Battery Runtime Calculator
Estimate how many hours a battery will run a given load. Enter capacity in Wh or Ah, voltage, system efficiency, and load in watts to find your runtime.
Result
10.8 hr
Estimated runtime is 10.8 hr, or about 648 minutes.
Quick Answer
A battery runtime calculator estimates operating time using three steps: Capacity (Wh) = Ah × Volts; Usable Wh = Capacity Wh × Efficiency; Runtime (hours) = Usable Wh / Load (watts). A 100Ah 12V battery has 1,200 Wh of capacity. At 85% efficiency powering a 150W load, the usable energy is 1,020 Wh, giving a runtime of 6.8 hours.
How A Battery Runtime Calculator Works, Step By Step
A battery's rated capacity in watt-hours represents stored energy under ideal conditions. Usable energy is lower due to inverter conversion losses, heat, and BMS cutoff. This calculator estimates realistic operating runtime.
The Battery Runtime Formula And How It Is Calculated
Capacity (Wh) = Ah × Volts Usable Wh = Capacity (Wh) × System Efficiency Runtime (hours) = Usable Wh / Load (watts)
- Capacity (Wh) = Ah × Volts
- Usable Wh = Capacity (Wh) × Efficiency
- Runtime (hrs) = Usable Wh / Load (W)
Getting Started With The Battery Runtime Calculator
Inputs
- Capacity: battery capacity in Ah or Wh
- Voltage: nominal voltage (e.g., 12V, 24V, 48V)
- Efficiency (%): inverter/converter efficiency (85% typical)
- Load (W): total continuous power draw of connected devices
Steps
- Check battery label for Ah rating and voltage.
- Enter system efficiency (use 85% for typical inverter).
- Enter total load in watts.
- Read estimated runtime in hours.
Battery Runtime Calculation In Practice: A Real Example
200Ah 12V lithium battery powering a 60W fan + 40W light (100W total) at 90% inverter efficiency.
- Calculate Capacity Wh: 200 Ah × 12V = 2,400 Wh.
- Calculate Usable Wh: 2,400 × 0.90 = 2,160 Wh.
- Calculate Runtime: 2,160 / 100 = 21.6 hours.
The battery runs the 100W load for approximately 21.6 hours.
When A Battery Runtime Calculator Gives The Right Answer
Use this calculator for sizing solar battery banks, UPS backup time, or camping power station requirements.
Assumptions
- Load is continuous and constant.
- Efficiency covers all hardware conversion losses.
Limitations
- Does not model Peukert effect for lead-acid batteries.
- Does not model extreme temperature effects or motor startup surge spikes.
In Practice
The most common error when estimating runtime is forgetting to account for multiple devices running simultaneously. Add up every active watt-consuming device before running the calculation.
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Frequently Asked Questions: Battery Runtime Estimation
What is the difference between Ah and Wh for a battery?
Ah measures charge capacity; Wh measures total energy. Wh = Ah × Volts.
Why does system efficiency reduce my runtime?
Inverters lose 8–20% of energy as heat during DC-to-AC conversion.
How do I find the wattage of my devices?
Check nameplate for W rating, or multiply Amps × Volts.
Why does this calculator not include depth of discharge?
The efficiency input already accounts for usable energy thresholds.
Can I use this for a UPS (uninterruptible power supply)?
Yes. Enter UPS battery Wh and connected load wattage.
Sources
Last updated: . Reviewed for accuracy against the formula shown above.