How Long Will A 100Ah Battery Last? 100W, 400W + Calculator

100Ah batteries are quite big. They can be used for RV, as solar batteries, or even car batteries. You can imagine that one of the most frequent questions regarding the 100 amp hours batteries is this one:

“How long will a 100Ah battery last?”

This can be quite easily calculated if you understand the basic electric power law:

Power (W) = Current (I) × Voltage (V)

A 100Ah battery can last anywhere from 120 hours (running a 10W appliance) to 36 minutes (running a 2,000W appliance). 100Ah 12V battery has a capacity of 1.2 kWh; that’s more than 2% of the capacity of the Tesla Model 3 car battery. You can check here how long does charging Tesla cars with much bigger batteries last here.

As you can see, how long will a 100 amp hour battery last depends primarily on how powerful the appliance you’re running. To fully answer how long will a 100Ah battery last, we will first look at how much capacity (or juice; in terms of Wh or Watt-hours) 100Ah 12V battery has.

We will also illustrate how you can calculate how long will a 100Ah battery run any appliance. On top of that, you will also find a ‘100Ah Battery Life Calculator‘ further on that makes all these calculations effortless.

Screenshot of the calculator: You just insert wattage and the calculator returns how many hours will a 100Ah battery last. You can use the calculator yourself further on.

Near the end, we also solve two real-life examples just to illustrate how you can use the calculator. These are:

  1. Example 1: How long will a 100Ah battery run an appliance that requires 400W?
  2. Example 2: How long will a 100Ah battery run an appliance that requires 100W?

The goal here is that once you finish this article, you will be able to determine how long will a 100Ah battery last yourself for any appliance.

Let’s start with looking at how much juice the 100Ah battery has:

100Ah Battery Capacity (In Terms Of Wh)

For starters, we need to determine how much electricity is in the 100Ah battery. Some might say, “It’s a 100Ah battery, that’s your capacity right there.”

Well, it’s not all that simple. “100Ah” only tells us the amount of electrical current the battery can provide. For example, a 100Ah battery can provide us with 100 amps current for 1 hour. It can also provide us with a 1 amp current for 100h.

To get to electrical capacity (or power, according to the P = I × V), we need to know the voltage as well.

Now, almost all batteries have a 12V output voltage. It doesn’t matter if you have a 100Ah lithium battery, 100Ah deep-cycle battery, or 100Ah LiFePO4 battery; all of them run on 12 volts or 12V.

With these two key metrics – 100Ah and 12V – we can precisely calculate how much electrical capacity (measured in Wh) a 100Ah battery actually has. Here is the equation we use:

Battery Capacity or Watt-Hours (Wh) = Amp-Hours (Ah) × Voltage (V)

In the case of a 100Ah 12V battery, we get:

100Ah 12V Battery Capacity = 100Ah × 12V = 1,200Wh

Now, this 1,200Wh battery capacity is the most useful piece of information when it comes to determining how long will a 100Ah battery last. It has 1.2 kWh of juice; for comparison, Tesla S model has a 100 kWh battery.

Here are just a few examples of how long will such a 100Ah battery run different appliances:

  • 100Ah battery will run a 1,200W appliance for 1 hour.
  • 100Ah battery will run a 600W appliance for 2 hours.
  • 100Ah battery will run a 400W appliance for 3 hours.
  • 100Ah battery will run a 100W appliance for 12 hours.
  • 100Ah battery will run a 1W appliance for 1,200 hours.

How did we calculate these times? Here is the simple equation pretty much everybody can use, together with the ‘100Ah Battery Life Calculator’:

100Ah Battery Life Calculator

Here’s how you look at this:

  1. You have an appliance you want to run. Let’s say a 100W television.
  2. You have a 100Ah battery. It has a capacity of 1,200Wh.

When will the battery run out of juice? Here’s how you calculate that:

100Ah Battery Run Time = Battery Capacity / Appliance Wattage

In our case, this is:

100Ah Battery Run Time = 1,200Wh / 100W = 12 Hours

Simple, right?

We even simplified it by designing an easy-to-use ‘100Ah Battery Life Calculator’. You can insert the wattage of the appliance you want to run, and the calculator will dynamically tell you how long will a 100Ah last.

Here is the calculator (you can play around with the numbers):


Using the calculator, you can determine how long any appliance will run on 100Ah 12V battery. If you need a running time for 200Ah batteries, you can use this similar 200Ah battery run time calculator.

You can also use this calculated chart of long will 100Ah battery power different appliances:

Appliance Power Draw (Watts): How Long Will 100Ah Battery Last:
10W 120 Hours
20W 60 Hours
30W 40 Hours
40W 30 Hours
50W 24 Hours
100W 12 Hours
150W 8 Hours
200W 6 Hours
250W 4.80 Hours
300W 4 Hours
350W 3.43 Hours
400W 3 Hours
450W 2.67 Hours
500W 2.4 Hours
600W 2 Hours
700W 1.71 Hours
800W 1.5 Hours
900W 1.33 Hours
1,000W 1.2 Hours
1,100W 1.09 Hours
1,200W 1 Hour
1,300W 0.92 Hours or 55 Minutes
1,400W 0.86 Hours or 52 Minutes
1,500W 0.8 Hours or 48 Minutes
2,000W 0.6 Hours or 36 Minutes

To illustrate how easy it is to manually calculate these times, we will solve two promised examples:

How Long Will A 100Ah Battery Run An Appliance That Requires 400W? (Example 1)

A lot of people who own RVs or caravans are interested in how long will a 100Ah battery last if you run a 400W appliance with it.

If you’re looking at 100Ah alone, it’s impossible to figure it out.

However, when you calculate the battery capacity of 100Ah and get 1,200Wh, you can quickly figure out how long the battery will last:

  1. You have a 1,200Wh battery.
  2. Appliance draws 400W.

You simply divide 1,200Wh by 400W and get 3 hours.

In short, a 100Ah 12V battery will run a 400W appliance for 3 hours.

How Long Will A 100Ah Battery Run An Appliance That Requires 100W? (Example 2)

The same principle applies here. We already know that a 100Ah battery will run a 100W appliance for 12 hours.

That’s because we know two metrics that make this calculation possible:

  1. You have a 1,200Wh battery.
  2. Appliance draws 100W.

Dividing 1,200Wh by 100W yields the result: 12h.

We hope that now everybody can calculate how long will a 100 amp-hours battery last. If you didn’t find your answer, you can use the comments below, give us some data about the battery and the appliance you want to run with it, and we’ll try to help you out.

19 thoughts on “How Long Will A 100Ah Battery Last? 100W, 400W + Calculator”

    • Nice for that now give when the load is consuming power,inverter is also on what will be the size of the battery enough to run the same load at night and what would be the panel siz

  1. Thank you for the very good article.
    What figures do I use for four 12 volt 100Ah wired in parralel/series for 24 volt output?
    100Ah at 12 volt or
    100Ah at 24 volt?
    Thank you in advance

    • Hi Peter, you start with the batteries: four 12V 100Ah have a combined capacity of 4,800 Wh. So, you use 100Ah at 12V. Be aware that you should also include the discharge rate because those 4,800 Wh are not 100% usable. The 24V output doesn’t play a major role here. Hope this helps.

      • I don’t believe this to be correct, if you take the voltage being 24 volt it means 2 batteries are connected in series to give you 24 volt output however the amperage of combined batteries connected in series are still only 100a/h then by stacking the 2 sets of batteries that are connected in series now in 2 x 24v parallel configuration that doubles and effectively gives you 200a/h between the 4 cells, which then means you effectively have 2400 Wh available to you, connecting the batteries purely in parallel configuration would give you the 4800Wh however only at 12v, as the voltage goes up so does the efficiency of the conversion process hence why 400w at 12 v is less efficient and draws more amps than 400w at 220v hence the big current difference between the 2 voltages, 400w / 12v = 33.3 amps whereas 400w / 24v = 16.6 amps so voltage definitely plays a part in the math.

        • Hi Justin, thank you for this explanation. You do have a point here; this is a better insight than ours, makes things much clearer.

    • Hi Clifford, if you vary voltage, you just need to remember that battery capacity in watt-hours (Wh) stays the same. Example: 100Ah 12V battery has 1,200 Wh capacity. That means it can run a 100W device for 12 hours and a 400W device for 3 hours. The wattage is important; voltage not at all.

      If you have a 400W 12V device, the amp draw will be 33.3 amps. If you have a 400W 220V inverter, the amp draw will be 1.8 amps. However, the wattage will be the same; and the true constant ‘juice’ in the battery is Wh, not Ah. Hope this makes at least a bit of sense.

      • Thanks dor the Q &A on this different voltages (12 vs 220v).
        You explain the amperage is different. This means that: you need THICKER cables for running in 12V because it draw more current/amperage than in 220v.


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