cold.

AGM batteries are sealed and maintenance-free. They suit a small bank on a tight budget, or a site that’s too cold for unheated lithium batteries.

Flooded lead-acid batteries are the cheapest to buy and can last a long time if they’re well looked after. They need regular watering and a ventilated space, because they give off hydrogen gas while charging.

Lead-acid batteries can only use about half their rated capacity, so a lead-acid bank needs roughly twice the rated capacity of a lithium bank to store the same usable energy. That narrows the price gap.

Step 4: Choose 12 V, 24 V or 48 V

The system voltage is the voltage of the whole bank, and the charge controller and inverter must match it. A higher voltage carries the same power at a lower current, which means thinner, cheaper cables and less energy lost in them:

• 12 V: suits small systems up to about 1,000 W of inverter capacity, and vans, boats and RVs that already run on 12 V.
• 24 V: a good middle ground for cabins and mid-sized systems up to about 3,000 W.
• 48 V: the usual choice for larger systems and whole-home off-grid builds.

For example, a 3,000 W load draws about 250 A from a 12 V bank but only about 63 A from a 48 V bank. Choose the voltage together with your inverter, since its DC input voltage has to match the bank.

Wiring batteries in series and parallel

Unless you buy a single battery at your full system voltage and capacity, you’ll connect several batteries together:

• Series connections (positive of one battery to negative of the next) add up the voltages while the amp-hours stay the same. Two 12 V 100 Ah batteries in series make a 24 V 100 Ah bank.
• Parallel connections (positive to positive, negative to negative) add up the amp-hours while the voltage stays the same. The same two batteries in parallel make a 12 V 200 Ah bank.
• Series-parallel combines both. Four 12 V 100 Ah batteries wired as two series pairs, with the pairs then connected in parallel, make a 24 V 200 Ah bank.

A few rules keep a bank healthy:

• Match the batteries. Use the same model, capacity and age throughout. Never mix chemistries, and avoid adding new batteries to an old bank, because the weakest battery drags down the rest.
• Balance the cables. In a parallel bank, use cables of equal length and size. Take the main positive and negative connections from opposite ends of the bank, or use busbars, so every battery charges and discharges evenly.
• Check the manufacturer’s limits. Many lithium batteries with a built-in BMS allow only a set number in series or in parallel. Check the manual before you plan the bank.
• Fuse the bank. Put a fuse or breaker on the main positive cable as close to the battery bank as possible.

Worked example

Here’s a hypothetical cabin system, with round numbers for illustration:

Capacity: 2,000 Wh × 2 ÷ 0.9 ≈ 4,400 Wh.

Amp-hours at 24 V: 4,400 Wh ÷ 24 V ≈ 185 Ah.

Building it: four 12 V 100 Ah LiFePO4 batteries, wired as two series pairs connected in parallel, make a 24 V 200 Ah bank. That stores 4,800 Wh, a little above the target.

With lead-acid instead: at 50% usable depth of discharge, the same need comes to 2,000 × 2 ÷ 0.5 = 8,000 Wh, or about 333 Ah at 24 V. That’s nearly twice the rated capacity, and far more weight.

With this bank in place, the next part to choose is the solar charge controller that will charge it.

Battery safety

A battery bank holds a lot of energy, and a short circuit can release it in an instant. Follow these rules:

• Fuse close to the battery. Choose a fuse or breaker rated for the cable and for the current the battery can deliver in a short circuit. Large lithium banks often need a fuse with a high interrupt rating, such as a class T fuse.
• Use a BMS with lithium batteries. Most LiFePO4 batteries have one built in. It protects the cells from overcharging, overdischarging, overcurrent and charging in the cold.
• Ventilate lead-acid batteries. Flooded batteries give off hydrogen gas while charging, so keep them in a ventilated space away from sparks and flames.
• Respect temperature limits. Keep batteries within their rated temperature range, and out of direct sun and freezing spaces where possible.
• Work safely. Use insulated tools, remove rings and watches, and cover the terminals.

Whole-home batteries installed with grid-tied systems are a different product: sealed units with their own inverter, usually fitted by an installer. This guide covers banks you build yourself.

Frequently asked questions

How many batteries do I need for solar?

Size the bank in watt-hours first, using the formula above, then divide by the capacity of the battery you plan to buy. For example, a 4,400 Wh bank built from 12 V 100 Ah batteries (1,200 Wh each) needs four batteries, wired to reach your system voltage.

How long will a solar battery bank last?

There are two answers. How long it runs between charges depends on its usable capacity and your loads: a bank with 4,400 Wh of usable energy runs a 200 W load for about 22 hours. How many years it lasts depends on its chemistry and how hard it’s used. LiFePO4 batteries are rated for thousands of cycles, while lead-acid batteries are rated for hundreds to about 1,500.

Can I mix old and new batteries in a bank?

It’s best not to. A new battery connected to old ones gets dragged down to their level and wears out sooner. If you need more capacity later, add a complete second bank, or replace the whole bank at once.

Is lithium or lead-acid better for solar?

For most new DIY systems, LiFePO4 lithium is the better choice. It lasts longer, lets you use more of its capacity, weighs less and needs no maintenance. Lead-acid can still make sense for a small bank on a tight budget, or where the batteries will be charged below freezing without heating.