Battery Backup Calculator
Calculate theoretical and practical battery backup runtime from voltage, capacity, and load power. Shows a capacity bar chart over time.
V
Ah
W
Results
Total Energy
1200 Wh
Theoretical Runtime
24.0 hours
Practical Runtime (85%)
20.4 hours
Remaining Capacity Over Time
0h
100%
2h
90%
4.1h
80%
6.1h
70%
8.2h
60%
10.2h
50%
12.2h
40%
14.3h
30%
16.3h
20%
18.4h
10%
20.4h
0%
How is Battery Backup Calculated?
Battery backup runtime is calculated by dividing the total energy capacity (Watt-hours) by the load power (Watts). Real-world runtime is reduced by about 15% due to inverter losses, battery discharge curves, and temperature effects.
Runtime = Total Wh / Load Power (W)
Practical Runtime = Runtime × 0.85 (efficiency)
Example
A 12V 100Ah battery has 1,200 Wh of energy. Powering a 50W load, it theoretically lasts 24 hours. After accounting for 85% efficiency, practical runtime is about 20.4 hours.
Common Use Cases
- Planning UPS (uninterruptible power supply) backup for critical equipment.
- Estimating how long a portable battery station can power devices during outages.
- Sizing battery banks for solar or RV setups.
FAQs
Why is practical runtime less than theoretical?
Real batteries lose efficiency from inverter conversion, heat, discharge rate, and the fact that voltage sags as the battery drains. The 85% factor is a common industry estimate.
Does temperature affect battery life?
Yes. Cold temperatures reduce available capacity, while excessive heat degrades battery health over time. Most batteries perform best between 50°F and 85°F.