With all batteries, we best know amp-hours (Ah) and watt-hours (Wh). This will allow us to estimate how long will a battery run an electric device or evaluate how many amp-hours do you need for running an electric device. That’s why we will look at how to calculate amp hours of a battery.
Here’s the thing:
With many batteries and power banks, you only get watt-hours (Wh) specified on the label. What you want to know, however, is how many Ah does the battery hold and how many Ah you need. This includes how many amp hours battery do you need to run an electric device with certain wattage for a specified time.
Example 1: How long will a 100Ah battery run an appliance that requires 1,000W? Simple. 100Ah battery running on 12V has a battery capacity of 1,200Wh. It will run a 1,000W appliance for 1.2 hours; that’s 1 hour and 12 minutes.
Example 2: I want to run a 400W device for 6 hours. How many amp hours battery do I need? This device will burn through 2,400Wh of electricity. You need a 2,400Wh battery. Given that most batteries run on 12V voltage, that means you will need a 200Ah battery to power a 400W device for 6 hours.
To help everybody with these calculations, we have designed a 12V Battery Amp Hour Calculator. You just input the wattage of a device and how long you want that device to be run by a battery, and the calculator will tell you how many amp-hours (Ah) does that battery hold.
You will find the calculator further on, complete with the Amp Hours Chart (tells you how many Ah you need to power different devices for 1h, 2h, 4h, and 8h).
Note: If you can’t find the answer in this article, you can use the comments below, specify what you want to run, and we will help you calculate amp hours.
Here is how to calculate battery amps hours from watt and how long can a battery power such a device manually. You can also use a calculator for easier calculation:
How To Calculate Ah Of A Battery?
The battery AH calculation is best explained via an example. Let’s say you want to power a 100W electric device by using a battery. Specifically, you want this battery to run a 100W device for 8 hours. How many amp hours battery do you need?
First, you need to calculate how much electricity is needed to power such a device using this simple equation:
Electricity Needed = Device Wattage × Time (In Hours)
In our example, we have:
Electricity Needed = 100W × 8h = 800 Wh
This means we need a battery that has an 800 Wh capacity. Of course, most battery capacities are not expressed in Wh but in amp-hours (Ah). You will now need to convert Wh to Ah (you can use this calculator for easier conversion) like this:
Ah = Wh / Voltage
Most batteries have a voltage of 12V. Here is how many amp hours battery you need to power a 100W device for 8 hours:
Ah = 800W / 12V = 66.67 Ah
This means you will need a battery with at least 66.67 amp-hours (Ah).
Here is the step-by-step procedure how to calculate Ah of a battery:
- Calculate the electricity needed to power an electronic device. That means you want to multiply the wattage by how many hours you want the device to run. Example: 100W × 8h = 800 Wh.
- When you have the Wh, you have to convert Wh to Ah. To do this, you have to divide watt-hours by voltage (12V for batteries). Example: 800Wh / 12V = 66.67 Ah.
- The calculated Ah is the minimum amp hours your battery should have to power your device for that time period.
If all this sounds a bit complex, don’t worry. You can use this calculator that does all these calculations automatically. Below the calculator, you will also find an Amp Hours chart that illustrates how many Ah you need to power various devices for 1h, 2h, 4h, and 8h:
12V Battery Amp Hours Calculator
You can use this calculator to figure out how many amp hours 12V battery you need to power a device with known wattage for a specified time:
Here is how this Ah calculator works:
Let’s say you have a 200W device and want the battery to power it for 5 hours. How many amp hours battery should you get? In the Ah calculator above, you just slide the wattage to ‘200’ and the hours to ‘5’. You get the result: To power a 200W device for 5 hours you will need a battery with at least 83.33 Ah.
That means you can use a 100 Ah battery, for example.
You can play around with the numbers to see how the battery Ah is calculated. Here is a chart with calculated amp-hours for different devices and running times:
Amp Hour Chart (1h, 2h, 4h, 8h Running Time)
|Device Wattage||Ah Needed For 1h Run Time||Ah Needed For 2h Run Time||Ah Needed For 4h Run Time||Ah Needed For 8h Run Time|
|10 Watt||0.83 Ah (830 mAh)||1.67 Ah||3.33 Ah||6.67 Ah|
|20 Watt||1.67 Ah||3.33 Ah||6.67 Ah||13.33 Ah|
|50 Watt||4.17 Ah||8.33 Ah||16.67 Ah||33.33 Ah|
|100 Watt||8.33 Ah||16.67 Ah||33.33 Ah||66.67 Ah|
|150 Watt||12.50 Ah||25.00 Ah||50.00 Ah||100.00 Ah|
|200 Watt||16.67 Ah||33.33 Ah||66.67 Ah||133.33 Ah|
|250 Watt||20.83 Ah||41.67 Ah||83.33 Ah||166.67 Ah|
|300 Watt||25.00 Ah||50.00 Ah||100.00 Ah||200.00 Ah|
|350 Watt||29.17 Ah||58.33 Ah||116.67 Ah||233.33 Ah|
|400 Watt||33.33 Ah||66.67 Ah||133.33 Ah||266.67 Ah|
|450 Watt||37.50 Ah||75.00 Ah||150.00 Ah||300.00 Ah|
|500 Watt||41.67 Ah||83.33 Ah||166.67 Ah||333.33 Ah|
|600 Watt||50.00 Ah||100.00 Ah||200.00 Ah||400.00 Ah|
|700 Watt||58.33 Ah||116.67 Ah||233.33 Ah||466.67 Ah|
|800 Watt||66.67 Ah||133.33 Ah||266.67 Ah||533.33 Ah|
|900 Watt||75.00 Ah||150.00 Ah||300.00 Ah||600.00 Ah|
|1,000 Watt||83.33 Ah||166.67 Ah||333.33 Ah||666.67 Ah|
|1,500 Watt||125.00 Ah||250.00 Ah||500.00 Ah||1,000.00 Ah|
|2,000 Watt||166.67 Ah||333.33 Ah||666.67 Ah||1,333.33 Ah|
|2,500 Watt||208.33 Ah||416.67 Ah||833.33 Ah||1,666.67 Ah|
|3,000 Watt||250.00 Ah||500.00 Ah||1,000.00 Ah||2,000.00 Ah|
|4,000 Watt||333.32 Ah||666.68 Ah||1,333.33 Ah||2,666.68 Ah|
|5,000 Watt||416.65 Ah||833.35 Ah||1,666.67 Ah||3,333.35 Ah|
|10,000 Watt||833.33 Ah||1,666.67 Ah||3,333.33 Ah||6,666.67 Ah|
Here is how you can use this chart:
Let’s say you have a 100Ah battery and want to know how many watts device you can run for 1h, 2h, 4h, or 8h. Just check the chart and you will see that:
- A 100Ah battery can run a 1,200-watt device for 1 h (this is not specified in the chart, you can calculate it).
- A 100Ah battery can run a 600-watt device for 2 h.
- A 100Ah battery can run a 300-watt device for 4 h.
- A 100Ah battery can run a 150-watt device for 8 h.
We hope this helps clarify some things about what size battery you need (in amp hours). If you have a question, just use the comments below and we’ll try to help you out with the calculations.