Solar & battery
Solar Charge Time Calculator
Works charge time from a solar array, where available sun rather than array size limits it.
Result
C rate is the charge current as a fraction of capacity, and every chemistry has a maximum. Lead acid should generally stay below 0.2C; lithium iron phosphate tolerates 0.5C and often more. Exceeding the manufacturer's limit shortens life and, on lithium, is a safety matter rather than a performance one.
Get a Free Business AuditHow the calculation works
Energy to be replaced divided by the energy the array actually delivers to the battery, which is array output times charging efficiency rather than the nameplate rating. Because the array only produces during useful sun hours, the constraint is frequently the number of those hours rather than the array's capacity.
What moves the number
- Energy to replace and array output.
- Useful sun hours available that day.
- Charging efficiency and controller losses.
- Whether loads are running during charging.
Where this one goes wrong
Loads running during charging consume part of the array's output, so a battery recovering while the house is in use charges much more slowly than a calculation of array against battery suggests. Account for the concurrent load. In winter the available sun hours can be too few to recover a deep discharge in a single day regardless of array size, which is the condition that makes multi-day cloudy periods the real design problem.
Before you quote
- Subtract concurrent loads from the array output available for charging.
- Use winter sun hours for the limiting case.
- Apply controller and charging efficiency losses.
Questions
About this calculator
Why is my battery charging so slowly?
Usually because loads are consuming part of the array's output at the same time, so less reaches the battery than the array is producing. The concurrent load has to be subtracted before the charging rate makes sense.
Can a bigger array always charge faster?
Only up to the controller and battery acceptance limits, and only during available sun hours. In winter there may be too few hours to recover a deep discharge in one day regardless of array size, which is a storage and generator question instead.
What is the real design problem?
Consecutive cloudy days, since that is when neither the array nor the available hours can recover the battery. Sizing for the single-day case while ignoring multi-day periods is how off-grid systems end up running a generator constantly.
The formula
hours = (capacity x depth of discharge / efficiency) / charge 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.
- How empty it is (%). Enter your own figure; the page starts at 50%.
- Charge current (A). Enter your own figure; the page starts at 40.
- Charging efficiency (%). Enter your own figure; the page starts at 90%.
The result updates as you type, so you can try a figure, see what it does to time to full, 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 |
| How empty it is | 50% |
| Charge current (A) | 40 |
| Charging efficiency | 90% |
What comes out
| Result | Value |
|---|---|
| Time to full main result | 2.8 |
| Amp hours to replace | 111.1 |
| Energy required | 5.3 |
| Charge rate | 0.2 |
Replacing 111.1 Ah at 40 A takes about 2.8 hours. That is a 0.2C rate, which is gentle and easy on the cells.
How sensitive the answer is
Holding everything else steady and changing only battery capacity (ah), this is what happens to time to full. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery capacity (Ah) | Time to full |
|---|---|
| 100 | 1.4 |
| 150 | 2.1 |
| 200 example above | 2.8 |
| 300 | 4.2 |
| 400 | 5.6 |
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.
- How empty it is (%)
- A percentage, between 0 and 100, starting at 50%.
- Charge current (A)
- A number, between 0 and 2000, starting at 40.
- Charging efficiency (%)
- A percentage, between 50 and 100, starting at 90%.
What the results tell you
| Result | What it is |
|---|---|
| Time to full | The headline result. A decimal figure. |
| Amp hours to replace | A decimal figure. |
| Energy required | A decimal figure. |
| Charge rate | A decimal 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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