Battery Runtime Calculator
Enter battery capacity, charge level, reserve, efficiency, and device power to estimate battery runtime and the time your reserve level will be reached.
Last updated: July 2026
Battery safety
- This estimate does not verify battery, inverter, wiring, or device compatibility. Confirm voltage, chemistry, current limits, connectors, fusing, and manufacturer requirements.
- Do not exceed continuous or peak current limits, and do not bypass a battery-management system or protection device.
- Do not use or charge a damaged, swollen, wet, unusually hot, or abnormal-smelling battery.
Estimated runtime
Energy and runtime breakdown
These are planning adjustments, not physically separate discharge stages.
Loss breakdown
Ideal versus estimated runtime
The final estimate follows the entered capacity and efficiency assumptions without an extra hidden safety factor.
Voltage × amp-hours gives nominal watt-hours. Actual available energy can differ from the battery label, especially under high load, low temperature, aging, voltage sag, or battery-management-system cutoff.
What this battery runtime calculator tells you
Estimate how long a steady average load can run before a battery reaches a selected reserve. Results include rated and delivered energy, adjustment losses, conservative whole-minute runtime, local reserve time, and charge consumption rates.
How battery runtime is calculated
The selected charge-window energy is adjusted by remaining-capacity and delivery-efficiency assumptions, then divided by average watts. Runtime is rounded down to a whole minute so the displayed duration and reserve time do not overstate the estimate.
Watt-hours versus voltage and amp-hours
Enter rated Wh directly, or calculate nominal energy as voltage × amp-hours. Voltage × Ah is a nominal label relationship; actual available energy can differ.
How reserve charge changes runtime
The reserve is excluded from planned use. A 100% current charge and 20% reserve creates an 80-percentage-point window. Energy below the reserve is intentionally kept, not treated as destroyed.
Remaining capacity and efficiency assumptions
Remaining capacity models retained capacity relative to the rating. Efficiency models inverter, controller, wiring, and conversion losses. Direct DC loads may be closer to 95–100%, while inverter systems may be lower, but actual efficiency varies.
Why actual battery runtime can differ
Battery age, chemistry, temperature, discharge rate, voltage sag, BMS cutoff, inverter idle use, changing loads, acceleration, hills, and Peukert effect where applicable can change real runtime. The calculator follows the entered assumptions and is not a compatibility or current-capability assessment.
Worked examples
48V 21Ah battery at 250W
Rated energy is 1,008Wh. From 100% to a 20% reserve, 806.4Wh is in the charge window. At 100% remaining capacity and 90% efficiency, delivered energy is 725.76Wh. Runtime is 2.90304 hours or 174.1824 minutes, displayed conservatively as 2 hours 54 minutes. Starting at noon on July 29, 2026 reaches reserve at 2:54 PM. The estimated rate is approximately 27.56 percentage points per hour, assuming a constant 250W load.
Frequently asked questions
How do I calculate runtime from watt-hours?
Multiply Wh by the usable charge fraction, capacity factor, and efficiency factor, then divide by average watts.
How do I calculate runtime from voltage and amp-hours?
Multiply nominal volts by Ah to obtain rated Wh, then apply the runtime calculation.
What load value should I enter?
Use the average total system load, including known inverter idle consumption.
Should I enter maximum or average power?
Use average operating power. Nameplate maximum power may not represent typical use.
Why leave a reserve charge?
A reserve prevents the plan from assuming every percentage point of label capacity is consumed.
What does remaining battery capacity mean?
It is a user-entered planning factor for retained capacity, not a battery-health diagnosis.
What efficiency should I use?
Use measured or manufacturer information when available. Efficiency varies with equipment and load.
Why is actual runtime shorter than Wh divided by watts?
Reserve, capacity loss, conversion loss, variable load, temperature, sag, and cutoff can shorten runtime.
Does battery voltage change during use?
Yes. The V × Ah mode uses nominal voltage for nominal energy.
Does this include inverter idle power?
Only when you include that overhead in the entered average load.
Can I use this for an e-bike?
Yes, as an energy estimate, but acceleration, hills, speed, temperature, and rider load make demand variable.
Can I use this for a power station, solar battery, RV, UPS, or backup battery?
Yes for planning with an average load, provided the inputs reflect the system.
Does it include Peukert effect?
No. It does not model chemistry-specific, load-dependent capacity curves.
Does it confirm the battery can safely supply the load?
No. It does not verify continuous-current capability, voltage, wiring, inverter, connectors, fusing, or compatibility.