300Ah Battery Backup Time: How Long It Will Last? + Calculator

how long will 300ah battery last?

12v 300ah lithium battery with a 100% DoD limit will last between 32 hours (running a 100-watt AC appliance) to 3 hours (running a 1000-watt AC appliance).

In this post, you’ll learn how long will a 300ah battery last running a different size AC or DC load (with examples).

Also, I’ll share the manual calculation method and some other important points to keep in mind that the other websites aren’t talking about.

Need an easy solution? use our battery runtime calculator ( given below ) and ” Enter the battery specs and the total load in watts ” to find out how long will a 300ah battery last

Battery Runtime Calculator

Battery Capacity
Battery Volts
Select Battery Type
Load Connected with inverter?
Total Load (watts)

Note! use my solar panel size calculator to find out what size solar panel you need to recharge your battery in desired hours with solar panels.


  • Inverter Efficiency: 90%
  • Lead-acid battery: 50% depth of discharge limit
  • Lithium (LiFePO4): 100% depth of discharge limit

Battery Depth of discharge DoD: is the percentage of the battery that has been discharged relative to the total battery capacity. The recommended depth of discharge limit for lead-acid, AGM, and gel batteries is about 50%. Only lithium (LiFePO4) batteries can be fully discharged to 100%.

How To Use The Calculator

  • Enter the battery capacity in amp-hours (Ah): in this case, it’ll be 300
  • Enter the battery voltage: is this a 12, 24, or 48-volt battery?
  • Select the battery type:  Select “Lead-acid” if you’re using a flooded or sealed (AGM or gel) lead acid battery. Select “Lithium (LiFePO4)” if you use a lithium-iron-phosphate battery.
  • load Connected with inverter?: Are you using an inverter to run the load or is the load connected directly through the battery?
  • Enter total load in watts: Enter the total load in watts (e.g 1000 for 1000 watts of output load)

After entering the information, click “Calculate” button to find out how long the battery last running a load.

if you’ve got some time keep reading… I’ll share a manual calculation method that you can use in the future to calculate any size battery runtime on any load and also some additional information about batteries that will affect the battery last time.

How Long Will A 300Ah Battery Run A Refrigerator? – Example

The average power consumption regular-size kitchen fridge is about 350 watts per hour.

How Long Will A 300Ah Battery Run A Refrigerator

12v 300ah lead-acid battery (with 50% DoD limit) will run a fridge for about 4 hours and 40 minutes

Video – All about battery basics (Must watch!)

Must watch this video to understand the basics of batteries (capacity, charge, and discharge mechanism)

Steps to calculate battery runtime

Follow these steps to calculate the battery life/runtime while running a load.


1- Converter battery ah into watt-hours (Wh). to calculate the battery capacity in watt-hours use this formula

Battery capacity in watt-hours = Battery amp-hours (Ah) * Battery Volts 

let’s say you have a 12v 300ah battery

300ah battery in watts = 300 * 12 = 3600 watt-hours 

2- Multiply the battery watt-hours by the battery depth of discharge limit. ( Default battery depth of discharge limit, Lead-acid battery: 50% DoD limit; Lithium: 100% DoD limit ). Let’s say you have a lead-acid type battery

Battery capacity in wh after DoD limit = 3600 * 50% = 1800wh

3- Multiply the battery capacity after the DoD limit with the inverter efficiency rate to calculate the AC watt-hours ( Most of the inverters are about 90% efficient)

Battery AC watts = 1800 * 90% = 1620 AC watt-hours 

4- Divide the battery AC watt-hours by the total load connected. Let’s say a 1000-watt appliance is connected to your 300ah battery

300ah battery runtime on 1000w appliance = 1620 / 1000 = 1.6 hours

How long will 300ah battery last? – chart

Here’s a chart on how long will a 12v 300ah lead-acid battery will last while running a different watt load.

Appliance Power Required300ah lead-acid Battery Runtime
20 watt81 hours
50 watt 32 hours
100 watt16 hours
150 watt11 hours
200 watt8 hours
300 watt5.5 hours
400 watt4 hours
500 watt3 hour
800 watt2 hours
1000 watt1.6 hours
1200 watt1.3 hours
1500 watt1 hour
2000 watt50 minutes
3000 watt33 minutes


  • 12v 300ah lead-acid battery with 50% DoD limit last anywhere from 16 hours to 50 minutes

Assumptions & Shortcomings of All These Methods

Here are some extra yet important points to keep in mind when figuring out how long will your 300ah battery will last running a load

Battery life cycle count:

  • The storage capacity of the battery will decrease with every cycle ( a cycle is when you charge and discharge the battery to its DoD limit ).
  • if you have used your battery for a few hundred cycles then the result will be different. ( this factor isn’t included in my above-given methods )

Batteries are rated for 5 amps over 20 hours of time:

  • Especially lead-acid type batteries are designed to be discharged at 20 hours. ( means 300ah lead-acid batteries are recommended to be discharged @ 15 amps in order to maintain the battery health and reduce the discharge efficiency ).
  • on the other hand, AGM and lithium (LiFePO4) can be discharged at 50% of their capacity.
  • Discharging your battery at a higher rate will increase heat in the battery’s internal cells which will cause power loss. (this factor isn’t included in my above-given methods).
  • For example, if you discharge your lead-acid battery with 100 amps then it’s will not last 1.5 hours ( keep in mind the 50% DoD limit ). but it may last about 0.8 – 1 hour
  • Check the battery discharge rate on the specs sheet of your battery or it’ll be mentioned on the battery (1h → 1C, 2h → 0.5C, 5h → 0.2C, etc.)

Related FAQ’s

How many watts is 300Ah?

300ah is equal to 3600 watts @12v, 7200 watts @24v, and 14,400 watts at @48v

How many kWh is a 300Ah battery?

300ah is equal to 3.6 kWh @12v, 7.2 kWh @24v, and 14.4 kWh at @48v

How long will a 300Ah AGM battery last?

12v 300ah AGM battery will last between 20 hours (running 160 watts of load) to 5 hours (running 650 watts of AC load)

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I'm an experienced Sales Consultant with a demonstrated history of working in the renewables and environment industry. Skilled in Photography, Customer Service, and Sales.