Solar & battery
Solar Battery Discharge Time Calculator
Works discharge time, where the discharge rate affects the capacity available.
Result
Lead acid delivers measurably less capacity as the discharge rate rises, an effect described by Peukert's law, and the approximation used here is deliberately mild. Lithium iron phosphate is far flatter, which is part of why it outperforms its nameplate comparison with lead. Cold weather reduces capacity in both.
Get a Free Business AuditHow the calculation works
Usable energy divided by the load, with the discharge rate noted against the rate the capacity rating assumes. Drawing faster than the rated rate reduces the total energy obtained, markedly in lead acid and more modestly in lithium chemistries, which is why a high-power load empties a battery faster than proportionally.
What moves the number
- Usable capacity after depth of discharge.
- Load, and the discharge rate it represents.
- Chemistry's sensitivity to discharge rate.
- Inverter efficiency at that load level.
Where this one goes wrong
A battery rated at a slow discharge rate delivers less total energy when discharged quickly, so a lead acid bank running a heavy load gives noticeably less than its rating suggests. Check the rate the rating assumes. Depth of discharge limits also exist to protect life rather than being arbitrary, and a system that regularly discharges deeper than the chemistry tolerates buys runtime now at the cost of replacing the bank sooner.
Before you quote
- Compare the actual discharge rate against the rating's assumed rate.
- Respect the chemistry's depth of discharge limit.
- Use inverter efficiency at the actual load.
Questions
About this calculator
Does discharging faster reduce capacity?
Yes, and markedly in lead acid. The capacity rating assumes a specific discharge rate, and drawing faster yields less total energy. Lithium chemistries are much less sensitive, which is one reason they suit high-power applications better.
Can I discharge deeper to get more runtime?
You can and it costs battery life. Depth of discharge limits protect longevity rather than being arbitrary, so regularly exceeding them buys runtime now in exchange for replacing the bank sooner, which is rarely a good trade.
Why does my runtime vary?
Load level, temperature and state of health all affect it. A cold battery delivers less, an aged battery delivers less than new, and a heavier load reduces the total energy obtained, so runtime from the same bank genuinely differs between occasions.
The formula
runtime = usable amp hours / load current
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How to use this calculator
- Battery capacity (Ah). Enter your own figure; the page starts at 200.
- System voltage. Enter your own figure; the page starts at 48 V.
- Load current (A). Enter your own figure; the page starts at 25.
- Battery type. Enter your own figure; the page starts at Lithium iron phosphate.
- Depth of discharge allowed (%). 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 runtime, 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 capacity (Ah) | 200 |
| System voltage | 48 V |
| Load current (A) | 25 |
| Battery type | Lithium iron phosphate |
| Depth of discharge allowed | 80% |
What comes out
| Result | Value |
|---|---|
| Depth of discharge applied | 80.0% |
| Runtime main result | 6.4 |
| Usable amp hours | 160.0 |
| Discharge rate | 0.1 |
| Effective capacity at this rate | 200.0 |
| Power draw | 1,200 |
At 25 A the bank runs for about 6.4 hours, a 0.1C discharge drawing 1,200 watts.
How sensitive the answer is
Holding everything else steady and changing only battery capacity (ah), this is what happens to runtime. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery capacity (Ah) | Runtime |
|---|---|
| 100 | 3.2 |
| 150 | 4.8 |
| 200 example above | 6.4 |
| 300 | 9.6 |
| 400 | 12.8 |
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 capacity (Ah)
- A number, between 0 and 100000, starting at 200.
- System voltage
- Choose from 12 V, 24 V, 48 V.
- Load current (A)
- A number, between 0 and 5000, starting at 25.
- Battery type
- Choose from Lithium iron phosphate, Lead acid.
- Depth of discharge allowed (%)
- A percentage, between 0 and 100, starting at 80%.
What the results tell you
| Result | What it is |
|---|---|
| Depth of discharge applied | A percentage. |
| Runtime | The headline result. A decimal figure. |
| Usable amp hours | A decimal figure. |
| Discharge rate | A decimal figure. |
| Effective capacity at this rate | A decimal figure. |
| Power draw | 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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