Nominal energy
E_nom = V_bank · C_Ah
A constant nominal voltage converts amp-hour capacity to watt-hours.
Battery Runtime Calculator
Estimate how long a battery bank can support a constant load after accounting for usable depth of discharge and end-to-end conversion efficiency.
Access: Free to use, no installation, and No account required. Calculations run in your browser.
Battery and load
Results
Runtime model
Nominal stored energy is estimated as E = V × Ah. Usable energy is then Eusable = E × DoD × η, and constant-load runtime is t = Eusable / Pload.
When the simple estimate becomes optimistic
Real battery runtime also depends on temperature, battery age, cell imbalance, voltage cutoffs, high C-rate behavior, inverter standby draw, dynamic load profiles, and chemistry-specific effects such as Peukert behavior in lead-acid batteries. Use the full DIY Power System Designer when those effects matter.
Engineering reference
Battery Runtime Calculator: background and worked detail
Use the Battery Runtime Calculator to estimate nominal battery energy, usable delivered energy, runtime, autonomy in days, and DC current from bank voltage, amp-hour capacity, usable depth of discharge, path efficiency, and load power.
Why real runtime falls short of capacity ÷ load
The model converts battery capacity from amp-hours to nominal watt-hours using bank voltage, then applies usable depth of discharge and an aggregate path efficiency before dividing by the load.
This is an energy-balance estimate. Real battery voltage, capacity, efficiency, temperature, age, discharge rate, inverter standby consumption, and battery-management limits can shorten or occasionally extend observed runtime.
48 V, 100 Ah battery powering 500 W
Use a 48 V, 100 Ah bank, 80% usable depth of discharge, 90% path efficiency, and a constant 500 W load.
- Nominal energy is 4.80 kWh.
- Usable delivered energy is 4.80 kWh × 0.80 × 0.90 = 3.456 kWh.
- Runtime is 3.456 kWh / 0.500 kW = 6.912 hours.
Result: The estimated DC current is about 11.6 A at the entered path efficiency.
Equations used by this calculator
Usable delivered energy
E_use = E_nom · DoD · η
Depth of discharge and path efficiency reduce the nominal energy available to the load.
Runtime
t = E_use / P_load
Runtime follows directly from usable energy divided by constant load power.
Where the estimate stops being reliable
Assumptions
- Load power is constant for the duration of the estimate.
- Bank voltage is represented by a nominal constant value.
- Depth of discharge and efficiency are independent scalar factors.
Limitations
- Does not model battery discharge curves, Peukert behavior, BMS cutoffs, temperature, aging, cell imbalance, or inverter idle consumption separately.
- Not a battery safety, fuse, cable, charger, or enclosure design calculation.
Battery Runtime Calculator FAQ
Why is actual battery runtime different?
Real batteries do not deliver their nameplate energy under every temperature, age, discharge rate, and voltage cutoff. The calculator is an energy-budget estimate.
Should inverter efficiency be included?
Yes. Use the path-efficiency input to represent inverter and wiring losses when the load is downstream of them.
Can I enter lithium and lead-acid batteries?
Yes as an energy estimate, provided you choose realistic usable depth-of-discharge and efficiency values for the battery and operating conditions.
Validation checks
Double the load
Case: Change 500 W to 1000 W with all else unchanged.
Expected: Estimated runtime should halve.
Double capacity
Case: Change 100 Ah to 200 Ah with all else unchanged.
Expected: Nominal energy, usable energy, and runtime should double.
Where this calculation comes from
The discharge model is shared with the DIY Power System Designer, which adds temperature derating and duty-cycle profiles.
Open the source workbench →Read calculation and source methodology →