How Long Will a 12v 18ah Battery Last?

18ah battery holds a small amount of power which is enough to run small appliances like bulbs, charging cell phones, & laptops. So the question begins how long will 18ah battery last while running appliances

The backup time of any battery size depends on

  • Battery’s depth of discharge limit (DoD)
  • Output AC/DC load
  • Inverter efficiency

In short, a 12v 18Ah battery will last 20 hours while running a 12v 0.9A (10.3 watts) output load which will fully drain the battery so multiply the hours by 0.5 for a lead-acid battery and 0.8 for a lithium 18ah battery

still not sure? it’s hard to tell how long your battery will last in a few lines because the calculation is not that simple and there are many things to keep in mind. So, keep reading the post for a better understanding

Now first of all let’s discuss the 18Ah battery capacity in watts which will make further calculations easy

What Does 12V 18Ah Battery Mean? – How Many Watts?

Battery capacity is measured in Amp-hours (Ah) but the capacity of appliances is measured in watts. So to make things clear let’s calculate the 18Ah in watts or Wh

Use this formula to calculate the battery capacity in watts (watts = battery Ah * Battery voltage)

So in this case we have a 12v 18Ah battery

18 * 12 = 216 watts

12v 18Ah means 216wh

but if you have a lead-acid battery then they are recommended to be discharged at only 50%. Discharging them below 50% will affect the lifespan of your battery

but if you have a lithium-ion battery that can be discharged between 80-100% but 80% is recommended

for lead-acid batteries multiply the watt capacity of the 18Ah battery by 0.5 and for lithium-ion batteries multiply it by 0.8

216 * 0.5 = 108 
216 * 0.8 = 172.8 

You can draw 108 DC watts from a fully charged 12v 18Ah lead-acid battery and 173 DC watts from a lithium battery

Video – Battery Capacity Explained

AC vs DC Watts & Inverter Efficiency

Batteries store power in DC (Direct current) but most of household appliances use AC (Alternating current) so to convert the DC into AC we use an inverter

DC appliances use 12v but AC appliances are designed to consume 110-120 volts.

A solar inverter is a device that converts the Direct current (12v) coming from the battery into Alternating Current (110-120V) to run your appliances.

100W AC appliance will draw 0.9 amps from your battery which is the ideal discharge rate for an 18Ah battery which I’ll discuss in a moment

While converting DC into AC there will be some power loss. Most of the inverters are 85-95% efficient which means a 100w inverter will draw 115 DC watts from the battery and will run a 100W AC appliance.

85% efficiency rate is normal in inverters so we’ll go with this number.

Make your calculations according to your inverter efficiency rate by reading the manufacturer’s instructions or product knowledge

108 * 0.85 = 91 AC watts, & 173 * 0.85 = 146 AC watts 

With an 85% efficiency rate inverter, you’ll get 91 AC watts from a 12v 18Ah lead-acid battery (50% DOD) and 146 AC watts from a lithium-ion battery (80% DoD)

your lead-acid 12v 18ah battery will last 1 hour while running the 90W AC appliance & lithium battery for one hour while running the 140W AC appliance

Battery Discharge rate & Backup time

Deep-cycle batteries are designed to supply a small amount of current for a long period of time.

Every size battery has its amps discharge rate (Depending on the brand) 18Ah batteries mostly have a discharge rate of 20 hours which means they are recommended to be discharged at 0.9 amps per hour

You can draw 0.9 Amps from an 18Ah battery with 12 volts for 20 hours. As the discharge amps will increase it will affect the battery backup time.

you can draw up to 2 amps per hour without making any difference in battery capacity but when the discharge rate will increase by 2 amps from an 18Ah battery the battery backup time will also decrease

For example: discharging your 18Ah battery with an 18A discharge rate your battery will last 30 minutes instead of 1 hour or 15 minutes if you have a lead-acid battery

This is why most of household appliances use 110-120v which will decrease the Amps draw (Amps = watts/volts)

and as the amps will increase thicker the cable size you’ll need can pass the current.

This is why most of our household appliances are designed in this way and our home electricity also uses 110-120 volts

Reminder: For better understanding read the instructions or about product knowledge on the backside of your battery or get this information from your manufacturer

How to calculate battery backup time (Step-by-step)

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 18ah battery

18ah battery in watts = 18 * 12 = 216 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 = 216 * 50% = 108 wh

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 = 108 * 90% = 98 AC watt-hours 

4- Divide the battery AC watt-hours by the total load connected. Let’s say a 50-watt AC appliance is connected to your 18ah battery

18ah battery runtime on 50w appliance = 98 / 50 = 1.9 hours

Frequently Asked Question

What does 12v 18ah 20HR mean?

12v 18Ah 20HR means you can draw 0.9A or 10.8 watts from your 18Ah battery for 20 hours with a 12v load ( 18/20 = 0.9). This is the recommended discharge rate for a Deep cycle 18Ah battery

12v 18ah battery how many watts?

12v 18ah battery is equal to 216 watts (Ah * volts = watts)

What size solar panel to charge a 12v 18ah battery in 5 hours?

50-watt solar panel will charge a 12v 18ah battery in 5 peak sun hours from 100% depth of discharge

How long will an 18ah lithium battery last?

12v 18Ah lithium battery with an 80% discharge limit will last 14 hours while running a 12v 1A input device

20ah 18ah battery last time?

Amp-hour (Ah) is the unit to measure the battery capacity so a 20Ah battery will last 10% longer than an 18Ah battery while running the same capacity load


how long will a 12v 18Ah battery last? the answer to this question is not that simple. There are many factors that will affect the battery backup time like battery DOD limit, Output AC/DC load, inverter efficiency & battery discharge rate which I have explained in detail

I hope this guide was helpful to you if you have any queries contact us. Thank You!

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.