12V Battery Runtime Calculator – Ah, Watts & Amps

How the 12V Battery Runtime Calculator Works

The 12V Battery Runtime Calculator estimates how long a 12V-class battery or matching parallel battery bank can support a DC load between a selected starting charge and planned minimum charge.

The estimate can account for battery condition, delivery efficiency, fixed controller or standby consumption, and an optional Peukert adjustment for lead-acid batteries.

Example: A 12 V, 100 Ah Battery Supplying a 120 W DC Load

Suppose:

The 50% minimum and Peukert settings are illustrative; use manufacturer data when available.

1. Calculate Available Battery Energy

Nominal battery energy:

12 V × 100 Ah = 1,200 Wh

The selected charge range is:

100% − 50% = 50%

At a 100% condition factor:

Selected energy = 1,200 Wh × 0.50 × 1.00

= 600 Wh

2. Calculate Battery-Side Demand

At 95% delivery efficiency:

120 W ÷ 0.95 = 126.3158 W

No separate controller or standby load is entered, so:

Battery-side demand = 126.3158 W

3. Calculate Simple Runtime

Simple runtime = 600 Wh ÷ 126.3158 W

= 4.75 hours

Estimated runtime:

About 4 hours 45 minutes

4. Apply the Optional Peukert Model

Using:

• rating period: 20 hours
• Peukert exponent: 1.20
• nominal battery current: ≈ 10.53 A

the adjusted lead-acid estimate is approximately:

4.09 hours

or:

About 4 hours 6 minutes

Example Results

The simple and Peukert results use different models. A Peukert result is useful only when its inputs reasonably represent the actual lead-acid battery.

Enter the Battery Bank Correctly

Use the battery system’s nominal label voltage, not its charging, float, cutoff, or temporary measured voltage.

Typical 12V-class values include:

• 12 V for many lead-acid batteries;
• 12.8 V for a four-cell LiFePO4 battery;
• another nominal value specified by the manufacturer.

A nominal 12 V lead-acid battery may charge above 14 V, but it remains a 12 V nominal battery for this calculation.

Parallel Batteries

For matching batteries connected in parallel:

Total bank Ah = Ah per battery × Number of parallel batteries

The nominal voltage stays the same.

For three 12 V, 100 Ah batteries:

100 Ah × 3 = 300 Ah

Nominal bank energy:

12 V × 300 Ah = 3,600 Wh

The bank remains a 12V-class system.

If you need to calculate combined voltage, Ah, or Wh from a battery arrangement first, use the Battery Series Parallel Calculator.

Parallel-bank estimates assume reasonably matched batteries. Significant imbalance can reduce practical runtime.

Higher-voltage series banks such as 24 V or 48 V systems are outside this calculator’s 12V-class model.

Battery Condition and Planned Minimum

The planned minimum charge controls how much of the displayed battery range is included in the runtime calculation.

The battery condition factor adjusts modeled capacity.

For the worked example:

Do not reduce the entered Ah value for degradation and then apply the same reduction again through the condition factor.

If the Ah value already represents reliable present-day capacity, a 100% condition factor may be appropriate.

The planned minimum is a calculation boundary, not a detected BMS cutoff or guaranteed remaining state of charge.

If starting charge and planned minimum are equal, selected energy and runtime are zero.

Enter a Representative Average Load

Use realistic average operating power, not only a startup surge, adapter maximum, controller rating, or brief peak reading.

The load can be entered as:

• average connected-load watts; or
• average connected-load current at the entered nominal battery voltage.

When current is used:

Connected-load watts = Nominal battery voltage × Entered current

A 5 V USB device drawing 2 A uses:

5 V × 2 A = 10 W

It should therefore be entered as a 10 W load, not as 2 A at 12 V.

Cycling Loads

For loads that cycle between different power levels, use a weighted average.

Suppose a device draws:

• 120 W for 30% of the time;
• 10 W for 70% of the time.

Average load:

120 × 0.30 + 10 × 0.70

= 43 W

Use approximately:

43 W

for the energy-runtime estimate.

Startup current may still be much higher than the average and is not calculated here.

Measured Battery-Side Power

If complete battery-terminal voltage and current are measured:

Measured battery-side watts = Actual measured voltage × Measured current

When this value already includes controller, wiring, and conversion losses, enter it as direct watts with:

• 100% delivery efficiency
• 0 W separate controller or standby load

This avoids counting the same losses twice.

Account for Delivery Losses and Fixed Overhead

Delivery efficiency and fixed overhead represent different types of battery demand.

Delivery efficiency accounts for proportional losses associated with the connected load, such as conversion and wiring losses.

Fixed overhead represents additional consumption from equipment such as:

• controllers;
• monitors;
• relay boards;
• distribution electronics;
• standby circuits.

The calculator treats these separately so proportional losses and fixed consumption are not confused.

Optional Peukert Adjustment for Lead-Acid Batteries

The simple energy model assumes rated Ah converts proportionally to available energy.

Lead-acid battery capacity can change with discharge rate, so the calculator provides an optional Peukert model for:

• flooded lead-acid batteries;
• AGM or gel lead-acid batteries.

It is not applied to LiFePO4 or other lithium batteries through this calculator.

Victron Energy’s official Battery Capacity and Peukert Exponent guide explains how discharge rate affects available lead-acid battery capacity and how the Peukert exponent is used.

The model uses:

• condition-adjusted battery Ah;
• nominal battery current;
• capacity-rating period;
• Peukert exponent;
• selected charge range.

Compare Discharge Current With Rating Current

Use:

Rating current = Condition-adjusted Ah ÷ Rating period

For the worked example:

100 Ah ÷ 20 h = 5 A

The nominal battery current is approximately:

10.53 A

Because the discharge current is above the illustrative 5 A rating current, the Peukert estimate is shorter than the simple runtime.

When a condition factor below 100% is used, rating current should be based on the condition-adjusted Ah value.

Higher discharge current generally shortens the Peukert estimate, while lower current can lengthen it. Use the manufacturer’s rating period and exponent when available; the selected charge-range adjustment remains a planning approximation.

Nominal Current, C-Rate, and Current Limit

Nominal current:

Nominal current = Battery-side demand ÷ Nominal voltage

Nominal discharge rate:

Nominal C-rate = Nominal current ÷ Total bank Ah

For the worked example:

10.53 A ≈ 0.11C

For a dedicated current-to-capacity calculation, use the Battery C-Rate Calculator.

Optional Continuous-Current Limit

If a continuous-current limit is known, use the rating that applies to the complete bank or shared output path.

Do not automatically multiply one battery’s rating by the number of parallel batteries unless the manufacturer and system design support that combined limit.

This comparison does not size wiring, fuses, breakers, or surge capacity.

Accepted Inputs

These are calculator validation limits, not recommended battery or load specifications.

Planned minimum charge cannot be higher than starting charge.

The optional Peukert model is limited to flooded, AGM, and gel lead-acid batteries.

Calculation Method

Total parallel-bank capacity:

Total bank Ah = Ah per battery × Parallel count

Selected battery fraction:

Selected fraction = (Starting % − Planned minimum %) ÷ 100

Condition-adjusted selected energy:

Selected Wh = Nominal voltage × Total bank Ah × Condition factor × Selected fraction

When load is entered as current:

Connected-load W = Nominal voltage × Entered current

Battery-side demand:

Battery-side demand W = Connected-load W ÷ Delivery efficiency + Fixed overhead

Simple runtime:

Simple runtime = Selected Wh ÷ Battery-side demand W

Nominal current:

Nominal current = Battery-side demand W ÷ Nominal voltage

Nominal C-rate:

Nominal C-rate = Nominal current ÷ Total bank Ah

For the optional lead-acid Peukert model:

Condition-adjusted Ah = Total bank Ah × Condition factor

Peukert runtime = Rating hours × [Condition-adjusted Ah ÷ (Nominal current × Rating hours)]^Exponent × Selected fraction

The calculator uses underlying unrounded values for calculations. Display precision changes only how results are shown.

Calculation Boundaries

The 12V Battery Runtime Calculator models DC runtime for one 12V-class battery or matching batteries connected in parallel.

It does not calculate:

• higher-voltage series-bank runtime;
• AC inverter-load runtime;
• battery charging time;
• required battery-capacity sizing;
• startup or surge-current requirements;
• wire, fuse, breaker, or connector sizing;
• exact BMS or low-voltage-disconnect behavior.