Quick answer
Runtime (hours) = capacity × voltage × usable fraction × efficiency ÷ load. A 100 Ah, 12 V battery with 80% usable capacity and a 90% efficient inverter powers a 200 W load for 4.32 hours (864 Wh usable).
Enter capacity, voltage, load, usable capacity and efficiency. The calculator returns runtime in hours and minutes and the usable energy.
Related tools: Ohm's Law Calculator: Solve voltage, current or resistance. Parallel Resistor Calculator: Equivalent resistance of resistors in parallel and in series. Watt-Hours Calculator: Energy in watt-hours from power and time, or from amp-hours and voltage.
How to use the Battery Runtime Calculator
- Enter battery capacity (Ah) (above 0).
- Enter battery voltage (V) (above 0).
- Enter load (W) (above 0).
- Enter usable capacity (%) (between 0 and 100). It starts at 100.
- Enter inverter or system efficiency (%) (between 0 and 100). It starts at 100.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Battery capacity | A number, in Ah | > 0 |
| Battery voltage | A number, in V | > 0 |
| Load | A number, in W | > 0 |
| Usable capacity | A number, in % | > 0, ≤ 100, starts at 100 |
| Inverter or system efficiency | A number, in % | > 0, ≤ 100, starts at 100 |
Formula and method
Formula
runtime (h) = capacity × voltage × usable% × efficiency% ÷ load
IEEE-754 double precision; shown to the decimals indicated.
Set the usable share to your battery's safe depth of discharge; real capacity falls with high loads and cold.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Battery capacity = 100; Battery voltage = 12; Load = 200; Usable capacity = 80; Inverter or system efficiency = 90.
| Result | Value |
|---|---|
| Runtime | 4.32 h |
| In minutes | 259.2 |
| Usable energy | 864 Wh |
Example 2
Inputs: Battery capacity = 50; Battery voltage = 24; Load = 150; Usable capacity = 100; Inverter or system efficiency = 100.
| Result | Value |
|---|---|
| Runtime | 8 h |
| In minutes | 480 |
| Usable energy | 1,200 Wh |
Example 3
Inputs: Battery capacity = 20; Battery voltage = 12; Load = 35; Usable capacity = 50; Inverter or system efficiency = 85.
| Result | Value |
|---|---|
| Runtime | 2.91 h |
| In minutes | 174.9 |
| Usable energy | 102 Wh |
The first line is the headline result (runtime). Below it the battery Runtime Calculator lists in minutes and usable energy, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The energy chain
100 Ah × 12 V = 1,200 Wh. Using only 80% leaves 960 Wh, and 90% inverter efficiency leaves 864 Wh. Dividing by the 200 W load gives 4.32 hours.
What it does not model
High loads shorten runtime beyond the simple estimate (Peukert effect in lead-acid batteries), and low temperatures cut capacity. Lead-acid batteries should not be discharged below about 50%, while lithium can use 80 to 90%.
How the result changes with battery capacity
Holding every other input at the first worked example, this table changes only battery capacity. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Battery capacity | Runtime | In minutes | Usable energy |
|---|---|---|---|
| 50 | 2.16 h | 129.6 | 432 Wh |
| 100 | 4.32 h | 259.2 | 864 Wh |
| 200 | 8.64 h | 518.4 | 1,728 Wh |
| 400 | 17.28 h | 1,036.8 | 3,456 Wh |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Battery capacity: entering "0" shows "Battery capacity must be greater than 0." beside that field and no result.
- Load: entering "0" shows "Load must be greater than 0." beside that field and no result.
Choosing between the Battery Runtime Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Ohm's Law Calculator](/engineering/ohm-s-law-calculator/) | Current | |
| [Parallel Resistor Calculator](/engineering/parallel-resistor-calculator/) | resistances | Parallel resistance |
| [Watt-Hours Calculator](/engineering/watt-hours/) | use | Energy |
| [LC Resonant Frequency Calculator](/engineering/resonant-frequency-lc/) | inductance and capacitance | Resonant frequency |
| [Wattage Calculator](/engineering/wattage/) | Power |
Background: electrical circuits
Ohm's law links voltage, current and resistance: V = I × R, so I = V ÷ R and R = V ÷ I. Power is P = V × I, which also equals I² × R and V² ÷ R.
These relations hold for ohmic components, where resistance is constant. Diodes and transistors are non-linear, and in AC circuits with reactive parts impedance (Z) takes the place of resistance. Check component ratings and local electrical code before relying on any calculation.
Assumptions and limitations
- Peukert and temperature effects are not modeled
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for battery capacity and load to confirm this.
Frequently asked questions
How do I use a smaller load?
Runtime rises in proportion: 100 W gives 8.64 hours.
What is depth of discharge?
The share of capacity you use.
Does a bigger inverter waste power?
A larger idle draw lowers efficiency at small loads.
What does the Battery Runtime Calculator calculate?
Runtime of a battery under a constant load. It returns Runtime, In minutes and Usable energy.
What formula does the Battery Runtime Calculator use?
runtime (h) = capacity × voltage × usable% × efficiency% ÷ load. Set the usable share to your battery's safe depth of discharge; real capacity falls with high loads and cold.
What do I need to enter in the battery Runtime Calculator?
You enter Battery capacity (Ah), Battery voltage (V), Load (W), Usable capacity (%) and Inverter or system efficiency (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Peukert and temperature effects are not modeled.
What is an example of a battery Runtime result?
With battery capacity 100, battery voltage 12, load 200, usable capacity 80 and inverter or system efficiency 90, the runtime is 4.32 h.
Where does the method come from?
The method follows: OpenStax, University Physics Volume 2 (Rice University): electricity, circuits and thermodynamics: electric power and energy (E = V × Ah); Usable capacity falls with load, temperature and battery age; see the battery datasheet.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Peukert and temperature effects are not modeled.
Related calculators
- Ohm's Law Calculator — Solve voltage, current or resistance.
- Parallel Resistor Calculator — Equivalent resistance of resistors in parallel and in series.
- Watt-Hours Calculator — Energy in watt-hours from power and time, or from amp-hours and voltage.
- LC Resonant Frequency Calculator — Resonant frequency of an LC circuit.
- Wattage Calculator — Solve power, voltage or current for a DC or resistive load.
- Ah ⇄ Wh Converter — Convert between amp-hours, milliamp-hours and watt-hours.
- Thread Pitch Calculator — Convert between threads per inch and pitch in millimeters.
Sources
- OpenStax, University Physics Volume 2 (Rice University): electricity, circuits and thermodynamics: electric power and energy (E = V × Ah)
- Usable capacity falls with load, temperature and battery age; see the battery datasheet
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.