🪫 Battery Bank Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

How much battery capacity an off-grid or backup solar system needs to ride out days without sun.

A battery bank's job is to store energy produced during sunny hours for use when the sun isn't producing — overnight at minimum, and through cloudy stretches for a system meant to run fully off-grid. Sizing it starts from the same daily energy need used to size the array, multiplied by how many days of backup ("autonomy") the system needs to survive with no solar input at all.

Batteries can't be run all the way to empty without damaging them or drastically shortening their lifespan — depth of discharge (DoD) is the fraction of a battery's total capacity that's actually safe to use. A battery with a 50% DoD limit needs twice its usable capacity in total rated capacity, since half of it is reserved as a buffer never meant to be drawn down.

Key formula

Energy needed = daily energy need × autonomy days
Usable capacity (Ah) = energy needed (Wh) ÷ system voltage
Required rated capacity = usable capacity ÷ depth of discharge

Variables

DoD

depth of discharge — the fraction of rated capacity actually safe to draw down

How to use the Battery Bank Sizing calculator

Use this to find how much battery capacity, in amp-hours at your system voltage, it takes to carry a daily energy need through a chosen number of days with no sun or no grid. It is the second step after the array in an off-grid or backup design, and the one that most controls the cost.

The key idea is that you cannot use all of a battery: discharging deeply shortens its life, so the calculator divides by the usable fraction, the depth of discharge. That is why the chemistry you choose changes the bank size so much.

Daily Energy Need (kWh)

The energy the loads use per day in kilowatt-hours, measured at the battery, including inverter losses if the loads run through one.

Days of Autonomy (days)

How many days the loads must run with no charging at all. One to two days is common with a generator or a good solar day to recover; three to five for critical loads with no backup.

Max Depth of Discharge (%)

The fraction of the battery you are willing to use each cycle. Lead-acid batteries last longest if kept near 50 percent; lithium iron phosphate batteries are routinely used to 80 percent or more.

System Voltage (V)

The nominal battery bank voltage: 12, 24 or 48 V. Higher voltage means lower current for the same power, which means thinner wire and smaller fuses, so bigger systems use 48 V.

Worked example: a cabin bank for two days

The same cabin uses 6 kWh per day and should ride through 2 days without charging. Lithium iron phosphate batteries will be used down to 80% depth of discharge on a 24 V bank. How much capacity?

Worked by hand:

1. Energy to store. 6 kWh × 2 days = 12,000 Wh (that is 6 × 2 × 1,000).
2. Convert to usable amp-hours. Amp-hours = watt-hours ÷ volts = 12,000 ÷ 24 = 500 Ah.
3. Allow for depth of discharge. Only 80% of the battery may be used, so divide: 500 ÷ 80% = 625 Ah.
4. Check in kilowatt-hours. 625 Ah × 24 V = 15.0 kWh of nominal storage.

The cabin needs a 625 Ah, 24 V lithium bank, which is 15 kWh nominal. Built from 12 V, 100 Ah batteries, that is two in series for the voltage in each of 7 parallel strings, 14 batteries in all. With lead-acid limited to 50 percent depth of discharge the same duty would need 1,000 Ah, and the same energy on a 48 V bank would need only 312.5 Ah.

Reading the result: what the number leaves out

The result is the capacity that must be installed to deliver the energy at the stated depth of discharge. Real banks lose a little on top of that, and treating the figure as a floor rather than a target is wise. Adding about ten percent, 688 Ah here, covers inverter losses, the round-trip efficiency of charging and an ageing battery.

• Cold weather cuts usable capacity, sharply for lead-acid, and many lithium batteries will not accept a charge below freezing without a heater, so a bank in an unheated shed needs a different design.
• Capacity is not the same as power. A bank must also deliver the peak current of the biggest load, and each battery has a maximum continuous discharge rate.
• Batteries in a bank should match in chemistry, age and capacity. Mixing them makes the weakest one set the limit, and parallel strings should be limited to what the manufacturer allows.
• Voltage is a design choice made once: a larger system on a higher voltage needs smaller cable, so bigger inverters generally call for 24 or 48 V.

Notes & limitations

• Depth of discharge varies significantly by battery chemistry — lead-acid batteries are often limited to 50% DoD for reasonable lifespan, while lithium (LiFePO4) batteries commonly tolerate 80–100% DoD, which is a major reason lithium banks can be smaller and lighter for the same usable capacity.
• System voltage is a real design choice, not just a number to plug in — higher-voltage battery banks (48V vs. 12V, for the same power) carry proportionally less current, which means thinner, less expensive cabling for the same power delivered.

Common mistakes

• Forgetting depth of discharge. Sizing to 100 percent of the nominal capacity over-discharges the bank, and it fails years early.
• Comparing amp-hours across different voltages. A 100 Ah battery at 12 V holds a quarter of the energy of a 100 Ah battery at 48 V; energy is amp-hours times volts.
• Basing autonomy on the average day. The energy you need on a dark, cold, high-use day is what decides whether the system survives.
• Ignoring inverter losses and standby draw, which can amount to a tenth or more of the stored energy.
• Mixing old and new batteries, or different chemistries, in one bank.

Frequently asked questions

Papers worth reading

A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries Doerffel, D. & Sharkh, S. A. (2006), Journal of Power Sources. A critical look at the Peukert equation, the rule behind “a battery gives less capacity at high discharge rates”, and where it stops being reliable.