Battery capacity sizing calculator
Estimate required battery capacity from entered load, runtime target, DC system voltage and reserve allowance for Australian battery planning records.
Required capacity Ah = (Load W x runtime h / 1000 x (1 + reserve)) x 1000 / system voltage- Required usable energy is load multiplied by runtime target.
- Reserve increases the stored-energy estimate.
- Required Ah divides stored Wh by entered DC voltage.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| Eload | Required usable energy | kWh | Entered load multiplied by target runtime and divided by 1000. |
| Estore | Stored energy with reserve | kWh | Required usable energy multiplied by one plus reserve. |
| R | Reserve allowance | ratio | Entered reserve percentage as a decimal. |
| Vdc | System voltage | V | Entered nominal DC battery voltage. |
| Ah | Required capacity | Ah | Stored Wh divided by system voltage. |
More
Battery capacity sizing calculator technical guide
Estimate required battery capacity from entered load, runtime target, DC system voltage and reserve allowance for Australian battery planning records.
Use this calculator when the work question starts with a target runtime. Instead of asking how long an existing battery may last, the page estimates the battery capacity required to support an entered load for an entered duration with a reserve allowance.
The calculation is intentionally transparent. Load and runtime produce required usable energy. Reserve increases the stored-energy estimate. DC voltage converts stored Wh into Ah. The result is a worksheet value, not a product recommendation.
Capacity Sizing Use Cases
| Work setting | Real question | Useful action from this page |
|---|---|---|
| Backup planning | What capacity is implied by a target runtime? | Enter load, duration, voltage and reserve. |
| Battery comparison | Which Ah range should be reviewed against products? | Use the Ah result as a planning record before product data. |
| Small system note | How much stored energy is implied by a load group? | Read required kWh before comparing battery options. |
| Current handoff | What current might the load imply on the DC side? | Use the average current output or move to the current worksheet. |
| Cost discussion | What energy quantity might feed a simple estimate? | Use kWh only with separately entered tariff or product costs. |
The strongest record names the load group and reserve basis. A capacity number without those assumptions is difficult to review.
Capacity Boundary
| Item | Included in the arithmetic | Boundary to keep separate |
|---|---|---|
| Load | Entered steady watts. | Variable load profile, starting events and inverter behaviour are not modelled. |
| Runtime target | Entered hours. | Required autonomy may come from project, safety or operational rules outside this page. |
| Reserve allowance | Entered percentage. | Battery chemistry, ageing, BMS and warranty limits can override the allowance. |
| System voltage | Entered nominal DC voltage. | Real battery voltage range and product configuration are outside the formula. |
| Australian installation context | Mentioned as a review boundary. | Product selection, protection, location and standards checks need separate review. |
This boundary prevents a sizing estimate from becoming an approval claim. It is a useful planning record only when the reserve and load assumptions remain visible.
Input Checklist
| Value | Where it normally comes from | Why it matters |
|---|---|---|
| Load | Load list, backup schedule or metered value | Sets required energy. |
| Runtime target | Project requirement, operating plan or backup objective | Sets the time multiplier. |
| System voltage | Battery or inverter DC system basis | Converts stored energy into Ah. |
| Reserve allowance | Project basis, product guidance or conservative assumption | Adds headroom above usable energy. |
| Sizing reference | Battery system, load group or project label | Keeps the estimate traceable. |
If the load is variable, create separate records for the scenarios that matter. A single average can hide the current and energy case that controls the design.
Review Workflow
- Name the battery system, load group or job reference.
- Enter the load in watts.
- Enter the target runtime in hours.
- Enter nominal DC system voltage.
- Enter the reserve allowance.
- Read usable energy before the Ah result.
- If reserve is high, confirm the basis.
- If required capacity is large, check product range, voltage basis and installation requirements.
- Use runtime and current calculators for neighbouring records.
- Keep manufacturer, BMS, site and Australian installation requirements outside this sizing-only estimate.
Worked Records
| Situation | Inputs | Result pattern | Interpretation |
|---|---|---|---|
| 48 V backup capacity | 1.5 kW, 4 h, 48 V, 20% reserve | 6 kWh usable, 7.2 kWh stored, 150 Ah | Useful planning capacity before product review. |
| Small backup load | 500 W, 6 h, 24 V, 25% reserve | Smaller Ah requirement | Useful for comparing small battery options. |
| Large capacity review | 6 kW, 8 h, 48 V, 60% reserve | Large-capacity review | Product range, installation context and reserve basis need review. |
Australian Context
Battery capacity estimates often become part of inverter, charger, cable, protection and enclosure decisions. Australia's electrical installation context, battery energy storage requirements, local authority expectations and manufacturer instructions remain outside this arithmetic page. Use this as an entered-data worksheet, then carry the result into the relevant electrical review.
Stop Points
- Load basis is unknown or averaged across unlike scenarios.
- Runtime target is not supported by project requirements.
- Reserve is copied without product or project basis.
- System voltage does not match the intended battery arrangement.
- Ah result is being treated as a product recommendation or installation approval.
48 V backup capacity estimate
A 1.5 kW backup load needs a 4 hour target runtime with 20% reserve on a 48 V DC basis.
- Reference
- BATT-SIZE-1
- Load
- 1500 W
- Runtime target
- 4 h
- Voltage
- 48 V
- Reserve
- 20%
- Usable energy6 kWh
- Stored energy with reserve7.2 kWh
- Required capacity150 Ah
7.2 kWh stored energy on the entered voltage basis.
The result gives a required stored-energy and Ah estimate before product selection.
- Load is entered as a steady value.
- Reserve is a planning allowance.
- Battery product selection remains external.
Small backup load
A 500 W load is estimated for 6 hours on a 24 V battery basis with 25% reserve.
- Reference
- BATT-SIZE-SMALL
- Load
- 500 W
- Runtime target
- 6 h
- Voltage
- 24 V
- Reserve
- 25%
- Usable energy3 kWh
- Stored energy with reserve3.75 kWh
- Required capacity156.25 Ah
3.75 kWh stored energy on the entered voltage basis.
The estimate helps compare battery capacity options using a visible reserve basis.
- Runtime target is entered by the user.
- System voltage is a planning value.
- No ageing model is included.
Large capacity review
A high load and long runtime target show when the required capacity needs product and installation review.
- Reference
- BATT-SIZE-REVIEW
- Load
- 6000 W
- Runtime target
- 8 h
- Voltage
- 48 V
- Reserve
- 60%
- Usable energy48 kWh
- Stored energy with reserve76.8 kWh
- Required capacity1600 Ah
76.8 kWh stored energy on the entered voltage basis.
The high reserve and capacity should be checked against product range and installation requirements.
- The large load is intentional.
- Reserve is high by design.
- Battery chemistry and manufacturer limits can override the worksheet.