Overall, AC is more efficient that DC, but that’s only because it is easier and more efficient to change the voltage of AC vs. DC.
Here’s an explanation:
After you generate the electricity, you want to boost the voltage to a high value before putting it on the long-distance transmission lines. That way, you minimize the amount of current on the transmission lines for a given amount of power. And the less current you have, the less metal you need in every foot of conductor for a given amount of I[sup]2[/sup]R loss. In other words, for a given amount of power, and a given amount of I[sup]2[/sup]R loss, higher voltage → lower current → higher resistance/ft. → smaller gage wire → lighter and cheaper wire.
Note that you want to boost the voltage to a high value before putting it on the long-distance transmission lines regardless of whether it’s AC or DC. Likewise, you’ll need to reduce the voltage at the customer’s location regardless of whether it’s AC or DC. So this begs the question: Is it more efficient to boost/reduce the voltage of AC or DC? The answer, of course, is that it is more efficient to boost/reduce the voltage of AC. The nifty device used to do this is the transformer. Unfortunately transformers don’t work w/ DC. To boost/reduce the voltage of DC, you need to send it through a complex and inefficient oscillator/switching/filtering gizmo. These are usually more costly, less efficient, less reliable, and noisier than a comparable transformer. But the technology is improving, and in the future the performance gap may be narrowed to the point where you’ll see more and more DC distribution schemes.
I’ve read every post above mine and don’t see any other discussion about safety. Why is it that DC is (or was argued to be) safer than AC? Touch the contacts of a 9v battery against your tongue if you think you can’t get a shock from DC.
One of the effects of AC on the body is that it causes your muscles to contract. Thus, if you were to inadvertently grab something that had become energized, your hand would clench around it, possibly tightly enough that you couldn’t let go. DC has the opposite effect; grabbing something that had become energized with DC would actually force your hand open.
When i was a kid, my little brother grabbed the post supporting a 120volt accent light that had become energized. We had to physically pry his hands off and it was a bit terrifying.
Old timers used to caution that if you were the least suspicious, your were supposed to touch things with the back of your hand. That way, if it was energized, the contraction of your muscles would actually pull your hand away. Hmm… i always figured that’s what they invented volt meters for.
When it comes to current going through your body, your household voltage (120 VAC RMS) would be more damaging than 120 VDC simply because the former has a higher peak voltage. (When studying electrocution, you have to look at peak voltages & currents, not RMS.)
Also, the electrical signals going to the heart can be disrupted when subjected to externally-applied electrical signals. I’ve heard claims that the level of disruption is a function of frequency, and that the heart is particularly susceptible to 60 Hz. Not sure if this is true… it’s just what I’ve heard.
Other than that, I can’t think of any inherent reason why AC is more “deadly” than DC. I think peak voltage is more important than anything else… as an example, 12 VAC RMS (with a peak of 17 V) is a lot safer than 120 VDC.