Solar & battery
Solar Battery Charge Time Calculator
Works charge time, where the final stage takes far longer than the arithmetic suggests.
Result
Charging slows as a battery fills, because the final stage tapers current to protect the cells, so real time to 100% is longer than this linear estimate. The house load matters more than people expect: whatever the house is using comes off the charging rate first.
Get a Free Business AuditHow the calculation works
Energy to be replaced divided by the charging power, adjusted for charging efficiency. That gives the bulk charging time, and it understates the total for chemistries whose final charging stage accepts current at a declining rate, which in lead acid can be a substantial additional period.
What moves the number
- Energy to replace and available charging power.
- Charging efficiency.
- Chemistry and its charging profile.
- Temperature, which can restrict charging.
Where this one goes wrong
Lead acid chemistries take a long absorption stage at declining current to reach full charge, so a simple energy divided by power calculation describes the bulk stage only and a battery left part charged repeatedly sulphates and loses capacity. Lithium chemistries accept current more uniformly and finish faster. Low temperature also restricts or prevents charging in many lithium batteries, which matters where the battery is in an unheated space.
Before you quote
- Allow additional time for the absorption stage in lead acid.
- Apply charging efficiency, not just the raw energy.
- Check low temperature charging restrictions for the chemistry.
Questions
About this calculator
Why does the last part of charging take so long?
Because lead acid chemistries have an absorption stage that accepts current at a declining rate, so the final portion takes far longer than the bulk. A simple energy divided by power calculation describes only the bulk stage.
Does it matter if a battery is not fully charged?
In lead acid, considerably: repeated partial charging causes sulphation and permanent capacity loss. Lithium chemistries are more tolerant and some prefer not to be held at full charge, so the right habit depends on the chemistry.
Can a battery be charged in the cold?
Many lithium batteries restrict or prevent charging below a temperature threshold, and charging them below it causes damage. Where a battery sits in an unheated space that restriction is part of the system design rather than a footnote.
The formula
hours = energy needed / (array output - house load)
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Battery capacity (kWh). Enter your own figure; the page starts at 13.5.
- Current state of charge (%). Enter your own figure; the page starts at 20%.
- Array size (kW DC). Enter your own figure; the page starts at 7.2.
- Peak sun hours per day. Enter your own figure; the page starts at Mid-Atlantic (4.5).
- System derate (%). Enter your own figure; the page starts at 80%.
- Charging efficiency (%). Enter your own figure; the page starts at 95%.
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 (kWh) | 13.5 |
| Current state of charge | 20% |
| Array size (kW DC) | 7.2 |
| Peak sun hours per day | Mid-Atlantic (4.5) |
| System derate | 80% |
| Charging efficiency | 95% |
| House load during charging (kW) | 0.5 |
What comes out
| Result | Value |
|---|---|
| Time to full main result | 2.2 |
| Energy to replace | 11.4 |
| Net charge rate | 5.3 |
| Days of sun needed | 0.5 |
Replacing 11.4 kWh at a net 5.3 kW takes 2.2 sun hours, about 0.5 days at 4.5 peak sun hours.
How sensitive the answer is
Holding everything else steady and changing only battery capacity (kwh), 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 (kWh) | Time to full |
|---|---|
| 6.75 | 1.1 |
| 10.13 | 1.6 |
| 13.5 example above | 2.2 |
| 20.25 | 3.2 |
| 27 | 4.3 |
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 (kWh)
- A number, between 0 and 5000, starting at 13.5.
- Current state of charge (%)
- A percentage, between 0 and 100, starting at 20%.
- Array size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Peak sun hours per day
- Choose from Pacific Northwest (3.5), Northeast, Midwest (4.0), Mid-Atlantic (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada (6.0).
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Charging efficiency (%)
- A percentage, between 60 and 100, starting at 95%.
- House load during charging (kW)
- A number, between 0 and 100, starting at 0.5.
What the results tell you
| Result | What it is |
|---|---|
| Time to full | The headline result. A decimal figure. |
| Energy to replace | A decimal figure. |
| Net charge rate | A decimal figure. |
| Days of sun needed | 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.
Keep going
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.