Several of the answers here seem to suggest that this only happens in first or business class.
While that was historically true, it's not so true anymore.
Airlines such as Cathay Pacific and Korean Air , for instance, have them at every seat in most (or all?)
of their long-haul aircraft.
Delta also has it in parts of economy on several of their long-haul configurations as well as in all of economy on their recently-acquired 717s and 737-900ERs
.
You can check with your specific airline or with sites like Seat Guru before booking to determine what is available on any given aircraft and at which seats.
As Lnafziger said, some airlines' systems require adapters, while others don't.
At least from personal experience, it seems that most of the recent ones do not.
On Korean Air, Cathay, and Delta, the plugs are designed to be able to fit any of the standard power plug types (e.g. North American, European, Asian, etc.)
Since most new electronic devices use auto-switching power supplies, they should accept the 110V/60Hz supplied by most of these systems, even if they're designed for 220V/50Hz markets.
Here's a picture of one of the outlets in economy on the Delta 717s.
Notice that the holes are cut such that any of the standard plug types will fit in it.
The slot on the bottom is a USB power port.
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This depends on the airline, but to answer your question: Yes, some airlines do have electrical outlets in the cabin.
It seems that since the FAA relaxed the rules on portable devices that the airlines are seeing the need to add them to more of their fleet.
For example: US Airways has a Business Tools webpage which tells which of their aircraft have outlets.
There are some in both coach and first class.
United has an Inflight connectivity and power webpage whichs shows that a lot of their fleet has electrical outlets and more are coming.
There are different types of outlets, and some of them require an adapter to use while others don't.
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Some EMB-145 models have a 120v AC outlet mounted in the cabin just below the cockpit door.
This was powered by an inverter
(EMB-145 is 100% DC busses).
It wasn't for passenger use (primarily for cleaning crews)
and I didn't trust the power coming out of it enough to plug in a laptop, though we sometimes did charge our phones with it.
Some larger airplanes have 120v AC in the cabin mounted at a subset of seats.
I can't recall which airlines, but I have been in a 757 and a corporate CRJ-700
that had this kind of power installed.
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To answer your question of "why don't you see them," I think it comes down to power requirements.
As mentioned, new-ish first/business class generally will have outlets available.
You can explore different aircraft here .
The reason it's not available in every seat is capacity.
If you have 150 people pulling 75W, that's 11.25 kW.
On a 737 NG, each of the two generators is rated at 90KVA.
However, they need most of that power to run the radio equipment, avionics, lights, galleys, window heat, entertainment system, and everything else on the plane that runs on electrical power.
So besides a possible upgrade of the generators/power system, this also adds the weight of running cables and having outlets at every seat.
It's a lot easier to do that sort of thing on a train car, where it doesn't have to fly and can get power from the locomotive.
You can find more about the 737 electric system here .
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They are universally available since maybe 10 to 15 years, but only in Business and First, and normally would accept both US and European plugs.
In Economy you are out of luck, but if you consider that most airlines have reduced Economy seating pitch to the minimum 31", laptop use is near impossible there, anyway.
I have seen one instance a few years ago on an older 747 where they had a proprietary socket and would hand out adapters for the different laptops.
Naturally, the adapter kit was of the same age and unusable on some modern laptops.
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Power Sockets are getting pretty common these days.
Most of the Widebodies (specially 777 and 380's have power outlets for 2 out of 3 Seats in each row).
The Power Ratings also are 110V 60Hz commonly.
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As we see trains nowadays with sockets for you to plug your laptop into, as well as Wi-Fi on-board both trains and planes, I'm wondering why you don't see electrical outlets on planes?  Do any airlines have sockets in the cabin? How about in first class?
Do commercial aircraft have electrical outlets?
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<airliner><passenger><electrical-system><electronics>
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aviation.stackexchange.com/users/97/Danny Beckett
Wide-bodied planes such as 777 and 380 have power outlets for most seats. Other planes, it seems, tend to have them only in first/business class. As the rules on using electronic devices has been relaxed, more airlines are seeing the need for power outlets. You can check for this information with the airline or on sites like Seat Guru. It depends on the airline, but some do now have electrical outlets. They have been available for around 10 to 15 years but only in business and first class. Specific airlines or websites such as Seat Guru should be able to tell you which seats have electrical outlets. United has an in-flight connectivity and power webpage. This shows that many of its planes now have outlets and there are plans for more to be added. The presence of electrical outlets in planes is increasing and they are now fairly common. This may be due in part to the FAA relaxing its rules on portable devices. Some larger airplanes, including the EMB-145, have a 120v AC outlet in the cabin. If a plane does not have them, it is usually a question of power requirements. There are not usually outlets in Economy, but it is very difficult to use a laptop in Economy in any case due to the reduced seating pitch. One of the respondents recalls seeing a proprietary socket on an older 747 that would be handed out with adapters for use with different laptops. Most wide-body aircraft have outlets for two of every three seats in a row.
Electrical outlets have been available for around 10 to 15 years but only in business and first class.  Specific airlines or websites such as Seat Guru should be able to tell you which seats have electrical outlets. United has an in-flight connectivity and power webpage. This shows that many of its planes now have outlets and there are plans for more to be added. It depends on the airline, but some do now have electrical outlets. The presence of electrical outlets in planes is increasing and they are now fairly common. This may be due in part to the FAA relaxing its rules on portable devices.  Some larger airplanes, including the EMB-145, have a 120v AC outlet in the cabin. If a plane does not have them, it is usually a question of power requirements. There are not usually outlets in Economy, but it is very difficult to use a laptop in Economy in any case due to the reduced seating pitch. One of the respondents recalls seeing a proprietary socket on an older 747 that would be handed out with adapters for use with different laptops. Most wide-body aircraft have outlets for two of every three seats in a row.