How long will 100 watts last on a car battery?

In our article discussing Ah (ampere-hours) and Wh (watt-hours), we got a ton of questions about the longevity of batteries. The question “How long does a battery last?” was a predominant one. To help everybody trying to calculate how long will a battery last, we have created a Battery Life Calculator.

It’s quite useful knowing when a battery will die on us. Example: If we go camping and depend on batteries for all our power needs, and we have no other means of generating electricity.

Before we check out the Battery Life Calculator, let’s note that figuring out how long will a battery last is pretty simple in theory (in practice, it’s actually quite difficult). We use this equation for battery drain time:

Battery Life (in hours) = Battery Capacity (in Ah) / Load Current (in A)

What does ah mean on a battery? It just means amp-hours. 1 Ah is a current of 1 amp running for 1 hour.

Example: How long will a 100 Ah (amp-hour) battery last if we hook it up to a 1 Ah electric device? Well, battery capacity = 100 Ah, load current = 1 A, thus such a battery will last for 100 Ah / 1 A = 100 hours.

Basically, a 100 Ah battery means that such a battery can provide 100 A of current for 1 hour. It can also provide 1 A current for 100 hours. Or 0.1 A or 100 mA for 1000 hours.

It seems quite simple, right?

If you have 100 capacity units (100 Ah) and you connect it to a device that requires 1 capacity unit (1 A) every hour, it will drain the battery in precisely 100 hours.

Why Calculating The Battery Life Is Not Exactly Easy

Here’s the deal:

In practice, we only need two numbers to calculate when the battery will die on us. These are:

1. Battery capacity (in Ah). This one is pretty easy to get; it’s written right on the battery. Typical AA battery has 2.5 Ah or 2500 mAh (milli-amp-hours) capacity, AAA battery has 1 Ah capacity, laptop battery has 2 Ah to 6 Ah, 100 Ah battery has Ah capacity, and so on. You can read more about battery capacities here.
2. Load Current or Amp Draw (in A). This is the tricky one; and the whole reason why calculating battery lifetime is difficult. Load current determines how fast the electrical capacity will be drawn from the battery, and depends on the power of the unit attached to it. 1000 W air conditioner, for example, will have a 10 times as big a load current than a 100 W personal evaporative cooler.

If you get these two numbers, you just divide battery capacity with load current and get how many hours a battery will last.

The problem is that questions about battery life are not posed in this way:

“I have a 100 Ah battery and want to run a camping light with a load current of 1 Ah with it. How long before the battery runs out?”

Most of us deal with watts (W). We don’t know what the load current of a 100 W light is. We just know that it’s a 100 W light, right. That’s why most questions about how long batteries last go along these lines:

“I have a 100 Ah battery and want to run a 100 W camping light with it. How long before the battery runs out?”

To adequately calculate the battery lifespan, we need to transform that 100 W into Ah. Here the voltage (V) plays the key role.

We want everybody to be able to determine how long will their battery life last. That’s why we feature 3 key sections that will help you out to do just that:

1. How to calculate the load current of any device. We start with knowing wattage (W) and voltage (V), and we’ll be able to calculate how many amps (A) does such a device needs to run. If you can calculate the amp draw (or load current), you can use the Battery Life Calculator.
2. Battery Life Calculator. You just input the battery capacity that’s written on your battery (in Ah) and the calculated amp draw (load current), and the calculator will tell you how many hours the battery will last.

Let’s start with the basics: How to get from watts to amps?

How To Calculate Load Current (Amps) From Wattage?

Imagine a simple enough scenario. You have a big 200 Ah lithium battery and want to run a small 800 W portable air conditioner with it. How long can you run such an AC before the battery dies out?

Well, we already know that we need 2 numbers:

1. Battery capacity. We have that; it’s 200 Ah.
2. Amp draw. That we don’t have; we have to calculate it.

To calculate amp draw (A) from watts (W), we also need to know the voltage (V). To calculate amps, we use the basic electric power equation:

P (in W) = I (in A) * V (in V)

Basically, electric power P (wattage) is calculated by multiplying electric current I (amps) with voltage V (volts). To calculate amps, you have to express the electric current I (amps) like this:

I (in A) = P (in W) / V (in V)

This basically tells us that we get the amps by dividing watts by volts.

Example: We have an 800 W AC unit that runs on a 120 V electric circuit. What’s the amp draw here? Easy, we just divide 800 W by 120 V and get 800W/120V = 6.67 A.

If you find this confusing a bit, you can use our watts to amps calculator here to help you out with the calculation.

In our example above, we have calculated the amp draw of the 800 W AC. It’s 6.67 A. Now we have both numbers; we have a 200 Ah battery and we know the AC has a 6.67 A draw. How long will a 200 Ah battery last if it has to power this AC? Let’s calculate:

200 Ah Battery Life = 200 Ah / 6.67 A = 30 hours

In short, a 200 Ah battery will be able to power an 800 W 120 V air conditioner for about 30 hours.

Now, it’s important that we feel the effect of different voltages. Let’s say that we have the same 200 Ah battery, the same power input 800 W unit, but it runs on a 240 V electrical circuit instead of a 120 V circuit.

Because the voltage is different, the amp draw – the amps required to run such an AC – will also change. Let’s calculate the new amp draw using the basic power equation:

Amps Draw (in A) = 800W/ 240V = 3.33 A

As we can see, the amp draw is no longer 6.67 A; it’s 3.33 A. When we increase voltage, we need fewer amps to get the same electrical power (wattage). Based on this, we can now calculate how long will a 200 Ah battery be able to power an 800 W 240 V air conditioner:

200 Ah Battery Life = 200 Ah / 3.33 A = 60 hours

As we can see, because the amp draw is halved, the battery life is increased. That’s because an 800 W air conditioner on 240V requires fewer amps than an air conditioner on 120V.

Now we know how to calculate the amps from watts. We can use this knowledge to calculate the second vital input into the Battery Life Calculator:

Battery Life Calculator (Insert Battery Capacity And Amp Draw)

When you figured out how big a battery you have (battery capacity in Ah), and how many amps does a device you want to hook on the battery runs on, you can input both numbers in this calculator. As a result, you will get how long will a battery last (in hours):

You can pretty much calculate the battery life for any kind of battery powering any kind of electric device.

• Why Calculating The Battery Life Is Not Exactly Easy
• How To Calculate Load Current (Amps) From Wattage?
• Battery Life Calculator (Insert Battery Capacity And Amp Draw)
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