Backup Autonomy Estimator
Calculate exactly how many hours your battery backup will last during a power outage. Accounts for depth of discharge, inverter efficiency, and multiple battery banks.
System Inputs
Total capacity of one battery
Used when capacity is in Ah
Total watts drawn
Results
How the Backup Autonomy Estimator Works
Define your battery bank
Enter capacity in Ah or Wh, the voltage, chemistry type, and the number of batteries in parallel or series.
Set real-world limits
Choose the depth of discharge and the inverter efficiency to reflect true usable energy — not just the sticker specs.
Get your backup hours
The tool divides usable watt-hours by your load in watts to return an accurate estimate of outage runtime.
Frequently Asked Questions
How do I calculate battery backup time during a power outage?
Battery backup time is calculated by dividing the usable energy of the battery (in watt-hours) by the total load (in watts). Usable energy equals battery capacity × voltage × depth of discharge × inverter efficiency × number of batteries. For example, a 100Ah 12V lithium battery at 90% DoD and 93% inverter efficiency delivers about 1,018 Wh, which powers a 300W load for roughly 3.4 hours.
What depth of discharge should I use for lithium vs lead-acid batteries?
For lithium-ion (LiFePO4) batteries, a safe depth of discharge is 80% to 90% to maximize cycle life. For lead-acid, AGM or gel batteries, limit DoD to 50% to avoid premature damage. Flooded batteries can go to about 60% but perform best around 40–50%. Always follow the manufacturer's recommendation to preserve warranty and longevity.