Battery Bank Calculator
This battery bank calculator sizes a battery bank for off-grid solar, backup power, RVs, and boats. Enter the daily energy use, the days of autonomy, the system voltage, the depth of discharge, and the inverter efficiency, and the battery bank calculator returns the bank capacity in amp hours, the stored and usable kWh, and the number of batteries in series and parallel.
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The Battery Bank Calculator is part of our Electronics collection. Browse more electronics to size, convert, and check your work alongside this tool.
What the battery bank calculator does
A battery bank must store a day or more of energy and still not be drained so deep that it wears out. The battery bank calculator multiplies the daily energy by the days you want to cover, divides by the share of capacity you allow yourself to use and by the system efficiency, and converts the result to amp hours at your system voltage. With a battery model entered, it shows how many to wire in series and in parallel.
Battery bank formula
The calculator sizes the bank, then the string layout:
η is the efficiency of the inverter and wiring. Stored energy is Ah × V; usable energy is that times the depth of discharge.
Battery bank calculator example
A cabin using 3,000 Wh per day, 2 days of autonomy, a 48 V lithium bank at 80% depth of discharge, and 90% efficiency:
That is 8.33 kWh stored and 6.67 kWh usable. With 12 V, 100 Ah batteries you need 8 batteries: 4 in series for 48 V, in 2 parallel strings.
How to use the battery bank calculator
Add up the daily energy use in watt hours: each load in watts times its hours per day. Enter the days of autonomy, usually 1 to 3, and the system voltage, 12, 24, or 48 V. Set the depth of discharge, about 50% for lead acid and 80 to 90% for lithium iron phosphate, and the inverter and wiring efficiency, about 85 to 95%. Add one battery voltage and capacity to get the battery count.
Reading the battery bank sizing
The capacity is the minimum at your system voltage; round up to available batteries. More days of autonomy carry you through cloudy weather but cost more and need more solar to recharge. Lead acid banks sized at 50% depth of discharge need twice the capacity of the same bank at 100%, which is why lithium banks are often smaller and lighter for the same usable energy. Cold reduces usable capacity, so oversize for winter use.
When to use a solar battery bank calculator
- Sizing an off-grid cabin or tiny house bank
- Planning backup power for outages
- Choosing RV or van batteries
- Comparing lead acid and lithium bank sizes
- Working out batteries in series and parallel
Common battery bank calculator mistakes
- Using full capacity. Respect the depth of discharge for the chemistry.
- Ignoring inverter losses. Include efficiency.
- Mixing old and new batteries. Use matched batteries.
- Sizing for summer only. Size for the worst month.
Tips for battery bank sizing
- Cut loads first; efficient appliances shrink the bank.
- Pick 48 V for systems over about 3 kW.
- Leave room in the layout to add capacity later.
Battery bank sizing for solar and off-grid
Start with a load list: each appliance, its watts, and its hours a day. A fridge may draw 100 W but run only a third of the time, so 800 Wh a day, while LED lighting may add 300 Wh and a laptop 200 Wh. Seasonal loads such as heating fans change the total, so size for the worst month. The bank then needs a solar array large enough to refill it in a normal day plus the daily use. Convert any battery rating between Ah and Wh with the amp hour calculator, and check how long a single load will run with the battery life calculator.
Battery bank formula for series and parallel
Series wiring adds voltage at the same capacity; parallel wiring adds capacity at the same voltage. Use identical batteries of the same age in a bank, keep parallel strings to a small number, and wire them so each string sees the same cable length, which keeps the current balanced. Lithium banks need a battery management system, and every bank needs fusing sized for the cable. To compare the stored energy with grid power, the energy cost calculator prices a kWh.
Battery bank sizing chart
Bank capacity for one day of autonomy at 90% efficiency:
| Daily use | 48 V lithium (80% DoD) | 12 V lead acid (50% DoD) |
|---|---|---|
| 1,000 Wh | 28.9 Ah | 185.2 Ah |
| 2,000 Wh | 57.9 Ah | 370.4 Ah |
| 3,000 Wh | 86.8 Ah | 555.6 Ah |
| 5,000 Wh | 144.7 Ah | 925.9 Ah |
| 10,000 Wh | 289.4 Ah | 1,851.9 Ah |
| 20,000 Wh | 578.7 Ah | 3,703.7 Ah |
Battery Bank Calculator FAQ
How many batteries do I need?
Size the bank in Ah, then divide the system voltage by the battery voltage for the series count and the bank Ah by the battery Ah for the parallel strings. 173.6 Ah at 48 V with 12 V 100 Ah batteries needs 4 × 2 = 8 batteries.
What depth of discharge should I use?
About 50% for flooded and AGM lead acid to protect cycle life, and 80 to 90% for lithium iron phosphate.
Why use a higher system voltage?
Higher voltage means lower current for the same power, so thinner cables and smaller losses. Larger systems usually run at 24 or 48 V.
How many days of autonomy do I need?
One day suits grid backup or sunny climates; two to three days suit off-grid homes in cloudy areas.