Amp Hours Vs Watt Hours

amp hours wattsMany times in the RC hobby the measurement of Amp hours or milliamp hours is given close attention. Amp hours (AH) is essentially the capacity of the battery. A better evaluation of the batteries capacity is given by the “Watt Hours” it provides. The only difference is that watt hours also takes into consideration voltage. Why does this matter with RC Cars and trucks? For one, because you may be using multi voltage packs in your vehicles: the most common two being 7.4 and 11.1 volt packs. If you’re curious which pack is going to have the longest run time you can’t just look at the milliamp hours. You have to take into consideration the total energy- the watt hours.

Luckily the math is easy. You just take the AH on the battery and multiply them by the voltage.

Two Packs with different Voltage

So if my first pack is a 7.4v 3300 mA battery and the second is a 11.1v 2200 mA battery which one is going to give me a longer run time if I run them both the same? Let’s see…

3.3 x 7.4 = 24.42 Wh

2.2 x 11.1 = 24.42 Wh

Turns out in these to batteries have the same capacity in watt hours.

Watt Hours for Charging

In this case I’ve got the 12v 22 AH jump starter battery I use to charge all my other batteries and my 2 MaxAmps 7.4v 4000 ma/hour or 4 AH battery.

22AH X 12 V = 264 Wh
2×4 AH X 7.4 V = 59.2 Wh

So you can see if I didn’t take into consideration energy lost in the charging process I could charge my 2 packs about 4 times before depleting all of the energy in the packs. In reality you don’t ever really want Lead Acid batteries to go below even 50%. So I shouldn’t charge this pair of batteries more than twice before charging the large battery.

Watt Hours for Energy Cost

Now here we are getting to be rather nerdy. If I take into consideration that I drain about 8 AH of battery every time I run my Emaxx, I can easily figure out how much it costs.

8 AH x 7.4 = 59.2 Wh
1 kwH = 1000 Wh

So doing the math by just moving the decimal 3 places I find that running the truck uses .0592 kwH of energy, it will take me about 16 runs to even use 1 kwH which runs about 13 cents around here. So know you know how much it costs to run your remote control car!


2 Responses to “Amp Hours Vs Watt Hours”

  1. car jumpstarter on October 8th, 2011 at 4:12 am

    Why does this matter with RC Cars and trucks? In this case I’ve got the 12v 22 AH jump starter battery I use to charge all my other batteries

  2. Good question. In your case Amp Hours Vs Watt hours doesn’t matter as much because you are comparing apples to apples: all of the jumpstarter units are 12 volt. In the case of RC Cars and trucks you may be running a 2 cell lipo or a 3 cell life. That’s voltages of 7.4 and 9.9 volts respectively. So that you can compare them on a level playing field it’s best to calculate the watt hours rather than just looking at the amp hours. Here’s a watt hour calculator to help you compare batteries.

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