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

**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 directscience.com 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 type | 18ah battery usable capacity (in ah) |
---|---|

Lead acid | 15ah |

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.

### Steps

**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 capacity | Hours of discharge |
---|---|

18ah | 20 hours |

16.2ah | 10 hours |

15.6ah | 8 hours |

14.7ah | 6 hours |

14.4ah | 5 hours |

12.6ah | 3 hours |

10.8ah | 2 hours |

9ah | 1 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

Appliance | 12v 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}

#### summary

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

### 12v 18ah Lithium battery

Appliance | 12v 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 |

#### summary

- 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