Solar & battery
Solar Battery Size Calculator
Sizes a battery, where usable capacity and nameplate capacity are different.
Result
Depth of discharge is how much of the nameplate you may actually use. Lithium iron phosphate tolerates 80% to 90%; lead acid should not go past 50% if you want it to last. Round-trip efficiency then takes another 5% to 15% on the way in and out. Cycle life figures are manufacturer ratings at favourable temperature and discharge rate; heat and deep fast cycling both shorten them considerably.
Get a Free Business AuditHow the calculation works
The energy to be supported divided by usable capacity, where usable is the nameplate capacity multiplied by the permitted depth of discharge and adjusted for round-trip efficiency and inverter conversion. Chemistry sets the depth of discharge, and the difference between chemistries is large enough to change the battery count.
What moves the number
- Energy to be supported and for how long.
- Chemistry, which sets permitted depth of discharge.
- Round-trip and inverter efficiency.
- Temperature, which affects available capacity.
Where this one goes wrong
Sizing from nameplate capacity rather than usable capacity undersizes the battery by the whole depth of discharge margin, and lithium iron phosphate and lead acid differ enough here that a swap between them is not a like-for-like substitution. Efficiency losses apply on the way in and the way out, so the energy drawn from the battery is less than the energy stored in it. Cold also reduces available capacity, which matters where the battery is in an unheated space.
Before you quote
- Size from usable capacity, not nameplate.
- Use the chemistry's actual depth of discharge limit.
- Allow for round-trip and inverter losses.
Questions
About this calculator
Why can I not use a battery's full capacity?
Because depth of discharge is limited by chemistry, and discharging beyond it shortens life sharply or is prevented by the management system. Lithium iron phosphate permits far deeper discharge than lead acid, which makes them very different in usable terms.
Do efficiency losses matter?
Yes, on both sides: energy is lost charging and lost again converting on discharge. The energy actually delivered to a load is meaningfully less than the energy sent to the battery, and sizing that ignores it falls short in practice.
Does cold affect battery capacity?
Substantially, which matters where a battery sits in an unheated garage or outdoors. Available capacity falls as temperature drops, and some chemistries also restrict charging below a threshold, so the installed location is part of the sizing.
The formula
nameplate = energy needed / (depth of discharge x (1 - reserve) x round-trip efficiency)
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How to use this calculator
- Energy to cover (kWh). Enter your own figure; the page starts at 12.
- Days of autonomy. How long it must last with no sun at all.
- Battery type. Enter your own figure; the page starts at Lithium iron phosphate (80% usable, 95% efficient).
- Reserve you will not touch (%). Enter your own figure; the page starts at 10.
- Installed cost per kWh ($). Enter your own figure; the page starts at $900.
The result updates as you type, so you can try a figure, see what it does to nameplate capacity 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 |
|---|---|
| Energy to cover (kWh) | 12 |
| Days of autonomy | 1 |
| Battery type | Lithium iron phosphate (80% usable, 95% efficient) |
| Reserve you will not touch (%) | 10 |
| Installed cost per kWh | $900 |
What comes out
| Result | Value |
|---|---|
| Nameplate capacity needed main result | 17.5 |
| Usable energy that gives | 12.6 |
| Delivered after losses | 12.0 |
| Installed cost | $15,789 |
| Rated cycle life | 6,000 |
To deliver 12.0 kWh you need a 17.5 kWh nameplate bank. Depth of discharge and round-trip losses mean only 12.0 kWh of it ever reaches the load.
How sensitive the answer is
Holding everything else steady and changing only installed cost per kwh, this is what happens to nameplate capacity needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Installed cost per kWh ($) | Nameplate capacity needed |
|---|---|
| $450 | 17.5 |
| $675 | 17.5 |
| $900 example above | 17.5 |
| $1,350 | 17.5 |
| $1,800 | 17.5 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Energy to cover (kWh)
- A number, between 0 and 5000, starting at 12.
- Days of autonomy
- How long it must last with no sun at all.
- Battery type
- Choose from Lithium iron phosphate (80% usable, 95% efficient), Lithium NMC (90% usable, 94% efficient), AGM lead acid (50% usable, 85% efficient), Flooded lead acid (50% usable, 80% efficient).
- Reserve you will not touch (%)
- A number, between 0 and 50, starting at 10.
- Installed cost per kWh ($)
- A money amount, starting at $900.
What the results tell you
| Result | What it is |
|---|---|
| Nameplate capacity needed | The headline result. A decimal figure. |
| Usable energy that gives | A decimal figure. |
| Delivered after losses | A decimal figure. |
| Installed cost | A money figure. |
| Rated cycle life | A number. |
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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