redzilla
All tools

IP Addressing

Layer 2

Cabling

Performance

Wireless

CCTV

Optical fiber

Power

Datacenter

AV

Voice

Diagnostics

Calculation

Security

Power

DC Battery Bank

Enter the load, system voltage (12/24/48 VDC), autonomy and depth of discharge and I compute the required capacity (Ah), the battery array (series for voltage, parallel for capacity) and the C-rate.

W
V
h

How much of the battery you use. Lead-acid 50 %; lithium 80–100 %.

Wiring and DC conversion losses. Typical 90 %.

Usable capacity vs. temperature. Cold uses 0.90; at 20–25 °C 1.00.

Bank battery
V
Ah

Specs of one battery. They go in series for voltage and in parallel for capacity.

Examples
redzilla.cl — bank 
 
  
 
 
Required capacity
 
—
 
at system voltage
 
 
 
Total batteries
 
—
 
series × parallel
 
 
 
Required energy
 
—
 
on the bank
 
 
 
Discharge C-rate
 
—
 
current / capacity
 
 
  
 
 Ah req. —  
 
 Array —  
 
 Batteries —  
 
  
 
Bank array
 
 
 — in series · — in parallel — 
 
 
 
—
 
  
 
Calculation breakdown
 
 
  
ItemValueDetail
   
 
 
 
 How it is computed · Wh, Ah and array 
 
 1. Required energy:
Wh = load_W × autonomy_h. If the load is AC downstream
            of an inverter, divide by the inverter efficiency.

 
 2. Capacity at the system voltage:
Ah = Wh / (Vsys × DoD × efficiency × tempDerate)
(DoD, efficiency and temperature as fractions).

 
 3. Array with batteries of
Vbat / Ah_bat:
series = Vsys / Vbat (for voltage) and
parallel = ceil(Ah_req / Ah_bat) (for capacity).
            The total is series × parallel.

 
 4. Discharge C-rate:
C = (load_W / Vsys) / Ah_installed. A high C-rate
            (> 0.5 C on lead-acid) lowers the real capacity via the Peukert effect.

 
 ! This is a sizing estimate.
            Check the maximum discharge current the battery allows, the balance of
            the parallel strings and the actual operating temperature.

 
 
 
 
 
Runs locally in your browser · no sign-up · nothing leaves your browser
  
 
 
How it works
 
The calculator sizes a DC battery bank by runtime: from the load (W), the system voltage (12/24/48 VDC or other), the backup hours and the depth of discharge (DoD), it derives the required energy Wh = W × hours (divided by the inverter efficiency if the load is AC) and the needed capacity with Ah = Wh ÷ (Vsys × DoD × efficiency × temperature factor). The DoD reflects the chemistry: typically 50 % for lead-acid and 80-100 % for lithium.
With the data of a single battery (voltage and Ah) it builds the series/parallel array: series strings reach the system voltage (series = Vsys ÷ Vbat) and parallel strings reach the capacity (parallel = ceil(Ah_req ÷ Ah_bat)), in 4S1P-style notation. It also reports the discharge C-rate (C = current ÷ installed Ah): above 0.5 C on lead-acid the real capacity drops due to the Peukert effect, and the array bar flags it as a warning.
 
 
Example: backing up 500 W for 4 hours at 48 V with 12 V / 100 Ah batteries
 
     
  1. Required energy: 500 W × 4 h = 2,000 Wh.
  2. Capacity with 50 % DoD and 90 % efficiency: 2,000 ÷ (48 × 0.5 × 0.9) = 92.6 Ah.
  3. Array: 48 ÷ 12 = 4 in series and ceil(92.6 ÷ 100) = 1 string in parallel: 4 batteries (4S1P).
  4. C-rate: (500 ÷ 48) ÷ 100 = 0.10 C, a gentle, safe discharge for lead-acid.
    5.  
 
 
 
 
Frequently asked questions
 
 
 How many batteries do I need to back up 1,000 W for 8 hours? 
At 24 V with 50 % DoD and 90 % efficiency, you need 8,000 Wh and about 741 Ah: with 12 V / 200 Ah batteries that is 2 in series and 4 parallel strings, 8 batteries total. With lithium at 90 % DoD the capacity drops to about 412 Ah, roughly half the units. Enter your exact values in the calculator for the precise array.
 
 What happens if I discharge a lead-acid battery beyond 50 %? 
You drastically shorten its service life: deep cycles degrade the plates, and a battery rated for 1,200 cycles at 50 % DoD may last only a few hundred if routinely discharged to 80-100 %. That is why lead-acid sizing uses 50 % DoD; lithium batteries (LiFePO4) do tolerate 80-100 % DoD without that penalty.
 
 What is the difference between connecting batteries in series and in parallel? 
In series the voltages add and the capacity stays: four 12 V / 100 Ah batteries in series give 48 V / 100 Ah. In parallel the capacities add and the voltage stays: the same four in parallel give 12 V / 400 Ah. Either way the total energy is the same (4,800 Wh); the array is chosen to match the system voltage and the runtime.
 
 Why does the discharge C-rate matter? 
The C-rate is the current relative to the capacity: 0.1 C on a 100 Ah bank is 10 A. In lead-acid, discharging above 0.5 C reduces the effective capacity (Peukert effect) and heats the battery, so the real runtime will be shorter than calculated. If your C-rate comes out high, add parallel strings or raise the system voltage to lower the current.
 
 
 
  
   
 Was this tool useful? 
      
   
 
 
 Disclaimer We take great care to keep every tool accurate and review it thoroughly; even so, we can't guarantee it is free of errors or take responsibility for how the results are used. We recommend double-checking anything critical. 
 Found an error? Let us know →