Solar & battery
Solar Battery Bank Calculator
Configures a battery bank, where matching the units matters as much as the total.
Result
Series raises voltage, parallel raises capacity. Every battery in a bank should be the same make, model, age and state of charge, because the weakest one sets the behaviour of the whole string. More than three or four parallel strings of lead acid is generally discouraged, since current shares unevenly between them.
Get a Free Business AuditHow the calculation works
The required usable energy converted into a count of batteries, arranged in series strings to reach the system voltage and in parallel to reach the capacity. The total energy follows from the count; the configuration follows from the voltage the inverter and charge controller require.
What moves the number
- Required usable energy and the per-unit capacity.
- System voltage the equipment requires.
- Series and parallel arrangement.
- Whether units are identical in age and type.
Where this one goes wrong
Mixing batteries of different age, capacity or chemistry in one bank causes the weakest unit to govern the whole, which wastes the capacity of the others and shortens the life of all of them. Build a bank from matched units and replace them together. Interconnection also matters more than it appears: unequal cable lengths between parallel strings cause unequal current sharing, which ages one string faster than the rest.
Before you quote
- Use matched units of the same age, type and capacity.
- Keep interconnection cables equal in length between parallel strings.
- Confirm the configuration matches the equipment's voltage requirement.
Questions
About this calculator
Can I add batteries to an existing bank?
It is generally a poor idea, because the oldest or weakest unit governs the behaviour of the whole bank. Adding new capacity to aged batteries wastes the new units and does not restore the bank, which is why banks are replaced together.
Does cable length matter within a bank?
Yes. Unequal cable lengths between parallel strings cause unequal current sharing, so one string works harder and ages faster. Keeping the interconnections symmetrical is a small detail with a real effect on bank life.
Can different chemistries share a bank?
No, they have different voltage profiles and charging requirements, and a charging regime suited to one will damage the other. Separate banks with separate charge control is the only workable arrangement where both are present.
The formula
series = target volts / battery volts; strings = target kWh / capacity per string
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How to use this calculator
- Battery voltage. Enter your own figure; the page starts at 12.
- Battery capacity (Ah). Enter your own figure; the page starts at 100.
- Target system voltage. Enter your own figure; the page starts at 48 V.
- Target capacity (kWh). Enter your own figure; the page starts at 10.
- Cost per battery ($). Enter your own figure; the page starts at $320.
The result updates as you type, so you can try a figure, see what it does to batteries needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Battery voltage | 12 |
| Battery capacity (Ah) | 100 |
| Target system voltage | 48 V |
| Target capacity (kWh) | 10 |
| Cost per battery | $320 |
What comes out
| Result | Value |
|---|---|
| Batteries needed main result | 12 |
| In series per string | 4 |
| Parallel strings | 3 |
| Bank capacity | 14.4 |
| Bank cost | $3,840 |
12 batteries: 4 in series to reach 48 V, with 3 strings in parallel. That gives 14.4 kWh for about $3,840.
How sensitive the answer is
Holding everything else steady and changing only cost per battery, this is what happens to batteries needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per battery ($) | Batteries needed |
|---|---|
| $160 | 12 |
| $240 | 12 |
| $320 example above | 12 |
| $480 | 12 |
| $640 | 12 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Battery voltage
- A number, between 0 and 100, starting at 12.
- Battery capacity (Ah)
- A number, between 0 and 10000, starting at 100.
- Target system voltage
- Choose from 12 V, 24 V, 48 V.
- Target capacity (kWh)
- A number, between 0 and 5000, starting at 10.
- Cost per battery ($)
- A money amount, starting at $320.
What the results tell you
| Result | What it is |
|---|---|
| Batteries needed | The headline result. A number. |
| In series per string | A number. |
| Parallel strings | A number. |
| Bank capacity | A decimal figure. |
| Bank cost | A money figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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