Battery Runtime Calculator

This battery runtime calculator estimates how long a battery will power a load before it reaches a safe discharge limit. Enter capacity in amp-hours and volts or directly in watt-hours, choose the chemistry, and type your load in watts or amps. The result shows runtime in hours and minutes.

It accounts for the two things that most often make real runtime shorter than a simple Ah times volts sum: the usable depth of discharge for your battery type, and inverter losses when you run AC appliances. It works for RV and van batteries, solar storage, trolling motors, CPAP backup and portable power stations.

How to Use the Battery Runtime Calculator

1
Enter the battery capacity
Use the Amp-hours + volts tab to enter Ah and battery voltage, or switch to Watt-hours if your battery or power station lists Wh.
2
Choose the chemistry
Pick flooded, AGM or gel lead-acid, LiFePO4 or lithium-ion NMC. The usable depth of discharge fills in automatically, and you can edit it.
3
Describe the load
Choose AC through an inverter or DC directly from the battery, then enter the load in watts or amps. For amps, also enter the load voltage.
4
Check inverter efficiency and read the runtime
For AC loads, keep 90% inverter efficiency or change it to your inverter's rating. Read the runtime, usable watt-hours and watt-hours delivered to the load.

How the battery runtime formula works

A battery's energy in watt-hours is its amp-hour rating times its nominal voltage. You should not use all of it. Lead-acid batteries, including AGM and gel, last far longer if you stop at about 50% depth of discharge. LiFePO4 batteries tolerate much deeper cycling, so 80 to 100% is commonly used; the calculator defaults to 90%.

If you run AC appliances, an inverter converts the battery's DC power and loses some energy as heat. The calculator multiplies by the inverter efficiency for AC loads and skips that step for DC loads.

Runtime (hours) = battery Wh x depth of discharge x inverter efficiency / load watts; Wh = Ah x volts; load W = amps x load volts

Worked example: a 100 Ah battery running a 100 W load

Compare a 12 V 100 Ah AGM battery with a 12.8 V 100 Ah LiFePO4 battery, each running a 100 W AC load through a 90% efficient inverter.

AGM: 100 Ah x 12 V = 1,200 Wh. At 50% usable, that is 600 Wh. After the inverter: 600 x 0.9 = 540 Wh. Runtime: 540 / 100 = 5.4 hours, or 5 h 24 min
LiFePO4: 100 Ah x 12.8 V = 1,280 Wh. At 90% usable, that is 1,152 Wh. After the inverter: 1,152 x 0.9 = 1,036.8 Wh. Runtime: 1,036.8 / 100 = 10.4 hours, or 10 h 22 min
Same amp-hour rating, but the lithium battery gives nearly twice the runtime because more of its capacity is usable

Runtime chart for AC loads (90% inverter efficiency)

Results from this calculator with the default depth of discharge for each chemistry. Real lead-acid runtimes at 500 W will be shorter because of the Peukert effect described below.

Battery	50 W load	100 W load	500 W load
12 V 50 Ah AGM (50%)	5 h 24 min	2 h 42 min	32 min
12 V 100 Ah AGM (50%)	10 h 48 min	5 h 24 min	1 h 5 min
12 V 200 Ah AGM (50%)	21 h 36 min	10 h 48 min	2 h 10 min
12.8 V 100 Ah LiFePO4 (90%)	20 h 44 min	10 h 22 min	2 h 4 min
12.8 V 200 Ah LiFePO4 (90%)	41 h 28 min	20 h 44 min	4 h 9 min

Why real runtime is often shorter

Peukert effect: lead-acid amp-hour ratings are usually measured over a slow 20-hour discharge. Draw power faster and the battery delivers less than its rating. The calculator shows a warning when a lead-acid runtime comes out under 10 hours.
Cold temperatures reduce available capacity, especially for lead-acid. Many LiFePO4 batteries also should not be charged below freezing.
Inverters draw some power even with no load. A large inverter left on overnight can use a noticeable share of a small battery.
Batteries lose capacity with age and cycles, so a battery a few years old may hold well below its label.
Appliances with compressors, like fridges, cycle on and off. Use their average draw over time, not the nameplate peak, for a fairer estimate.

Battery safety

Follow the battery maker's limits for discharge current, depth of discharge and charging, and use correctly sized cables and fuses between the battery and inverter. Flooded lead-acid batteries vent hydrogen while charging and need ventilation. For home backup systems tied into house wiring, have a licensed electrician do the installation.

Features

Capacity in amp-hours plus volts, or watt-hours
Chemistry presets for usable depth of discharge (lead-acid 50%, LiFePO4 90%, NMC 80%), editable
Load in watts or amps, for AC loads through an inverter or DC loads
Adjustable inverter efficiency (default 90%)
Runtime in hours and minutes, plus usable and delivered watt-hours
Peukert warning for heavy loads on lead-acid batteries

Frequently Asked Questions

How long will a 100Ah battery last?

It depends on the chemistry and the load. A 12 V 100 Ah AGM battery holds 1,200 Wh, but only about 600 Wh is usable at 50% depth of discharge. Running a 100 W AC load through a 90% efficient inverter, that is about 5 h 24 min. A 12.8 V 100 Ah LiFePO4 battery at 90% usable lasts about 10 h 22 min on the same load.

How do I convert amp-hours to watt-hours?

Multiply amp-hours by the battery's nominal voltage. A 12 V 100 Ah battery holds 100 x 12 = 1,200 Wh, and a 12.8 V 100 Ah LiFePO4 battery holds 1,280 Wh. Watt-hours make it easy to compare batteries of different voltages and to divide by an appliance's watts to estimate runtime. Remember that not all of those watt-hours are usable.

Why should I only use 50% of a lead-acid battery?

Deep discharges wear out lead-acid batteries much faster. Stopping around 50% depth of discharge is a widely used rule that balances usable energy against cycle life, and it applies to flooded, AGM and gel types. You can discharge deeper in an emergency, but regular deep cycling shortens the battery's life. The calculator uses 50% by default and lets you change it.

How much usable capacity does a LiFePO4 battery have?

LiFePO4 batteries handle deep discharge well, so 80 to 100% of rated capacity is commonly used. The calculator defaults to 90% as a middle ground that leaves a small reserve. Most LiFePO4 batteries have a built-in battery management system that cuts off power before damage, so check your battery's manual for its recommended depth of discharge.

How long will a battery run a CPAP machine?

Find your CPAP's average power draw in watts, which varies with pressure and whether the humidifier is on. Running it on DC without the humidifier avoids inverter losses and uses much less power. For example, at a 40 W DC draw, a 12 V 100 Ah AGM battery at 50% usable gives about 15 hours. Check with your equipment supplier for the right battery setup.

Does using an inverter reduce battery runtime?

Yes. An inverter loses energy converting DC to AC, typically around 10% for a good unit and more at very light loads. The calculator applies your inverter efficiency to AC loads only. For example, a 600 Wh usable battery delivers 540 Wh at 90% efficiency, so a 100 W AC load runs 5.4 hours instead of 6 hours on DC.

Last reviewed September 21, 2026