12V Battery Run Time Calculator

Calculator Inputs

Battery voltage per unit

Capacity per battery

Capacity unit

Batteries in series

Parallel battery strings

Load entry method

Load power

Load current

Duty cycle percent

Standby or inverter idle watts

Depth of discharge percent

Extra reserve percent

Inverter or controller efficiency percent

Cable loss percent

Battery health percent

Temperature capacity factor percent

Peukert exponent

Capacity rating hours

Surge watts

Target runtime hours

Example Data Table

Battery	Capacity	Load	Efficiency	DoD	Estimated Runtime
12V AGM	100Ah	60W	90%	50%	About 9 hours
12V LiFePO4	100Ah	60W	95%	80%	About 15 hours
12V Deep Cycle	200Ah	120W	90%	70%	About 12 hours

Formula Used

System voltage = battery voltage × batteries in series.

Bank capacity = single battery Ah × parallel strings.

Nominal energy = system voltage × bank capacity Ah.

Effective DoD = depth of discharge − reserve percent.

Usable Wh = nominal Wh × effective DoD × efficiency × cable factor × health × temperature factor.

Average load W = load watts × duty cycle + standby watts.

Runtime hours = usable Wh ÷ average load W.

Peukert runtime = H × (C ÷ (I × H))^k.

Here H is the capacity rating time. C is usable amp hours. I is battery current. k is the Peukert exponent.

How to Use This Calculator

Enter the battery voltage. Use 12 for a normal 12V battery.

Add the capacity shown on the battery label. Choose Ah, mAh, or Wh.

Set series and parallel counts. Keep both at one for a single battery.

Enter the load as watts or amps. Add standby watts for inverter idle draw.

Adjust depth of discharge, reserve, efficiency, cable loss, health, and temperature.

Use Peukert exponent near 1.00 for lithium. Use a higher value for lead acid.

Press Calculate. The result appears above the form and below the header.

Use CSV or PDF buttons to download the same calculated result.

12V Battery Runtime Planning Guide

Why Battery Run Time Matters

A 12V battery can power lights, fans, pumps, routers, tools, and small inverters. Runtime is not only capacity divided by load. Real systems lose energy through wiring, conversion, heat, age, and discharge limits. This calculator treats those details as adjustable inputs. That makes the estimate more useful for field work.

Key Electrical Ideas

Battery capacity is often shown in amp hours. Energy is shown in watt hours. A 12V 100Ah battery stores about 1200Wh before losses. The usable part depends on depth of discharge. Lead acid batteries often need a larger reserve. Lithium batteries can usually use more stored energy. The load also matters. A steady 60W load is easier to plan than a motor with a high starting surge.

Derating And Efficiency

No battery gives perfect output forever. Inverters waste some energy while converting DC to AC. Long cables also lose power. Cold weather can reduce available capacity. Older batteries may deliver less than their label states. This calculator lets you enter each factor. It then builds an effective energy value. The result is closer to a real installation.

Peukert Effect

Many lead acid batteries lose effective capacity at high discharge current. This behavior is called the Peukert effect. A higher exponent means runtime falls faster as current rises. Lithium batteries have a smaller effect. For simple estimates, use an exponent close to one. For flooded lead acid, a higher value may be suitable.

Planning A Safe System

Use the result as a planning estimate, not a promise. Check the battery manual before final design. Compare the calculated current with battery and cable ratings. Leave reserve capacity for unexpected loads. Include inverter idle draw when using AC appliances. If the load starts a compressor or pump, review the surge current too. A good backup system is sized with margin. It should run the load without deep cycling the battery every day.

Practical Output Review

The calculator reports runtime in hours, minutes, and days. It also shows bank energy, average load, battery current, and target capacity. These values help compare different batteries. Try several scenarios before buying equipment. Small changes in load or reserve can change runtime a lot during real use and field testing.

FAQs

How do I calculate 12V battery runtime?

Convert battery capacity to watt hours, apply usable depth of discharge and losses, then divide by average load watts. The calculator performs these steps and adds optional Peukert adjustment.

Why is actual runtime lower than the label suggests?

Battery labels show nominal capacity. Real output is reduced by inverter losses, wiring losses, cold temperature, aging, high current, and safe discharge limits.

What depth of discharge should I use?

Use the battery manual first. Many lead acid systems use 50 percent or less. Many lithium systems allow deeper discharge, often near 80 percent or more.

What is Peukert exponent?

It describes capacity loss at higher discharge current. A value near 1.00 has little effect. Higher values reduce runtime more as current rises.

Should I enter inverter idle draw?

Yes. Add it as standby watts. Small idle loads can matter during long backup periods, especially with low power devices.

Do parallel batteries increase runtime?

Yes. Parallel batteries increase amp hour capacity at the same voltage. Series batteries increase voltage, but amp hours remain the same for one string.

Can this calculator size a solar battery bank?

It can help with storage estimates. Solar design also needs charging hours, panel output, controller limits, weather, and daily energy use.

Is the PDF export created without extra libraries?

Yes. The file creates a simple text PDF from the calculated results. It is meant for quick records and field reports.