Battery bank size calculator

Enter how much energy you use per day, how many days the batteries should last without charging, and the batteries you plan to use. The calculator works out the bank capacity, the amp-hours at your system voltage, and how many batteries to wire in series and parallel.

Don't know your daily watt-hours yet? List your devices in the power station size calculator. It adds them up, and the number works here too.

How the calculation works

Depth of discharge is how much of the rated capacity you plan to use before recharging. Lead-acid batteries wear out much faster when they're regularly run below about half charge, so 50% is the usual planning figure. LiFePO4 batteries handle deep cycles far better. Planning on 80% leaves a reserve for cloudy days and cold weather, and keeps the battery away from its low-voltage cut-off. Your battery maker's datasheet shows how cycle life changes with depth of discharge.

Efficiency covers the energy lost between the battery and your devices. An inverter turning 12 V DC into 120 V AC typically loses 10–15% at moderate loads, and more when it's lightly loaded. DC loads on a fuse block lose only a little in the cables. If you don't know, the 85% default is a fair, slightly cautious figure.

Batteries in series add their voltages: two 12.8 V batteries make a 25.6 V (nominal "24 V") string. Strings in parallel add their amp-hours. Our series vs. parallel guide explains the wiring and the rules for mixing batteries.

Worked examples

A 12 V lithium bank for 1,200 Wh a day, two days of backup

1,200 × 2 = 2,400 Wh delivered. Divide by 80% usable and 85% inverter efficiency: 2,400 ÷ 0.8 ÷ 0.85 ≈ 3,529 Wh. At 12.8 V that's 3,529 ÷ 12.8 ≈ 276 Ah. With 100 Ah batteries, round up to three in parallel: 300 Ah, or 3,840 Wh. That bank actually covers about 3,840 × 0.8 × 0.85 ÷ 1,200 ≈ 2.2 days.

The same loads on lead-acid

At 50% usable: 2,400 ÷ 0.5 ÷ 0.85 ≈ 5,647 Wh, or 5,647 ÷ 12 ≈ 471 Ah at 12 V. That takes five 100 Ah batteries in parallel instead of three. Lead-acid is cheaper per rated amp-hour, but you need more of them, and they're much heavier.

Moving to 24 V

The same 3,529 Wh at 24 V uses two 12.8 V batteries in series per string (25.6 V). 3,529 ÷ 25.6 ≈ 138 Ah, which needs two strings of 100 Ah: four batteries, 5,120 Wh in total. Here the rounding works against you, because batteries are added in pairs. Larger batteries, such as 150 Ah, or a single 24 V battery may fit better. Higher voltage still pays off on current: a 2,000 W load draws about 2,000 ÷ (25.6 × 0.85) ≈ 92 A from a 24 V bank, compared with about 184 A at 12.8 V. That means thinner cables and less strain on each battery.

Check the current, not just the capacity

Each lithium battery has a battery management system (BMS) that shuts it off above a set current, usually listed as "max continuous discharge" on the spec sheet. With parallel strings, the current is shared, roughly evenly if the cables are equal lengths. Enter your biggest load and the battery's limit above to check it. A 1,500 W microwave on a 12 V bank pulls around 1,500 ÷ (12.8 × 0.85) ≈ 138 A, which is more than many 100 Ah batteries are rated for on their own. Motors can also briefly draw several times their running watts as they start. The running vs. surge watts guide explains why.

Other things that change the size

• Cold. Lead-acid batteries deliver noticeably less capacity when they're cold. Most LiFePO4 batteries must not be charged below freezing unless they have built-in heating. If the bank lives outdoors in winter, add margin and check the datasheet's temperature limits.
• Aging. All batteries lose capacity over the years. Sizing 10–20% above the minimum keeps the system working as the bank ages.
• Charging. Days of backup only help if you can refill the bank afterwards. Check that your solar array can recharge it in a reasonable time with the solar charging time calculator.
• Matching batteries. Batteries in one bank should be the same model, capacity and age. Adding a new battery to old ones usually leaves the new one doing most of the work.

Frequently asked questions

How many batteries do I need for 1,000 Wh a day?

For one day on 12 V LiFePO4 at 80% usable with an inverter: 1,000 ÷ 0.8 ÷ 0.85 ≈ 1,471 Wh, or about 115 Ah at 12.8 V. That's two 100 Ah batteries, or a single battery of 120 Ah or more. For two days, double it. Lead-acid needs about 2,353 Wh (196 Ah at 12 V) for the same single day.

Is one large battery better than several small ones?

Fewer, larger batteries mean fewer connections and no current-sharing problems, so they're simpler. Several smaller batteries are easier to lift, and you may be able to keep running if one fails. Either works if the wiring and fusing are done properly.

Should I size for days of backup or just one night?

It depends on how you charge. With a generator or shore power you can recharge any time, so one day plus a reserve may be enough. With solar only, two or three days of backup covers runs of cloudy weather. More days means a much bigger and more expensive bank, so adding solar panels is often cheaper than adding batteries.

Does this work for a portable power station?

A power station is a battery bank with the inverter built in, so the daily use and efficiency parts apply. Skip the battery count and use the power station size calculator instead, which also checks the inverter's continuous and surge watts.