Batteries allow us to store energy for a variety of purposes. They come in different capacities, depending on the device’s needs.
Of course, most devices are commonly rated in terms of watts, so knowing how to calculate amp hours of a battery we need will help us buy the right one. We can calculate amp hours from watts, voltage, and operating time of an appliance.
Table of Contents
Steps to Calculate Amp Hours of a Battery
What to prepare
First, we need to look at the equations we will use.
Then, we need to know the following measurements to calculate battery capacity based on what is available. These measurements may be:
- Power of the appliance in watts
- Energy in watt hours
- Voltage of the battery
- Operating time of the appliance in hours
The power of the appliance and voltage of the battery are normally listed on their respective labels, while the operating time is determined by the user.
Detailed Steps To Calculate Battery Amp Hours
Before we begin, we need to derive our useful equation. Let’s determine our battery calculation formula with the definition of battery capacity:
\begin{equation}
\text{Battery Capacity (Ah)} = \text{Current (amps)} \times \text{Time (hour)}
\end{equation}
We also have the definition of power shown below. We can re-arrange it to isolate current:
\begin{equation}
\text{Power (Watts)} = \text{Voltage (Volts)} \times \text{Current (Amps)}
\end{equation}
\begin{equation}
\text{Current (Amps)} = \frac{\text{Power (Watts)}}{\text{Voltage (Volts)}}
\end{equation}
Then, we can substitute this to our definition of battery capacity to find amp hours of a battery:
\begin{equation}
\text{Battery Capacity (Ah)} = \left[\frac{\text{Power (Watts)}}{\text{Voltage (Volts)}}\right] \times \text{Time (Hours)}
\end{equation}
The term “power × time” is actually electrical energy, commonly known as watt hour and is defined as:
\begin{equation}
\text{Energy (Wh)} = \text{Power (Watts)} \times \text{Time (Hours)}
\end{equation}
If we substitute this to our equation, we would get:
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Power (Watts)} \times \text{Time (Hours)}}{\text{Voltage (Volts)}} = \frac{\text{Energy (Wh)}}{\text{Voltage (Volts)}}
\end{equation}
This relationship may help us determine the amp hour of a battery using its watt hour and voltage, and may also estimate the amp hour needed for a device.
Let’s try to find the battery capacity needed to run a pair of 55 watt halogen headlights for a total of 110 watts in 4 hours. Note that we’re using a 12 volt car battery.
Step 1: Know what you’re looking for and choose the right battery calculation formula.
In this example, we are looking for the battery capacity to run the headlights. Hence, we can use this equation from our derived formulas earlier:
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Power (Watts)} \times \text{Time (Hours)}}{\text{Voltage (Volts)}}
\end{equation}
Step 2: Identify the measurements needed.
In this example, we need the power of the appliance, voltage of the battery, and operating time. We listed down these variables:
- Power of appliance – 110 watts
- Voltage of battery – 12 volts
- Operating time – 4 hours
Step 3: Use the equation to get the amp hours.
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Power (Watts)} \times \text{Time (Hours)}}{\text{Voltage (Volts)}} = \frac{110\text{ (Watts)} \times 4\text{ (hours)}}{12\text{ (volts)}} = 36.7\text{ Ah}
\end{equation}
This means a pair of halogen headlights with a combined 110 watts will use 36.7 Ah from a 12 volt battery in 4 hours.
You can look at this battery amp hours chart for 12 volts for easier estimation:
| Power of device (in watts) | Battery capacity needed to run device (in Ah) for a 12 volt battery | ||||
| for 1 hr | for 2 hr | for 4 hr | for 6 hr | for 8 hr | |
| 10 | 0.8 | 1.7 | 3.3 | 5.0 | 6.7 |
| 20 | 1.7 | 3.3 | 6.7 | 10.0 | 13.3 |
| 30 | 2.5 | 5.0 | 10.0 | 15.0 | 20.0 |
| 40 | 3.3 | 6.7 | 13.3 | 20.0 | 26.7 |
| 50 | 4.2 | 8.3 | 16.7 | 25.0 | 33.3 |
| 100 | 8.3 | 16.7 | 33.3 | 50.0 | 66.7 |
| 150 | 12.5 | 25.0 | 50.0 | 75.0 | 100.0 |
| 200 | 16.7 | 33.3 | 66.7 | 100.0 | 133.3 |
| 300 | 25.0 | 50.0 | 100.0 | 150.0 | 200.0 |
| 400 | 33.3 | 66.7 | 133.3 | 200.0 | 266.7 |
| 500 | 41.7 | 83.3 | 166.7 | 250.0 | 333.3 |
| 750 | 62.5 | 125.0 | 250.0 | 375.0 | 500.0 |
| 1000 | 83.3 | 166.7 | 333.3 | 500.0 | 666.7 |
| 1250 | 104.2 | 208.3 | 416.7 | 625.0 | 833.3 |
| 1500 | 125.0 | 250.0 | 500.0 | 750.0 | 1000.0 |
| 2000 | 166.7 | 333.3 | 666.7 | 1000.0 | 1333.3 |
| 3000 | 250.0 | 500.0 | 1000.0 | 1500.0 | 2000.0 |
| 4000 | 333.3 | 666.7 | 1333.3 | 2000.0 | 2666.7 |
| 5000 | 416.7 | 833.3 | 1666.7 | 2500.0 | 3333.3 |
| 10000 | 833.3 | 1666.7 | 3333.3 | 5000.0 | 6666.7 |
We can try another example.
How many amp hours do we have for a 12-volt battery at 1,200 Wh?
Step 1. Know what you’re looking for and choose the right battery calculation formula.
We are looking for the Ah of a 12V battery. For this, we will need:
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Energy (Wh)}}{\text{Voltage (Volts)}}
\end{equation}
Step 2. Identify the variables we know.
In this case, we have:
- Energy – 1,200 watt hour
- Voltage – 12 volts
Step 3. Substitute the watt hours and the voltage to get:
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Energy (Wh)}}{\text{Voltage (Volts)}} = \frac{1200 \text{ Wh}}{12 \text{ Volts}} = 100 \text{ Ah}
\end{equation}
Note that if you link two of these 12V batteries in series, the Ah will be the same. In a parallel connection, however, the Ah will be 200.
Alternatively, you can use a battery amp hour calculator available online if you don’t want to compute. Just input the voltage and the watt hours.
Frequently Asked Questions
What is an amp hour on a 12 volt battery?
The amp hour is the battery capacity. It is the number of amps the 12 volt battery can give in an hour.
How many amp hours are on a 1000 watt hour battery?
It depends on the voltage of the battery. Please check the sticker of the battery for the voltage.
If you were able to get the voltage (for instance, 12 volts), then you can calculate Ah battery capacity using:
\begin{equation}
\text{Battery Capacity (Ah)} = \frac{\text{Energy (Wh)}}{\text{Voltage (volts)}}
\end{equation}
Can you measure amp hours in a battery?
Yes, you can. Here are the basic steps for doing so.
- Charge the 12V battery until it’s full. It should have a voltage of 12.6 if you measure the terminals.
- Connect the battery to a 1 ohm, 200W resistor. Use the multimeter to check the battery voltage; do so once every 60 minutes until the battery is discharged to 50%.
- The amp hours = Battery amperage x Hours for battery to reach 12V x 2
- Can I convert amps to amp hours?
To get the amp hours from amps, you can multiply the number of hours the battery is used with the current needed during that time. You won’t need an amps to amp hours calculator for this.
Can we use the same formulas with lithium ion batteries?
Yes, you can use the same formulas. A lithium battery amp hour calculator is also available online if you don’t feel like doing the math.
Conclusion
Knowing how to calculate amp hours of a battery will help you choose the best one that fits your needs. As long as you know the watt hours of your power bank and the amperage of your device, using the formulas above should be straightforward.
We hope you found this article helpful. Thanks for reading.
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I am Edwin Jones, in charge of designing content for Galvinpower. I aspire to use my experiences in marketing to create reliable and necessary information to help our readers. It has been fun to work with Andrew and apply his incredible knowledge to our content.