Battery Size Calculator

Inputs

Fill fields, then calculate. Values can include commas.

Formula Used How to Use

Load type

AC uses inverter efficiency. DC bypasses inverter.

Input mode

Choose power for quickest sizing.

Load power *

Average real power (not surge).

System voltage (V) *

DC bus voltage for the battery bank.

Runtime (hours) *

Total backup duration per day.

Autonomy days

Use >1 for multi-day storage needs.

Inverter efficiency (%)

Ignored for DC loads.

DC path efficiency (%)

Cables, connections, converters.

Battery efficiency (%)

Round-trip behavior approximation.

Depth of discharge (%)

Typical: lead-acid 50, LiFePO₄ 80–90.

Temperature factor (%)

Use 90% for cold derating scenarios.

Aging margin (%)

Accounts for capacity fade over time.

Reserve margin (%)

Extra buffer for uncertainty and growth.

Peukert adjustment

Helpful when discharge current is high.

Single battery voltage (V)

Used for configuration estimate.

Single battery capacity (Ah)

Use datasheet rating (e.g., 20h).

View Example Table

Example Data Table

Sample scenario to verify the calculator workflow.

Scenario	Load	System	Runtime	Assumptions	Recommended Capacity	Example Bank
Home backup	800 W AC	24 V	4 h × 1 day	Inv 92%, DC 97%, Bat 95%, DOD 80%, Margins 20%+10%	≈ 230 Ah	24 V bank: 2S × 3P of 12 V 100 Ah (6 units)
Telecom DC	20 A DC @ 48 V	48 V	6 h × 1 day	DC 98%, Bat 95%, DOD 50%, Temp 90%	≈ 321 Ah	48 V bank: 4S × 4P of 12 V 100 Ah (16 units)
Field kit	150 W AC	12 V	10 h × 2 days	Inv 90%, DC 96%, Bat 94%, DOD 85%, Margins 15%+10%	≈ 380 Ah	12 V bank: 1S × 4P of 12 V 100 Ah (4 units)

Results are approximate and depend on real discharge rate, temperature, and battery aging.

Formula Used

1) Power and energy

AC load Battery-side power: P_batt = P_load / η_inv
DC load Battery-side power: P_batt = P_load
Energy for runtime and days: E = P_batt × t × d
Loss-adjusted energy: E_adj = E / (η_dc × η_bat)

2) Capacity and deratings

Base capacity: Ah_base = E_adj / V_dc
DOD and temperature: Ah = Ah_base / (DOD × T)
Margins: Ah_final = Ah × (1+m_age) × (1+m_res)
Average current: I ≈ P_batt / (V_dc × η_dc × η_bat)

3) Optional Peukert correction (20-hour style)

When enabled, the calculator converts the required capacity to an equivalent 20-hour rated capacity using exponent k.

Ah₂₀ = [ Ah_final × (20 × I)^(k−1) ]^(1/k)

This is an approximation; consult battery datasheets for exact curves and allowable discharge rates.

How to Use This Calculator

Choose AC if using an inverter, otherwise choose DC.
Enter your load as power in watts, or as current with voltage.
Set system voltage, then enter the required runtime and autonomy days.
Adjust efficiencies, DOD, temperature factor, and margins to match your design standard.
Optional: enable Peukert for lead-acid high-rate discharge situations.
Provide a typical single-battery voltage and capacity to estimate series/parallel count.
Press Calculate. Results appear above the form and can be exported.

Engineering Notes

Use the average load for energy sizing; handle surge separately via inverter and cable ratings.
Cold temperatures reduce capacity; use the temperature factor to derate accordingly.
For mission-critical systems, follow local standards and manufacturer recommendations.

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FAQs

1. What load value should I enter: average or peak?

Use the average real load for energy sizing. Size peaks separately by checking inverter surge rating, cable ampacity, and protective devices. If the load varies, estimate duty cycle or use logged measurements over at least one day.

2. How do I choose depth of discharge?

Select DOD based on chemistry and life target. Flooded lead-acid often uses 50%, AGM around 60%, and LiFePO₄ commonly 80–90%. Lower DOD improves cycle life but increases required capacity and cost.

3. Why does temperature factor change the result?

Battery capacity drops in cold conditions, so the temperature factor derates usable energy. For example, 90% near 0°C or 80% near −10°C means you must install more nameplate capacity to achieve the same runtime.

4. When should I enable Peukert correction?

Enable it for lead-acid banks with high discharge current or short runtimes. A higher Peukert exponent increases the required 20-hour rated capacity, reflecting reduced effective capacity at high C-rates.

5. How accurate is the series/parallel battery count?

It is a planning estimate using your selected unit voltage and Ah rating. Always validate against manufacturer limits for maximum series strings, recommended parallel count, BMS constraints, and available physical space and cabling.

6. Can this calculator size for surge loads?

It focuses on energy and average power. For motor starts or inverter surges, verify peak watts, starting current, and inverter surge duration. Then confirm batteries can supply the peak current without excessive voltage drop.