Calculator Form
Formula Used
Raw required energy: Daily load Wh × autonomy days.
Reserved energy: Raw required energy × (1 + reserve ÷ 100).
Adjusted energy: Reserved energy ÷ inverter efficiency ÷ temperature derating.
Required Ah: Adjusted energy ÷ system voltage ÷ depth of discharge.
Series batteries: Ceiling of system voltage ÷ battery voltage.
Parallel strings: Ceiling of required Ah ÷ single battery Ah.
Total batteries: Series batteries × parallel strings.
Nominal bank energy: Actual bank voltage × total bank Ah.
Usable energy: Nominal energy × discharge limit × efficiency × temperature factor.
How to Use This Calculator
Enter the total watt hours used each day. Add the number of backup days needed. Choose the system voltage used by the inverter or DC bus.
Enter inverter efficiency, discharge limit, battery voltage, and battery amp hours. Add temperature derating and reserve values for practical sizing.
Press Calculate to view required capacity, series count, parallel count, total batteries, usable energy, and runtime estimate. Use CSV or PDF download for records.
Example Data Table
| Example | Daily Load | Autonomy | Voltage | Battery | DOD | Approx Result |
|---|---|---|---|---|---|---|
| Small cabin | 2,000 Wh | 2 days | 24 V | 12 V, 100 Ah | 50% | 8 batteries |
| Telecom backup | 3,500 Wh | 2 days | 48 V | 12 V, 100 Ah | 80% | 12 batteries |
| Workshop backup | 6,000 Wh | 1.5 days | 48 V | 12 V, 200 Ah | 80% | 8 batteries |
Battery Bank Planning Guide
A battery bank is more than a group of cells. It is the reserve that keeps equipment running when charging stops. Good sizing starts with daily watt hours. This value should include lights, pumps, routers, tools, controls, and standby loads. Small hidden loads can drain a bank overnight. Add them early.
Why Autonomy Matters
Autonomy is the number of days the system must run without charging. Off-grid systems often need more autonomy than backup systems. A workshop backup may need only a few hours. A remote site may need several days. Higher autonomy increases required capacity, cost, space, and recharge time.
Efficiency and Discharge Limits
Real systems lose energy. Inverters waste some power as heat. Cables, fuses, and battery chemistry also affect delivery. The calculator adjusts the load for inverter efficiency. It also limits discharge depth. This protects batteries from deep cycling. Lead acid banks usually need shallow discharge. Lithium banks often allow deeper use.
Voltage, Series, and Parallel Strings
System voltage controls current. Higher voltage lowers current for the same power. Lower current can reduce cable losses. Batteries are placed in series to raise voltage. Parallel strings increase amp hour capacity. The calculator rounds both values upward. This makes the design practical. Fractional batteries cannot be installed.
Temperature and Aging Reserve
Cold weather reduces usable capacity. Hot conditions can shorten battery life. Aging also lowers capacity over time. A reserve percentage helps cover these losses. It also supports future loads. Use honest values. Oversizing wastes money. Undersizing causes early shutdowns and frequent cycling.
Practical Load Review
List each appliance with watts and daily hours. Convert motor ratings carefully. Starting surge may exceed running watts. Check inverter surge capacity. Separate essential loads from comfort loads. This reduces bank size. It also makes operation easier. Recheck the list when equipment changes. Battery planning improves when load data stays current.
Design Checks
After sizing, compare the result with charger output. A very large bank needs a strong charger. It also needs proper protection. Use correct fuses, breakers, disconnects, and cable sizes. Follow manufacturer data. For critical systems, have a qualified electrician review the plan. This calculator gives planning estimates. Final designs must match local rules and equipment ratings.
FAQs
What is a battery bank?
A battery bank is a connected group of batteries. It stores energy for backup, off-grid, solar, marine, RV, or industrial electrical systems.
What does autonomy mean?
Autonomy means how long the bank should run loads without charging. It is usually entered in days, but decimals can show partial days.
Why is depth of discharge important?
Depth of discharge limits how much battery energy is used. Lower discharge can extend life, especially with lead acid batteries.
Why are batteries placed in series?
Series wiring increases voltage. Four 12 volt batteries in series create about 48 volts while keeping the same amp hour rating.
Why are batteries placed in parallel?
Parallel wiring increases amp hour capacity. It keeps voltage the same while adding more stored energy for longer runtime.
Should I include reserve capacity?
Yes. Reserve helps cover aging, cold weather, future loads, and real operating losses. Many designs use 10 to 30 percent reserve.
Can this calculator size lithium batteries?
Yes. Enter the lithium battery voltage, amp hours, and allowed discharge percentage. Always follow the battery maker’s data sheet.
Is the result a final electrical design?
No. It is a planning estimate. Final wiring, fusing, breakers, ventilation, and code checks should be reviewed by a qualified professional.