Solar & battery
Solar Battery Backup Time Calculator
Works backup duration, where deciding what to back up is the real design.
Result
Enter usable capacity, not nameplate, or the answer will be a third too generous. Averages matter more than peaks here: a fridge rated 400 W draws that only while the compressor runs, so its daily average is far lower.
Get a Free Business AuditHow the calculation works
Usable battery energy divided by the backed-up load gives duration, with solar recharge during the outage added where the system supports it. The sensitive input is the load, and reducing what is backed up extends duration far more cheaply than adding battery capacity.
What moves the number
- Which circuits are backed up.
- Whether solar can recharge during the outage.
- Usable capacity and inverter efficiency.
- Whether high-power loads are included.
Where this one goes wrong
Backing up the whole house is expensive and backing up a critical loads subpanel is not, and the difference in cost is substantial while the difference in practical outcome is often small. Design the backed-up load list deliberately. Whether the system can recharge from solar during a grid outage is the other question worth confirming, because a battery that cannot be recharged while the grid is down gives one cycle rather than indefinite operation, and not every configuration supports it.
Before you quote
- Decide the backed-up load list deliberately.
- Confirm the system can recharge from solar during an outage.
- Exclude high-power loads unless they are genuinely needed.
Questions
About this calculator
Should I back up the whole house?
It is substantially more expensive than backing up a critical loads subpanel, and the practical difference is often small. Deciding the load list deliberately extends duration far more cheaply than adding battery capacity.
Can the battery recharge during an outage?
Only in configurations that support it, and it is worth confirming rather than assuming. A system that cannot recharge from solar while the grid is down gives one cycle of backup rather than ongoing operation, which is a very different product.
Which loads should be excluded from backup?
High-power ones that are not essential: electric ranges, dryers, air conditioning and electric water heating consume in hours what the rest of the house uses in days. Excluding them is usually what makes a backup system affordable.
The formula
hours = usable kWh x 1000 / total load in watts
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How to use this calculator
- Usable battery (kWh). Enter your own figure; the page starts at 10.8.
- Fridge and freezer (W average). Enter your own figure; the page starts at 150.
- Lighting (W). Enter your own figure; the page starts at 120.
- Router, phones, small electronics (W). Enter your own figure; the page starts at 60.
- Well or sump pump (W average). Enter your own figure; the page starts at 200.
- Anything else (W). Enter your own figure; the page starts at 100.
The result updates as you type, so you can try a figure, see what it does to backup time, 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 |
|---|---|
| Usable battery (kWh) | 10.8 |
| Fridge and freezer (W average) | 150 |
| Lighting (W) | 120 |
| Router, phones, small electronics (W) | 60 |
| Well or sump pump (W average) | 200 |
| Anything else (W) | 100 |
| Solar recharge per day (kWh) | 0 |
What comes out
| Result | Value |
|---|---|
| Backup time main result | 17.1 |
| Total load | 630 |
| Energy used per day | 15.1 |
| Days with the solar recharge | 0.7 |
17.1 hours on battery alone at 630 W. With 0 kWh of daily recharge it stretches to 0.7 days.
How sensitive the answer is
Holding everything else steady and changing only usable battery (kwh), this is what happens to backup time. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Usable battery (kWh) | Backup time |
|---|---|
| 5.4 | 8.6 |
| 8.1 | 12.9 |
| 10.8 example above | 17.1 |
| 16.2 | 25.7 |
| 21.6 | 34.3 |
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
- Usable battery (kWh)
- A number, between 0 and 5000, starting at 10.8.
- Fridge and freezer (W average)
- A number, between 0 and 5000, starting at 150.
- Lighting (W)
- A number, between 0 and 5000, starting at 120.
- Router, phones, small electronics (W)
- A number, between 0 and 2000, starting at 60.
- Well or sump pump (W average)
- A number, between 0 and 10000, starting at 200.
- Anything else (W)
- A number, between 0 and 20000, starting at 100.
- Solar recharge per day (kWh)
- Zero if the array is offline or the sky is dark.
What the results tell you
| Result | What it is |
|---|---|
| Backup time | The headline result. A decimal figure. |
| Total load | A number. |
| Energy used per day | A decimal figure. |
| Days with the solar recharge | 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.
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.