Solar & battery
Solar Battery Quantity Calculator
Works battery quantities, where the configuration constrains the count.
Result
Modules come in fixed sizes, so the installed capacity almost always overshoots the target. Check the manufacturer's limit on how many can be stacked on one system, and whether the inverter supports the full stack; both are common constraints that do not appear in the arithmetic.
Get a Free Business AuditHow the calculation works
Required usable energy divided by per-unit usable capacity gives a raw count, adjusted so the units arrange into complete series strings at the system voltage. Partial strings are not an option, so the practical count rounds up to a whole multiple of the string length.
What moves the number
- Required usable energy and per-unit usable capacity.
- System voltage and therefore string length.
- Whether the units must be identical.
- Space, weight and ventilation at the installed location.
Where this one goes wrong
The count rounds up to complete strings, which means the delivered capacity frequently exceeds the requirement and the cost step between sizes is coarse. Plan around that rather than being surprised by it. Weight and space are the practical constraints people forget, since a battery bank of any size is heavy enough to matter structurally and some chemistries have ventilation and clearance requirements that the installed location has to satisfy.
Before you quote
- Round up to complete series strings.
- Check weight, space and clearance at the installed location.
- Use matched units of the same type and age.
Questions
About this calculator
Why does my battery count round up?
Because series strings have to be complete to reach the system voltage, so the count rounds up to a whole multiple of the string length. That makes the cost steps coarse and the delivered capacity frequently above the requirement.
What practical constraints get forgotten?
Weight, space and clearance. A bank of any size is heavy enough to matter structurally, and some chemistries have ventilation and clearance requirements, so the installed location has to be checked before the count is settled.
Can the bank be expanded later?
Poorly, because mixing new units with aged ones means the weakest governs the whole bank. Planning the eventual capacity at the outset, or planning to replace the bank as a unit, is more realistic than incremental expansion.
The formula
modules = (target usable / usable share) / capacity per module, rounded up
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How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Capacity per module (kWh). Enter your own figure; the page starts at 5.
- Spares and breakage (%). Enter your own figure; the page starts at 0%.
- Cost per module installed ($). Enter your own figure; the page starts at $4,200.
- Target usable capacity (kWh). Enter your own figure; the page starts at 20.
- Usable share of nameplate (%). Enter your own figure; the page starts at 80%.
The result updates as you type, so you can try a figure, see what it does to modules, 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 |
|---|---|
| System size (kW DC) | 7.2 |
| Capacity per module (kWh) | 5 |
| Spares and breakage | 0% |
| Cost per module installed | $4,200 |
| Target usable capacity (kWh) | 20 |
| Usable share of nameplate | 80% |
What comes out
| Result | Value |
|---|---|
| Modules main result | 5 |
| Before rounding | 5.0 |
| Material cost | $21,000 |
| Cost per kW | $2,917 |
5 modules give 25 kWh nameplate and 20 kWh usable, costing about $21,000 installed.
How sensitive the answer is
Holding everything else steady and changing only cost per module installed, this is what happens to modules. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per module installed ($) | Modules |
|---|---|
| $2,100 | 5 |
| $3,150 | 5 |
| $4,200 example above | 5 |
| $6,300 | 5 |
| $8,400 | 5 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Capacity per module (kWh)
- A number, between 0 and 100000, starting at 5.
- Spares and breakage (%)
- A percentage, between 0 and 25, starting at 0%.
- Cost per module installed ($)
- A money amount, starting at $4,200.
- Target usable capacity (kWh)
- A number, between 0 and 5000, starting at 20.
- Usable share of nameplate (%)
- A percentage, between 20 and 100, starting at 80%.
What the results tell you
| Result | What it is |
|---|---|
| Modules | The headline result. A number. |
| Before rounding | A decimal figure. |
| Material cost | A money figure. |
| Cost per kW | 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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