Have you ever found yourself on a hot and humid day, without power, and wondering just how long your car battery could power a fan to keep cool in a pinch?
I’ve had that same question and sought out the numbers to get some answers!
A car battery will power a typical household fan on high speed between 2.5 and 6 hours and still be able to start the car. Double these times for full capacity. The ranges depend on the type of fan being used.
How Long Can a Car Battery Power a Fan?
| Fan Type | 100% Discharge (Total Time in Hours) | 50% Discharge time in Hours (to still start your car) |
|---|---|---|
| Desk Fan | 12.76 | 6.38 |
| Tower Fan | 9.45 | 4.73 |
| Window Fan (Both fans on High) | 9.1 | 4.54 |
| Box Fan (High) | 5.31 | 2.66 |
| Box Fan (Med) | 6.21 | 3.11 |
| Box Fan (Low) | 7.97 | 3.98 |
| Oscillating Fan | 6.21 | 3.11 |
| USB Fan (3-watt) | 200 | 100 |
A car battery can power a fan — many different types of fans, in fact. We’ll take a look at the most common types you’ll probably run into, how I reached these numbers, and the best fan for conserving your battery’s life.
Before we cover each of the fans individually I’m going to briefly go over the car battery and the inefficiency factor of using an inverter.
This article will not be taking into account Peukert’s Law.
If you want though, you can also skip to any of the specific fans by clicking them below but you can refer to the battery section if you have any questions about the numbers I used.
How much Energy does my Car Battery have to Power my Fans?
For all of these examples below, we are going to assume your car battery has an ampere-hour (AH) rating of 50 hours. This is a common estimation in the battery world since car batteries are “starter batteries” which are rated in CCA (Cold Cranking Amps) and not AH like other “deep-cycle” batteries.
Theoretically, a car battery with a 50AH equivalency should be able to release 1 amp for 50 hours, 2 amps for 25 hours, etc.
With the exception of the USB fan, we’re looking to power household fans that run on alternating current (AC). Batteries supply direct current (DC).
In order to switch the current type, we need to use an inverter as an intermediary between the battery and the fan so that the direct current can be changed into alternating current. A 400w or 500w inverter would be suitable for any of the fans that we’re going to cover here, and this inverter is one with very solid reviews from Amazon at a great price point.
Due to the fact that the inverter is being used, we are going to lose about 15% of our efficiency overall when powering our fans. There’s not much you can do about it, it just comes with the territory when inverting DC current into AC.
With that inefficiency of 15%, remember to take the final amps required by your fan and divide them by 0.85 to get the actual total draw from the battery so that it will still be able to provide the required amps to the fan after passing through the inverter which will remove 15%.
The formula will be:
Estimated AH of the Car battery (50) / (Direct Current Amps drawn by the fan / 0.85)
Now, follow along and let’s get started!
How Long can a Car Battery Power a Personal Desk Fan?
When looking at different models, the average watts required by either the 9” or 12” versions was 40 watts.
If we want to know how many amps are required of direct current from your car battery, we need to divide 40-watts by 12, since we’re dealing with a 12-volt battery. Remember to factor in the 15% inefficiency of the inverter!
amps * volts = watts
(40W / 12V = 3.333 amps) ==> 3.333 / 0.85 = 3.92 amps required by the battery to pass through the 85% efficient inverter and still supply the fan with 3.333 amps.
Battery AH / (Amps being drawn by fan / 0.85) ⇒ 50 / (3.92) = 12.76 hours until fully discharged ⇒ 12.76 * 0.5 = 6.38 hours until the battery is at the lowest allowable limit before it won’t start the car any longer.
How Long can a Car Battery Power a Tower Fan?
A 42” tower fan uses 54 watts. 54 watts / 12 volt (car battery) = 4.5 amps
4.5 amps / 0.85 = 5.29 amps required by the battery to pass through the 85% efficient inverter.
A rough estimation would be:
50AH car battery / 5.29 amp draw from the fan = 9.45 hours until the battery is dead. 9.45 * .5 = 4.725 hours until the battery is at the lowest limit allowable before not being able to start the car.
How Long can a Car Battery Power a Window Fan?
A 9” twin window fan uses 56 watts with both fans on high speed. 56 watts / 12 volts (car battery) = 4.7 amps.
4.7 amps / 0.85 = 5.5 amps are required by the battery before passing through the 85% efficient inverter.
A rough estimation would be: 50AH car battery / 5.5 amps drawn from the fan = 9.1 hours until the battery is fully discharged ⇒ 9.1 * .5 = 4.54 hours until the battery is at the lowest allowable limit before it won’t start the car.
How Long can a Car Battery Power a Box Fan?
A 20” box fan uses 96 watts on high speed, 82 on medium, and 64 on low. When divided by a 12-volt car battery, they come out to 8 amps, 6.833 amps, and 5.33 amps, respectively.
After dividing them by 0.85 for the inverter’s 15% inefficiency, they are now: 9.41, 8.04, and 6.27.
High: 50AH Car Battery / (9.41) = 5.31 hours until the battery is dead ⇒ 5.31 * .5 = 2.66 hours until the car is at the lowest allowable limit before it can’t start the car.
Medium: 50AH Car Battery / (8.04) = 6.21 hours until the battery is dead ⇒ 6.21 * .5 = 3.11 hours until the car is at the lowest allowable limit before it can’t start the car.
Low: 50AH Car Battery / 6.27) = 7.97 hours until the battery is dead ⇒ 7.97 * .5 = 3.98 hours until the car is at the lowest allowable limit before it can’t start the car.
How Long can a Car Battery Power an Oscillating Fan?
An 18” Oscillating fan uses 82 watts. 82 watts / 12 volts = 6.833 amps.
6.833 / 0.85 = 8.04 amps required from the battery to pass through the 85% efficient inverter.
50AH Car Battery / (8.04) = 6.21 hours until the battery is dead ⇒ 6.21 * .5 = 3.11 hours until the car is at the lowest allowable limit before it can’t start the car.
How Long can a Car Battery Power a personal USB fan?
A typical 6” USB personal fan uses only 3 watts! This is also equivalent to .25 amps (3 watts / 12 watt battery). This is by far the best method for stretching out the life of your battery, though it won’t be as effective as larger fans at cooling you.
I think a fantastic option for everyday life as well as during power outages is this versatile mini-desk fan seen here on Amazon. It can be powered with a USB cord or the cord can also charge the fan’s internal battery. This thing sips power when running on USB, and a car battery should be able to recharge this fan’s battery 9 full times and still be able to start the car. Based on the fan speed, you can get 2.5-6 hours of use out of each battery charge.
For this example we don’t need to factor in the 15% inefficiency since an inverter is not required to run this fan.
With a set of clamps that have a DC plug on the end, you can simply insert a USB charger like you would in your car’s cigarette lighter port and you’ll be in business.
A rough estimation would tell you that: 50AH battery * 12-volts = 600 watt hours ⇒ 600 watt hours / 3 watts from the fan = 200 hours of operation until the battery is dead ⇒ 200 * .5 = 100 hours (4+ days) until the car battery is halfway dead and at the lowest allowable limit before it can’t start the car.
How many Times can I use my Car Battery to Power a Fan Before the Battery Dies?
Assuming your battery is healthy and new, it should be able to withstand a 50% discharge about 100 times before you kill the battery, or a 100% discharge about 10-12 times before it dies.
Remember, car batteries are not engineered to “deep-cycle” and they build up sulfation at a fast rate on the plates every time you discharge them. If you’re looking for a battery to do tasks like running a fan or lights, the ideal option would be to use deep-cycle marine batteries or golf cart batteries which are engineered for such a task.