Solar & battery
Solar Battery Capacity Calculator
Works battery capacity in amp hours and kilowatt hours, where the voltage links them.
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. Amp hours mean nothing without a voltage: 200 Ah at 12 V is 2.4 kWh, and the same 200 Ah at 48 V is 9.6 kWh. Comparing banks on amp hours alone is the classic mistake.
Get a Free Business AuditHow the calculation works
Capacity in amp hours multiplied by nominal system voltage gives energy in watt hours, and dividing by a thousand gives kilowatt hours. Converting correctly matters because battery specifications are commonly quoted in amp hours at a stated voltage while loads and solar production are in kilowatt hours.
What moves the number
- Amp hour rating and the nominal voltage it is quoted at.
- Series and parallel configuration.
- Depth of discharge for usable energy.
- The discharge rate the rating assumes.
Where this one goes wrong
Amp hour ratings are quoted at a specified discharge rate, and drawing faster than that rate yields less total energy, which is a real effect in lead acid and smaller in lithium chemistries. Check the rate the rating assumes against how the battery will actually be used. Series and parallel configuration is the other place errors occur, since series raises voltage while parallel raises amp hours, and the total energy is the same either way.
Before you quote
- Note the discharge rate the amp hour rating assumes.
- Convert using the correct nominal voltage.
- Apply depth of discharge for the usable figure.
Questions
About this calculator
How do amp hours convert to kilowatt hours?
Multiply amp hours by the nominal voltage to get watt hours, then divide by a thousand. The voltage matters, so an amp hour figure quoted without it cannot be converted, and mixing voltages between batteries and loads is where conversion errors start.
Does discharge rate affect capacity?
Yes, and the rating assumes a specific rate. Drawing faster yields less total energy, which is a pronounced effect in lead acid and smaller in lithium chemistries. Checking the assumed rate against actual use is part of sizing honestly.
Series or parallel?
Series raises voltage and parallel raises amp hours, and total energy is the same either way. The configuration is chosen to match the inverter and charge controller input requirements rather than to change the capacity available.
The formula
kWh = amp hours x volts / 1000
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How to use this calculator
- Enter capacity as. Enter your own figure; the page starts at Amp hours.
- Capacity (Ah). Enter your own figure; the page starts at 200.
- Capacity (kWh). Enter your own figure; the page starts at 10.
- System voltage. Enter your own figure; the page starts at 48 V.
- Batteries in the bank. Enter your own figure; the page starts at 1.
- Battery type. Enter your own figure; the page starts at Lithium iron phosphate (80% usable, 95% efficient).
The result updates as you type, so you can try a figure, see what it does to total nameplate, 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 |
|---|---|
| Enter capacity as | Amp hours |
| Capacity (Ah) | 200 |
| Capacity (kWh) | 10 |
| System voltage | 48 V |
| Batteries in the bank | 1 |
| Battery type | Lithium iron phosphate (80% usable, 95% efficient) |
What comes out
| Result | Value |
|---|---|
| Total nameplate main result | 9.6 |
| Total capacity | 200 |
| Usable at rated depth | 7.7 |
| Delivered after losses | 7.3 |
| Usable share | 80.0% |
9.6 kWh nameplate, which is 200 Ah at 48 V. Only 7.7 kWh is usable at 80.0% depth, and 7.3 kWh actually reaches the load.
How sensitive the answer is
Holding everything else steady and changing only capacity (ah), this is what happens to total nameplate. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Capacity (Ah) | Total nameplate |
|---|---|
| 100 | 4.8 |
| 150 | 7.2 |
| 200 example above | 9.6 |
| 300 | 14.4 |
| 400 | 19.2 |
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
- Enter capacity as
- Choose from Amp hours, Kilowatt hours.
- Capacity (Ah)
- A number, between 0 and 100000, starting at 200.
- Capacity (kWh)
- A number, between 0 and 5000, starting at 10.
- System voltage
- Choose from 12 V, 24 V, 48 V, 51.2 V LFP.
- Batteries in the bank
- A number, between 0 and 500, starting at 1.
- 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).
What the results tell you
| Result | What it is |
|---|---|
| Total nameplate | The headline result. A decimal figure. |
| Total capacity | A number. |
| Usable at rated depth | A decimal figure. |
| Delivered after losses | A decimal figure. |
| Usable share | A percentage. |
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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