Categories⚡ Engineering & EnergyBattery Runtime Calculator

Battery Runtime Calculator

Calculate battery discharge runtime in hours based on capacity (Ah), voltage, and load.

Estimated Battery Runtime
8.5 hours
Total Energy: 1200 Wh | Usable (85%): 1020 Wh
Guide & Educational Reference
How to Use the Battery Runtime Calculator
  1. 1Enter your battery capacity in Amp-hours (Ah).
  2. 2Enter nominal battery Voltage in Volts (V).
  3. 3Enter connected electrical load in Watts (W).
  4. 4Enter discharge efficiency percentage (defaults to 85%).
  5. 5View estimated total energy (Wh), usable energy (Wh), and estimated runtime in hours.
Understanding Battery Energy & Runtime

Battery capacity is often specified in Amp-hours (Ah), which indicates electrical charge rather than total energy. Energy capacity in Watt-hours (Wh) is calculated by multiplying Ah by Voltage (V).

Formulas Implemented by THE CALC
Total Energy (Wh)

Calculates theoretical energy stored in the battery.

Total Wh = Capacity (Ah) × Voltage (V)
Usable Energy (Wh)

Applies discharge efficiency factor (default 85%).

Usable Wh = Total Wh × (Efficiency % / 100)
Estimated Runtime (Hours)

Divides usable energy by load power in Watts.

Runtime (hours) = Usable Wh / Load (Watts)
Practical Real-World Example
100 Ah, 12V Battery powering a 120W Load (85% efficiency)
Total Wh = 100 × 12 = 1,200 Wh Usable Wh = 1,200 × (85 / 100) = 1,020 Wh Runtime = 1,020 / 120 = 8.50 hours
→ Total Energy = 1,200 Wh | Usable Energy = 1,020 Wh | Runtime = 8.50 hours
100 Ah, 12 V battery, 120 W load, 85% efficiency
Total Wh = 100 × 12 = 1,200; Usable = 1,200 × 0.85 = 1,020; Runtime = 1,020 / 120
→ ≈ 8.5 hours
50 Ah, 24 V battery, 200 W load, 90% efficiency
Total Wh = 1,200; Usable = 1,080; Runtime = 1,080 / 200
→ ≈ 5.4 hours
Important Battery Modelling Assumptions
  • Default efficiency assumption: 85% is a built-in modelling default to account for inverter conversion loss and internal resistance, not a universal law for all battery chemistries.
  • Real-world runtime variability: Actual runtime depends on discharge rates (Peukert effect), battery chemistry (Lead-Acid vs LiFePO4), ambient temperature, depth of discharge (DoD), and inverter cutoff voltage.
  • Linear model: The calculator uses a linear efficiency factor rather than non-linear Peukert curves.
Frequently Asked Questions

How do I convert Amp-hours (Ah) to Watt-hours (Wh)?

Multiply Amp-hours by Voltage: Wh = Ah × V. For example, 100 Ah × 12 V = 1,200 Wh.

Why is discharge efficiency defaulted to 85%?

85% accounts for real-world inverter inefficiency (typically 85–90%) and thermal losses during battery discharge.

How is usable capacity calculated?

Capacity (Ah) × voltage (V) gives total watt-hours, then that is multiplied by the efficiency percentage to account for conversion and discharge losses.

Does this account for battery chemistry or depth of discharge?

No. It is a straightforward energy-divided-by-load estimate; real runtime also depends on chemistry, temperature, age, and how deeply you discharge the pack.

Related Engineering & Energy Calculators

Appliance Electricity Cost

Estimate annual electricity power consumption costs for home appliances.

Open tool
Carbon Footprint Estimator

Estimate annual household CO2 emissions and environmental footprint.

Open tool
Ohm’s Law Calculator

Calculate Voltage (V), Current (I), Resistance (R), and Power (W).

Open tool