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:
- 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.
- 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:
- 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.
- 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:
- Battery capacity. We have that; it’s 200 Ah.
- 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.
I have a 1/3 hp sump pump that draws 6.5 amps. If I have a 12v battery with 170ah, will the pump run continuously for 26 hours?
Hello Fernando, 1/3 HP is about 250W of power. 12V battery with 170Ah has 12V*170Ah = 2040 Wh capacity. That will run 250W sump pump for 2040Wh/250W = 8.16h at 100% output. If you have an average output of 25%, the battery will run this pump for 32.64 hours. Pretty much it all depends on the output percentage. If you want it to run for 26 hours, you would have to run it at about 78W; that’s 31% average output. Hope that helps.
If I have a 33Ah battery and draw 350 Watt(Amp =350/220 equals 1.6Amps)…thus 33ah/1.6=20hrs. Can’t be…where is my mistake???
Hello Bert, a 33Ah 12V battery has a capacity of 33Ah*12V = 396Wh. A 350W device will draw 350Wh worth of electricity every hour. You can calculate how long the battery will last like this: 396Wh/350W = 1.13h. That’s 1 hour and 8 minutes.
Your mistake was the 220V voltage. Batteries don’t produce that amount of voltage; most batteries operate on 12V. 220V vs 12V is about 20x difference; that’s why you get 20 hours when the correct result is closer to 1 hour. Hope this helps.
Adding 12v in parallel will mean to increase Ah let say for four 12v – 170 Ah in parallel the capacity will be 680 Ah
Now the 12v X 680Ah = 8160Wh
Longevity using 250W pump must now be 8160Wh / 250W allows you to run for approximately 36 hrs
Because three batteries in parallel longevity is at 24 hrs which is still less desirable for Fernando.
I think the Acid battery of 170 AH should be divided by 2 because we usually use 50% of that battery.
I have 8 battery of 12 v each with 200 ah each in Parallel.
Imverter is 48v and can use 200X 8 /2
I have continuous load of 800 watt
How would you calculate that?
Hi Georges, yes, we should account for the 50% discharge rate for acid batteries. So, 12V batteries with a combined 1600Ah amp-hours has a capacity of 12V*1600Ah = 19,200 Wh or 19.2 kWh. Accounting for a 50% discharge rate, you get 9,600 Wh juice from those batteries.
Now, the 800W load will draw 800 Wh every hour. Here is how you can calculate how long will it continuously run:
9,600Wh / 800W = 12 hours.
In short, the 800W device will run for 12h with these batteries. Hope this makes sense.
If I have a 60 volt battery and 40 amps in that battery how many ah is that. I am trying to power a 2500 watt motor. I am only 14 so plz help
Hi Caleb, nice setup. So, you can calculate wattage by multiplying amps and volts. In your case, this is: 40A × 60V = 2,400W. So, this battery can power a 2,400-watt motor for 1 hour. You will need a bit bigger battery for a 2,500-watt motor; 80 amp or 100 amp battery. Ah are basically just amps; so your battery has 40 Ah. The important thing is the voltage. Hope this makes sense.
I’m looking at running 2 18w heat pads in a greenhouse on a solar panel system what type of battery would allow me to run them constantly
Hello Allen, 2 18W heat pads require 36 Wh to run for 1 hour. If you have a 12V battery, that’s 3 Ah per running hour. So, a big 100 Ah battery will allow both pads to work at 100% output for about 33 hours. Hope this helps.
Very helpful with clear and concise explanation. Thanks for sharing.
Hey i have 2 29ah deep cycle batteries and its not holding charge i only have a 2amp load on is it a fault battery or wrong setup
Hi Nolan, it might be the voltage change. Battery output is 12V DC, electric devices are 120V AC. Maybe you would need an inverter somewhere in between.
Hi, i appreciate the help
If i have a battery station of 256Wh, and im using it to power a telescope mount that uses 14.4v over 2.5amp…. how long will the battery station last?
Thank you
Hi Jan, the 14.4V 2.5A telescope will run on 36 watts. Here is the calculation: 256Wh / 36W = 7.11 hours. That’s 7 hours and about 7 minutes. Hope this helps.
Hi, thanks in advance for any help.
I have a 900 watt continuous running portable air conditioner, plugged into a 2000 watt 12V to 120V inverter, that’s hard wired to a 200ah deep cycle battery.
How many hours can this setup power the air conditioner while keeping the deep cycle battery at approximately 50% discharge?
Hi Bob, alright, the 50% discharge 200Ah deep cycle battery has 100Ah of useful juice in it. If this is a 12V battery, the total useful capacity is 1,200 Wh. Now, running a 900-watt portable air conditioner for 1 hour will consume 900 Wh of electricity. 1,200 Wh / 900 W = 1 hour and 20 minutes. That’s how long the portable AC will run on this battery. Hope this helps.
Hi I have a 500w garage motor door and my inveter is 1000w which battery I can use that will last 5 hours in my garage door
Hi there, hm, this is difficult to calculate since the garage motor door doesn’t drain the battery with 500W power draw continuously. If that were the case, you would need a battery with 2,500Wh capacity for 5 hours. In practice, the power draw is much lower. The first thought is that a 100Ah 12V lithium battery should suffice.