Inverter backup, solar, RV, UPS and more

Battery Runtime Calculator

This battery runtime calculator shows how long your battery will last under any load, or what size battery you need for the backup time you want. It accounts for battery chemistry, depth of discharge, inverter losses and series or parallel banks.

Runtime and battery size
Lead-acid and lithium
Inverter efficiency
Battery banks
Runtime curve

Battery backup calculator

Formulas

How to Calculate Battery Runtime

Battery runtime depends on how much energy the battery stores, how much of it you can safely use, the losses on the way and how much power your appliances draw.

Battery runtime

Hours = Ah × V × DoD × Efficiency ÷ Watts

A 12 V 100 Ah lead-acid battery at 50% depth of discharge, through a 90% efficient inverter, runs a 100 W load for 100 × 12 × 0.5 × 0.9 ÷ 100 = 5.4 hours.

Battery size needed

Ah = Watts × Hours ÷ (V × DoD × Efficiency)

To run 1,000 W for 1 hour from a 12 V LiFePO4 bank at 90% depth of discharge and 90% efficiency, you need 1,000 × 1 ÷ (12 × 0.9 × 0.9) = 103 Ah.

Amp-hours to watt-hours

Wh = Ah × Volts

Watt-hours let you compare batteries of different voltages. A 12 V 200 Ah battery and a 24 V 100 Ah battery both store 2,400 Wh.

Series and parallel banks

Series adds volts, parallel adds Ah

Two 12 V 100 Ah batteries in series make a 24 V 100 Ah bank. In parallel they make a 12 V 200 Ah bank. Both store 2,400 Wh.

Know your load. Add up the watts on each appliance label. If only amps are shown, convert them with the watts to amps calculator.

Reference

How Long Will a 12V Battery Last? Runtime Chart

Approximate runtimes for popular 12 V battery sizes through a 90% efficient inverter. Lithium batteries last much longer because more of their capacity is usable.

Lead-acid runtime (50% depth of discharge, 90% inverter efficiency)
12 V battery	50 W	100 W	200 W	300 W	500 W	1,000 W
50 Ah	5 h 24 min	2 h 42 min	1 h 21 min	54 min	32 min	16 min
100 Ah	10 h 48 min	5 h 24 min	2 h 42 min	1 h 48 min	1 h 5 min	32 min
150 Ah	16 h 12 min	8 h 6 min	4 h 3 min	2 h 42 min	1 h 37 min	49 min
200 Ah	21 h 36 min	10 h 48 min	5 h 24 min	3 h 36 min	2 h 10 min	1 h 5 min
300 Ah	32 h 24 min	16 h 12 min	8 h 6 min	5 h 24 min	3 h 14 min	1 h 37 min

LiFePO4 lithium runtime (90% depth of discharge, 90% inverter efficiency)
12 V battery	50 W	100 W	200 W	300 W	500 W	1,000 W
50 Ah	9 h 43 min	4 h 52 min	2 h 26 min	1 h 37 min	58 min	29 min
100 Ah	19 h 26 min	9 h 43 min	4 h 52 min	3 h 14 min	1 h 57 min	58 min
150 Ah	29 h 10 min	14 h 35 min	7 h 17 min	4 h 52 min	2 h 55 min	1 h 27 min
200 Ah	38 h 53 min	19 h 26 min	9 h 43 min	6 h 29 min	3 h 53 min	1 h 57 min
300 Ah	58 h 19 min	29 h 10 min	14 h 35 min	9 h 43 min	5 h 50 min	2 h 55 min

Chemistry

Battery Types and Usable Capacity

Draining a battery too deeply shortens its life. These are typical safe depths of discharge and cycle lives for daily use.

Typical usable capacity and cycle life by battery type
Battery type	Usable capacity	Typical cycle life	Best for
Flooded or tubular lead-acid	50%	500 to 1,200 cycles	Home inverters, budget backup
AGM lead-acid	50%	300 to 700 cycles	UPS units, sealed spaces
Gel lead-acid	50 to 60%	500 to 1,000 cycles	Deep cycle use, hot climates
LiFePO4 lithium	80 to 100%	2,000 to 6,000 cycles	Solar, RV, daily cycling
Lithium-ion NMC	80%	1,000 to 2,000 cycles	Portable power stations, EVs

Why ours

Why This Battery Runtime Calculator Is Better

Most battery calculators divide capacity by load and stop. That overstates runtime, sometimes by half.

Real usable capacity

Depth of discharge is set for your battery chemistry, so lead-acid and lithium results reflect what you can actually use.

Two calculators in one

Find how long a battery lasts, or flip the question and find the battery size you need for a target backup time.

Inverter losses included

AC appliances lose energy in the inverter. Turn it on for AC loads or off for DC devices like fridges in campervans.

Battery banks

Model several batteries in series or parallel and see the bank voltage, capacity and current at a glance.

Smart warnings

Get alerts for high discharge rates that cut lead-acid capacity and for deep discharges that shorten battery life.

Runtime curve

A live chart shows how runtime changes as you add or remove load, so you can decide what to keep on during an outage.

Guide

How to Use the Battery Runtime Calculator

Have your battery label and appliance wattages handy. Results update as you type.

Choose what to calculate
Pick How long it lasts for runtime, or Battery size needed for a target backup time.
Select the battery type
The usable percentage fills in automatically. Adjust it if your manufacturer recommends a different depth of discharge.
Enter battery details
Add the amp-hour rating and voltage from the label, plus how many batteries you have and how they are connected.
Add your load
Type the total watts or tap the quick-add buttons for common appliances. Set the inverter efficiency for AC loads.
Read and plan
Check the runtime, warnings and curve, then copy or share your result.

Benefits

Who Uses a Battery Runtime Calculator

Anyone who depends on stored power can plan better with accurate runtime numbers.

Home inverter and UPS owners

Know how long fans, lights and the TV will run during a power cut, and whether a second battery is worth it.

Solar and off-grid systems

Size a battery bank for overnight use and compare it with your panel output from the solar rooftop calculator.

RV, van and boat owners

Plan how long you can stay off-grid before running the engine or a generator, then size one with the generator size calculator.

Medical and critical devices

Make sure CPAP machines, routers and security systems stay powered through the night when the grid goes down.

FAQ

Battery Runtime FAQ

How long will a 100Ah battery last?

A 12 V 100 Ah battery stores 1,200 Wh. With a lead-acid battery used to 50% and a 90% efficient inverter, it runs a 100 W load for about 5.4 hours. A 12 V 100 Ah LiFePO4 battery used to 90% runs the same load for about 9.7 hours.

How do I calculate battery runtime?

Multiply the battery amp-hours by its voltage to get watt-hours, then multiply by the usable depth of discharge and the inverter efficiency. Divide the result by your load in watts to get runtime in hours.

What is depth of discharge?

Depth of discharge is the percentage of a battery’s capacity you use before recharging. Lead-acid batteries last longest when used to about 50%, while lithium iron phosphate batteries can safely use 80% to 100% of their capacity.

What size battery do I need to run 1000 watts for 1 hour?

Through a 90% efficient inverter you need about 1,111 Wh of usable energy. That is roughly 185 Ah of 12 V lead-acid at 50% depth of discharge, or about 103 Ah of 12 V LiFePO4 at 90% depth of discharge.

How long will a 150Ah inverter battery last?

A 12 V 150 Ah lead-acid inverter battery running a 300 W load lasts about 2.7 hours if used to 50%. Many home inverters drain batteries to about 80%, which stretches runtime to about 4.3 hours but shortens battery life.

What is the difference between series and parallel batteries?

Connecting batteries in series adds their voltages while the amp-hours stay the same. Connecting them in parallel adds their amp-hours while the voltage stays the same. Total stored energy in watt-hours is the same either way.

Why is real battery runtime shorter than calculated?

Batteries lose capacity with age and in cold weather, lead-acid batteries deliver less at high discharge rates because of the Peukert effect, and inverters draw some standby power even with no load. Plan for 10% to 20% less runtime to be safe.

Does an inverter waste battery power?

Yes. Inverters are typically 85% to 95% efficient, so part of the battery energy is lost as heat. They also draw standby power, often 10 to 30 watts, whenever they are switched on.

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

Multiply amp-hours by the battery voltage. A 12 V 100 Ah battery stores 1,200 Wh, and a 48 V 100 Ah battery stores 4,800 Wh. Watt-hours make it easy to compare batteries with different voltages.

Never Get Caught Without Power

Use the battery runtime calculator before you buy a battery, inverter or solar system, then size the rest of your backup setup with our related tools.

Calculate my runtime

Results are estimates. Real runtime varies with battery age, temperature, discharge rate and inverter standby power.