Engineering
Battery Runtime Calculator
Estimate usable energy, idealized runtime, approximate current, and nominal energy held outside the modeled delivery. The battery-gauge visual separates protected capacity, conversion loss, and energy delivered to the load.
Estimated usable energy (Wh)-
Idealized runtime (hours)-
Approximate battery current (A)-
Nominal energy not delivered (Wh)-
Decision view
Battery energy reserve and load runtime
Battery energy reserve and load runtimeNominal energy is reduced by the protected depth-of-discharge reserve and conversion loss before reaching the load.
Exact scenario comparisonContinuous load (W) changes while all other entered assumptions remain constant.
| Continuous load (W) | Estimated usable energy (Wh) | Idealized runtime (hours) | Approximate battery current (A) | Nominal energy not delivered (Wh) |
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How to use Battery Runtime Calculator
- Use capacity specified at a discharge rate close to the intended application.
- Enter the allowed depth of discharge for the battery chemistry and cycle-life objective.
- Add inverter standby, surge, temperature, aging, and low-voltage cutoff before relying on runtime.
Calculator guide
Understanding Battery Runtime Calculator
Battery runtime begins with stored watt-hours, then reduces them for usable depth of discharge and system efficiency before dividing by load. Chemistry and discharge conditions determine whether those allowances are realistic.
Calculation method
How the calculation works
Convert amp-hours to nominal watt-hours, apply usable depth of discharge and system efficiency, and divide by continuous load. Multiply amp-hours by nominal voltage for watt-hours, apply usable depth of discharge and system efficiency, divide usable energy by continuous load, and calculate current as watts divided by voltage.
Battery behavior
Power capability and energy capacity are different
A pack can contain enough energy but still fail to support a demanding load.
Worked situations
Practical examples
- 100 Ah at 12 V contains 1,200 nominal Wh.
- At 80% depth of discharge and 90% efficiency, about 864 Wh is modeled usable.
- A constant 250 W load gives about 3.46 idealized hours and roughly 20.8 A at nominal voltage.
Better inputs
Useful tips
- Use watt-hours from the manufacturer when voltage changes substantially across discharge.
- Size for surge power separately from energy runtime.
- Apply an aging reserve when the battery must meet runtime after years of use.
Before relying on the result
Limitations and common mistakes
- Voltage, load, efficiency, and available capacity are treated as constant.
- Peukert effect, BMS behavior, chemistry, temperature, aging, surge, standby, wiring loss, and cutoff are simplified.
- Series and parallel pack configuration must be converted to the correct total voltage and amp-hours.
Reference
Key terms
- Amp-hour
- Charge capacity; it becomes energy only after voltage is included.
- Watt-hour
- Electrical energy equal to one watt delivered for one hour.
- Depth of discharge
- Share of nominal capacity intentionally used.
- System efficiency
- Share of discharged battery energy delivered to the modeled load.
Important note
Calculated from the entered values using the displayed engineering relationship. Confirm design values, load cases, safety factors, standards, and field conditions with a qualified professional.
Frequently asked questions
Why not divide amp-hours by watts?
Amp-hours measure charge, so voltage is required to convert them to watt-hours.
Does a 100 Ah battery always deliver 1,200 Wh?
No. That is nominal energy at 12 V; usable energy depends on conditions and limits.
Can this model an inverter?
Use system efficiency to approximate conversion loss, then verify inverter standby and surge separately.
Why might real runtime be shorter?
Temperature, aging, discharge rate, voltage sag, cutoff, and varying load reduce available energy.