Battery Runtime Calculator
Estimate how long your battery can power a device using battery capacity, voltage, depth of discharge, and load.
What this battery running time calculator does
This calculator estimates battery backup time based on practical energy math. You enter your battery size, voltage, and load, and it returns an estimated runtime in hours and a readable format (days, hours, and minutes).
It also includes two important real-world corrections that many simple calculators ignore:
- Depth of discharge (DoD): how much of the battery you actually plan to use.
- System efficiency: losses from inverters, converters, and wiring.
Battery runtime formula
Step 1: Convert to usable energy
Usable watt-hours are estimated as:
Usable Wh = Capacity (Ah) × Voltage (V) × DoD × Efficiency
Where DoD and Efficiency are used as decimals (for example, 80% = 0.80).
Step 2: Divide by load power
If you know power directly:
Runtime (hours) = Usable Wh ÷ Load (W)
If you know current in amps:
Runtime (hours) = Usable Ah ÷ Load Current (A)
with Usable Ah = Capacity (Ah) × DoD × Efficiency.
Example calculation
Suppose you have a 12V, 100Ah battery powering a 60W device, with 80% DoD and 90% efficiency:
- Nominal energy: 100 × 12 = 1200Wh
- Usable energy: 1200 × 0.80 × 0.90 = 864Wh
- Runtime: 864 ÷ 60 = 14.4 hours
So your expected battery backup time is roughly 14 hours 24 minutes.
Why real runtime may be shorter
1) Battery chemistry and discharge rate
Lead-acid batteries can deliver less effective capacity at high discharge rates (Peukert effect). Lithium batteries are generally more stable under load.
2) Temperature
Cold weather can reduce available capacity significantly. Very high temperatures can shorten battery lifespan.
3) Battery age and health
As batteries age, actual amp-hour capacity drops. If your battery is older, reduce expected runtime accordingly.
4) Hidden loads
Inverters and electronics often consume standby power. Fans, routers, and chargers can also add up.
Tips for better runtime planning
- Use a realistic DoD based on chemistry and cycle-life goals.
- Measure actual load with a watt meter when possible.
- Add a safety margin of 15% to 30% for mission-critical systems.
- For off-grid and UPS designs, size for worst-case temperature and peak load.
Quick FAQ
Is this calculator good for solar battery banks?
Yes. It is useful for solar storage planning, RV systems, marine setups, and home backup. For final system design, also account for charging profile and daily solar production.
Should I use 100% depth of discharge?
Usually no. Most systems avoid full discharge to improve battery life. Use chemistry-specific guidance from the manufacturer.
Can I enter mAh instead of Ah?
Yes. Select mAh in the calculator and it will convert automatically.