Estimate how long a battery can run a device using the capacity, voltage, load, efficiency, and on-time you know.
Table of contents
How to use our Battery Runtime Calculator
- Enter Battery capacity and choose the Capacity unit printed on the label: Ah, mAh, or Wh.
- Enter Battery voltage (V), then enter Device load and choose the Load unit: W, A, or mA.
- Open Advanced options if you want to change Usable battery amount (%), Power path efficiency (%), Time the device is on (%), or Number display.
- Select Calculate to see Estimated runtime, Runtime in hours, Usable energy, Average load used, Average battery current, and Load compared with capacity.
- Sanity-check the result: if Estimated runtime looks too high, confirm that Device load is the running load, Battery voltage matches the battery bank, and Time the device is on (%) is not hiding standby power.
Definitions
Battery capacity: The amount printed on the battery label. It may be charge capacity in Ah or mAh, or energy capacity in Wh.
Ah: Amp-hour, a charge rating. A 100 Ah battery can supply 100 amps for 1 hour only under ideal rating conditions.
mAh: Milliamp-hour. 1000 mAh equals 1 Ah.
Wh: Watt-hour, an energy rating. It combines voltage and amp-hours, so it is often the easiest unit for runtime.
V: Volts, the battery voltage used to convert between amp-based and watt-based values.
W: Watts, the power a device uses while running.
A and mA: Amps and milliamps, current drawn by the device or battery. 1000 mA equals 1 A.
Usable battery amount: The percent of the rated battery capacity you plan to use before stopping or recharging.
Power path efficiency: The percent of battery energy that reaches the device after inverter or converter losses.
Time the device is on: The duty cycle, or the percent of time the listed load is actually running.
Load compared with capacity: The C-rate. It compares average battery current with the battery's Ah capacity.
Common mistakes and quick fixes
Mistake: Entering Battery capacity in Ah while leaving Capacity unit set to Wh.
Fix: Match Capacity unit to the label exactly, such as Ah, mAh, or Wh.
Mistake: Using Device load in amps but leaving Load unit set to W.
Fix: Change Load unit to A or mA, or convert the device label to watts before entering it.
Mistake: Guessing Battery voltage from the charger instead of the battery or battery bank.
Fix: Use the nominal Battery voltage (V), such as 12 V, 24 V, 48 V, 3.7 V, or the correct series bank voltage.
Mistake: Setting Power path efficiency (%) to 100 for an inverter or converter system.
Fix: Use a lower Power path efficiency (%) if power passes through an inverter, DC regulator, or other conversion step.
Mistake: Using Time the device is on (%) for an on-off load but forgetting standby power.
Fix: If standby power matters, include it in Device load or run a separate estimate for the standby part.
Mistake: Treating Estimated runtime as a guaranteed shutoff time.
Fix: Use Estimated runtime as a planning number and leave a reserve with Usable battery amount (%) when the battery must not be fully drained.
Limitations & Key Assumptions / Boundary Conditions
- The estimate assumes a steady running load. Startup surge, motor cycling, and changing brightness or speed are not modeled unless you average them into Device load or Time the device is on (%).
- Battery voltage is treated as nominal and constant. Real battery voltage drops as the battery discharges.
- Battery chemistry is not inferred. Lead-acid, lithium-ion, LiFePO4, NiMH, and other batteries can behave differently at the same Ah and voltage.
- High Load compared with capacity can reduce real runtime, especially for lead-acid batteries and small cells.
- Cold temperature, battery age, cell imbalance, cutoff voltage, and battery management systems can stop a device sooner than the estimate.
- Power path efficiency (%) is a single average. Real inverter and converter efficiency can change with load level.
- If Time the device is on (%) is very low, standby power is excluded unless you include it in Device load.
Methodology
How the calculator works
The calculator converts all battery capacity entries to amp-hours and watt-hours, converts all load entries to watts, then divides usable energy by average load.
Capacity conversion
capacity_ah = capacity_value when Capacity unit is Ah.
capacity_ah = capacity_value / 1000 when Capacity unit is mAh.
capacity_ah = capacity_value / battery_voltage when Capacity unit is Wh.
rated_energy_wh = capacity_ah x battery_voltage
If Capacity unit is Wh, rated_energy_wh is the entered Battery capacity value. Battery voltage is still used for Average battery current and Load compared with capacity.
Load conversion
load_w = load_value when Load unit is W.
load_w = load_value x battery_voltage when Load unit is A.
load_w = (load_value / 1000) x battery_voltage when Load unit is mA.
Usable energy and runtime
average_load_w = load_w x (duty_cycle_percent / 100)
usable_energy_wh = rated_energy_wh x (usable_capacity_percent / 100) x (power_path_efficiency_percent / 100)
runtime_hours = usable_energy_wh / average_load_w
The hours-and-minutes result is made by keeping the whole hours and multiplying the remaining decimal hour by 60 minutes.
Battery current and C-rate
battery_current_a = average_load_w / battery_voltage
c_rate = battery_current_a / capacity_ah
For example, a 100 Ah battery at 12 V has 1200 Wh rated energy. With a 200 W load, 100% usable battery amount, 85% power path efficiency, and 100% on-time, usable energy is 1020 Wh and runtime is 1020 / 200 = 5.1 hours, or about 5 hours 6 minutes. Average battery current is 200 / 12 = 16.67 A, and Load compared with capacity is 16.67 / 100 = 0.167 C.