PART 2
**How many of these TEG thingies do I actually need?**
Great, we have a TEG selected that we'll use! Now, how many do we need? As you can see, the TEG listed above puts out 7.5W. Two of them in series will create 15W. This is not an amazing amount of power by any means. A standard alternator puts out 13.5V and lets say 80 amps for a total of 1080W. To match that output, we would need 144 TEGs! That is a bit much, and actually probably not required the vast majority of the time. The alternator is sized to be able to handle every electrical load on the car at the same time. The liklihood of this happening is small and if you're aware of it, you can ensure it never happens. So, the next step really is to figure out how much power you want your TEGs to generate. The best way of doing this is to figure out what kind of electrical load your car has while its in operation. As a rough guide, MetroMPG has posted some information on

typical electrical loads. The best way is to measure it with an ammeter though.

Once you know what your typical electrical load is, you can figure out how many TEGs you need. For my example, I am going to say that I need 15A (200W) of charging. We already know that we need 2 TEGs in series, and that series of 2 TEGs gives us 15W. So, the math is simple at this point. We take 200W of generation divided by 15 which gives us 13.3 TEGs in parallel. I'm going to round that up to 14 to keep the numbers even. 14 in parallel times 2 in series gives us 28 TEGs. That sounds a lot more doable.

**Nope, wait! You need more.**
BUT, this is their rated output at maximum temperature differential. We surely won't be operating at maximum temperature all of the time. So, we will need to derate them. By how much is a total guess unless you've done testing to see what kind of exhaust temperatures you'll see consistently. Then you can reference the data sheet for the TEG and guess what kind of power output you'll get. For this example I am going to take a total guess and say we will derate them by a third. Now we need 36 TEGs to get our 200W.