Battery runtime calculator
Estimate battery runtime from entered Ah capacity, nominal DC voltage, load and usable fraction for Australian battery planning records.
Runtime = (Battery Ah x nominal voltage x usable fraction / 1000) / load kW- Stored energy is Ah multiplied by nominal DC voltage.
- Usable energy applies the entered usable fraction.
- Runtime divides usable energy by the entered steady load.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| Estored | Stored battery energy | kWh | Battery Ah multiplied by nominal DC voltage and divided by 1000. |
| Eusable | Usable battery energy | kWh | Stored energy multiplied by the entered usable fraction. |
| Pload | Load | kW | Entered load divided by 1000. |
| Fusable | Usable fraction | ratio | Entered usable fraction as a decimal. |
| t | Runtime | h | Usable energy divided by load kW. |
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Battery runtime calculator technical guide
Estimate battery runtime from entered Ah capacity, nominal DC voltage, load and usable fraction for Australian battery planning records.
Use this calculator when the work question is runtime: how long an entered battery record may support an entered load. It is useful for backup notes, small battery systems, communications loads and preliminary energy records. It is not a battery product selector and it does not approve an installation.
The page keeps the arithmetic visible. Ah capacity and nominal voltage create stored kWh. The usable fraction creates usable kWh. The load converts runtime into hours and minutes. If any of those values are assumptions, keep them visible in the exported record.
Battery Runtime Use Cases
| Work setting | Real question | Useful action from this page |
|---|---|---|
| Backup load review | How long can the entered load be supported? | Enter Ah capacity, voltage, load and usable fraction. |
| Small communications battery | Is the backup window plausible for a known steady load? | Use the runtime result as a planning note. |
| Battery replacement discussion | Does a proposed capacity materially change runtime? | Compare records using the same load and usable-fraction basis. |
| Current handoff | Does the load imply a battery current check? | Move to the charge/discharge current worksheet. |
| Cable handoff | Does high DC current need cable drop review? | Move to the battery cable voltage-drop calculator. |
The strongest record names the battery system and load group. A generic entry is harder to review later than "BATT-RUN-1, 200 Ah, 48 V, 1.2 kW, 80% usable".
Runtime Boundary
| Item | Included in the arithmetic | Boundary to keep separate |
|---|---|---|
| Battery capacity | Entered Ah value. | Manufacturer capacity basis, ageing and temperature derating remain external. |
| Nominal voltage | Entered DC voltage. | Battery voltage changes during charge and discharge are not modelled. |
| Usable fraction | Entered percentage. | Chemistry, BMS, depth-of-discharge limits and warranty rules can override it. |
| Load | Entered steady load. | Starting, cycling, inverter behaviour and variable load profiles are outside the formula. |
| Australian installation context | Mentioned as a review boundary. | Battery location, protection, isolation, ventilation and standards checks need separate review. |
This boundary matters because a runtime value can look precise even when its inputs are planning assumptions. The calculator is useful because it exposes those assumptions, not because it hides battery behaviour.
Input Checklist
| Value | Where it normally comes from | Why it matters |
|---|---|---|
| Battery capacity | Battery datasheet, BMS record or project schedule | Sets stored energy with voltage. |
| Nominal voltage | Battery system record | Converts Ah into kWh. |
| Load | Load list, backup schedule or metered value | Runtime is directly proportional to load. |
| Usable fraction | Product data, BMS setting or conservative planning basis | Controls how much stored energy is assumed available. |
| Runtime reference | Battery system, backup load or scenario label | Keeps the estimate traceable. |
If values are guessed, record that outside the calculator result. Runtime planning is only as strong as its battery and load assumptions.
Review Workflow
- Name the battery system, backup load or scenario.
- Enter the Ah capacity basis used by the record.
- Enter nominal DC voltage.
- Enter the steady load in watts.
- Enter the usable fraction from product data or planning basis.
- Read stored energy before using the runtime result.
- If runtime is very short, check whether the load or capacity basis is realistic.
- If usable fraction is high, confirm chemistry and BMS support.
- Use current and cable calculators when the runtime record becomes an electrical design input.
- Keep manufacturer, site and Australian installation requirements outside this runtime-only estimate.
The workflow keeps a backup-duration question separate from product selection and installation approval.
Worked Records
| Situation | Inputs | Result pattern | Interpretation |
|---|---|---|---|
| 48 V backup battery | 200 Ah, 48 V, 1.2 kW, 80% usable | 7.68 kWh usable and 6.4 h runtime | Useful as a transparent backup-duration record. |
| Small communications backup | 100 Ah, 24 V, 300 W, 75% usable | Longer small-load autonomy | Useful for planning notes while battery ageing remains outside the worksheet. |
| Short runtime review | 100 Ah, 24 V, 5 kW, 95% usable | Short-duration review | Check load basis, usable fraction and product data before relying on the value. |
Australian Context
Australia's 230/400 V a.c. supply context becomes relevant when the battery runtime estimate feeds chargers, inverters, circuits, switchboards or backup supply design. This page stays on the DC energy and load arithmetic. Installation requirements, product instructions, BMS limits, local authority expectations and competent-person review remain outside the calculator.
Stop Points
- Battery capacity basis is unknown or mixed between usable and nominal values.
- Nominal voltage does not match the battery system record.
- The load is variable but entered as a steady value.
- Usable fraction is copied without product or BMS support.
- Runtime is being treated as product selection or installation approval.
48 V backup battery record
A 200 Ah, 48 V battery is checked against a 1.2 kW backup load with 80% usable fraction.
- Reference
- BATT-RUN-1
- Battery capacity
- 200 Ah
- Voltage
- 48 V
- Load
- 1200 W
- Usable fraction
- 80%
- Stored energy9.6 kWh
- Usable energy7.68 kWh
- Runtime6.4 h
7.68 kWh usable energy from the entered basis.
The result gives a transparent runtime estimate before product and BMS limits are checked.
- Capacity is entered in Ah.
- Nominal DC voltage is the planning voltage.
- Usable fraction is a project assumption.
Small communications backup
A 100 Ah, 24 V battery is checked for a 300 W communications load.
- Reference
- BATT-RUN-COMMS
- Battery capacity
- 100 Ah
- Voltage
- 24 V
- Load
- 300 W
- Usable fraction
- 75%
- Stored energy2.4 kWh
- Usable energy1.8 kWh
- Runtime6 h
1.8 kWh usable energy from the entered basis.
The estimate is useful for backup notes, while autonomy requirements remain external.
- Load is a steady worksheet value.
- Usable fraction is deliberately conservative.
- No battery ageing model is included.
Short runtime review
A high load is compared with a small 24 V battery to show when the record needs review.
- Reference
- BATT-RUN-REVIEW
- Battery capacity
- 100 Ah
- Voltage
- 24 V
- Load
- 5000 W
- Usable fraction
- 95%
- Stored energy2.4 kWh
- Usable energy2.28 kWh
- Runtime0.46 h
2.28 kWh usable energy from the entered basis.
The short duration and high usable fraction should be checked before the record is used.
- The high load is intentional.
- The usable fraction requires product support.
- Temperature and BMS behaviour can override the arithmetic.