How Long Will a 12v 18ah Battery Last?

When using a 12v 18ah battery to power devices, it's important to understand how long 18ah battery will last before needing to be recharged.

Most of the answers out there will give you a 'theoretical' answer. But in real-world conditions, battery runtime is affected by several factors, such as battery discharge efficiency, speed of discharge, and temperature.

Generally, a 12v 18ah battery will last around 20 hours when powering a device that draws 0.9 amps of current. However, this can vary depending on the load and how fast the battery is being discharged.

Need an easy solution? Use our battery runtime calculator to find out your 18ah battery's runtime.

amp-hour, watt-hours, and Battery discharge Efficiency

Power is measured in watts. Here are some formulas to remember:

  • Watts = Volts × Amps
  • Amps = Watts ÷ Volts
  • Volts = Watts ÷ Amps
Watch this video to understand the basics of battery capacity and what it means in real life

Amp-hours (Ah) is a unit of measurement that tells us how much electrical charge (in amps) a battery can provide over a certain amount of time. If a battery has a capacity of 1 Ah, it means that it can deliver a current of 1 amp for one hour.

Watt-hours (Wh) is the energy used by an electrical device over time. It is equivalent to the energy used by a 1-watt device for one hour.

Battery depth of discharge limit: Lead acid, AGM, and gel batteries are usually recommended to be only discharged 50% only lithium batteries can be 100% discharged. Meaning you can only use 9 amps from a 18ah lead acid battery.

Battery discharge efficiency: Unfortunately, batteries are not 100% efficient when discharging. The efficiency rate will depend on the chemistry of the battery.

Based on data:

  • Lead-acid batteries discharge efficiency ≈ 80 − 85%
  • Lithium-ion batteries discharge efficiency ≈ 90 − 95%

Here’s a table on 18ah battery usable amp-hours after considering discharge efficiency (Lead acid: 85%, and Lithium: 95%).

Battery type18ah battery usable capacity (in ah)
Lead acid15ah
Lithium (LiFePO4)17ah
Table: 18ah battery usable capacity after its discharge efficiency rate

What Does 12v 18ah Mean On Battery?

18ah or 18 amp-hours battery means it can provide 1 amp for 18 hours or 18 amps for 1 hour “theoretically”.

In order to determine how much power a battery can actually hold, we measure its capacity in watt-hours.

Formula: Battery capacity Wh = Ah × Volts

12v 18ah battery is equal to 216 watt-hours and 24v 18ah battery is equal to 432 watt-hours. This means 18 amp-hour 24v battery stores double the power than a 12v same-capacity battery.

Now let’s dive into the calculation part and at the end, I’ll share why is it impossible to get a 100% accurate runtime.

Related post: Battery Amp Hours To Watts (Ah To Watts) Calculator

5 Steps To Calculate 12v 18ah Battery Runtime

Follow these steps to calculate your 12v 18ah battery runtime.


1. Converter battery amp-hours (Ah) into watt-hours (Wh). Let’s say you have a 12v 18ah battery.

18ah battery in watts = 18 × 12 = 216 watt-hours 

2. If you're using a lead-acid battery, remember to multiply its watt-hour capacity by 0.5 to adjust for the depth of discharge limit. For example, if you have 18ah lead-acid battery:

Battery capacity in wh after DoD limit = 216 × 50% = 108wh

3. To account for battery discharge efficiency, multiply the battery capacity after the depth of discharge (DoD) by 0.85 for lead-acid batteries and 0.95 for lithium-ion batteries.

Battery usable watt-hours: 108 × 0.85 = 91.8wh

4. To consider the inverter efficiency rate, multiply the battery's usable capacity by the inverter's efficiency rate, which is usually around 90%.

Battery AC watts = 92 × 90% = 82.8 AC watt-hours 

5. Divide the battery AC watt-hours by the total load connected. Let’s say you’re running a 25-watt bulb with a 12v 18ah battery.

18ah battery runtime on light bulb = 83 ÷ 25 = 3.3 hours

Turns out, a 12v 18ah lead acid battery will run 25-watt LED light for about 16 hours.

Note: Use our solar panel size calculator to recharge your battery in desired hours.

How Discharging Rate Impacts Your Battery's Usable Capacity

Peukert's law states that if a battery is used at a faster rate, it won't be able to provide the same amount of energy as it's labeled.

This is because when a battery is discharged quickly, more energy is wasted as heat instead of being used to power devices.

However, this law only applies to lead-acid batteries. On the other hand, lithium batteries can be safely discharged at 50% of their capacity without any power loss. Meaning you can discharge your lithium battery 2 hours without causing any power losses.

The table shows the usable capacity of a 18ah lead-acid battery in amp-hours based on the rate of discharge.

Usable 18ah lead acid battery capacityHours of discharge
18ah20 hours
16.2ah10 hours
15.6ah8 hours
14.7ah6 hours
14.4ah5 hours
12.6ah3 hours
10.8ah2 hours
9ah1 hour
Note: This table doesn’t take into account lead acid’s 50% DoD limit.

How Temperature Affects Battery Performance

The temperature can impact how well your battery works. Generally, batteries perform best when they are between 68°F and 77°F (20°C to 25°C). If the temperature goes above or below this range, it can affect the battery's performance. As a general rule, a temperature increase to 77ºF or 25º C can cut battery performance by 50%.

To ensure that your battery works efficiently, it's important to keep it at the right temperature. Place your battery in a spot where it's neither too hot nor too cold.

How long will a 12v 18ah battery last? - chart

Here are charts on how long will 12v 18ah lead acid and lithium batteries will last running different small household appliances.

12v 18ah lead acid battery

Appliance12v 18ah battery runtime
Small LED bulb (10w)8 hrs
Medium LED bulb (25w)3 hrs
TV (60w)1 hrs
Laptop (120w)40 minutes
Wifi router (20w)4 hrs
Small table fan (20w)4 hrs
Note: This table doesn't take into account Peukert's law and temperature


  • 12v 18ah lead acid battery will last between 8 hours to 40 minutes while running different appliances.

12v 18ah Lithium battery

Appliance12v 18ah battery runtime
Small LED bulb (10w)18.5 hrs
Medium LED bulb (25w)7.4 hrs
TV (60w)3 hrs
Laptop (120w)1.5 hrs
Wifi router (20w)9 hrs
Small table fan (20w)9 hrs


  • 12v 18ah lithium battery will last between 18 to 2 hours while running different appliances.

18ah battery: FAQs

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)

How long will an 18ah lithium battery last?

12v 18Ah lithium battery will last about 18 hours while running a 1amp or 12w 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

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Chris Tsitouris is a renewable energy professional with 10+ years of experience as Director of Engineering at Solar Spectrum, previously working as Project Manager at SunPower and Energy Analyst at the National Renewable Energy Laboratory. As a thought leader, Chris has authored numerous articles and research papers.

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