Battery Autonomy Calculator
Engineers designing remote telemetry, RTU, or solar sites use this to estimate how long a battery bank can carry a load with no charging. Enter the bank capacity, voltage, a depth-of-discharge limit, and the load, and it returns usable energy and the run time in hours and days.
A battery's rated capacity is not all usable: draining it fully shortens its life, so a depth-of-discharge limit reserves part of the charge. This calculator applies that limit, converts to watt-hours, and divides by the load to give autonomy. It is the starting point for the reserve-days figure in a solar or backup design.
The Calculator
Result
Depth of discharge reserves part of the charge to protect battery life.
The Formula
Usable energy is the rated amp-hours times voltage times the depth-of-discharge fraction; runtime is that energy divided by the load:
Worked example. A 200 Ah, 12 V bank at 0.5 DoD holds 200 × 12 × 0.5 = 1,200 Wh usable. Against a 10 W load that is 1,200 / 10 = 120 hours, or 120 / 24 = 5.0 days of autonomy with no charging.
Assumptions and Limits
- The default depth of discharge is 0.5, a common lead-acid figure. Many lithium chemistries tolerate deeper discharge; use the value the battery manufacturer recommends for the cycle life you want.
- The calculation assumes a constant load and ignores temperature, aging, and inverter or converter efficiency, all of which reduce real autonomy.
- Rated capacity is usually quoted at a specific discharge rate. A faster discharge yields less usable capacity than the rating suggests (the Peukert effect).
- Results are engineering estimates. Verify against the battery datasheet and add margin for temperature and end-of-life derating.
FAQ
What depth of discharge should I use?
It depends on chemistry and target cycle life. Flooded and AGM lead-acid banks are commonly designed around 0.5 to protect life; many lithium iron phosphate systems are rated for deeper regular discharge. Always use the manufacturer's recommendation.
Why does temperature matter?
Cold reduces the capacity a battery can actually deliver, and heat shortens its life. A bank rated at 25 C can deliver noticeably less at freezing, so cold-site designs add margin beyond this ideal figure.
Does this include the charging source?
No. This is autonomy with no charging, which is what you want for sizing reserve days. Pair it with the solar array sizing estimator to check that the charging source can refill the bank between low-sun periods.
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