My USB power bricks work fine here in Japan. Absolutely seamlessly for my cellphone, laptop, tablet and smartwatch. Nothing caught on fire.

I checked this reddit FAQ and this Power Plugs website, but I am getting some confusing information.

No surprise. Japan is one of the more confusing countries out there when it comes to the power grid and power plugs. Half the country is on 50 Hz, half is on 60 Hz, and all of it is on a lower voltage than most of the world (100 V), but the JIS C 8303 plugs are almost (but not exactly) equivalent to NEMA 1-15 and 5-15 plugs.

I would imagine that the frequency/voltage difference shouldn’t matter as long as the prong fits in. I guess the USB bricks would have some stabilization mechanism built in. Right?

The good ones should handle this fine. The problem is that you may not have good ones. I still occasionally see cheap USB power adapters that do not properly support anything except the local line frequency and voltage.

In general most devices with NEMA 1-15 or 5-15 plugs will mechanically work with Japanese JIS C 8303 sockets. You may have issues with devices with polarized plugs in older buildings, as older JIS C 8303 sockets were unpolarized.

Electrically, many, but not all, solid-state power adapters for modern electronics sold in the US will work just fine in Japan. You should check each individual device, it should have information somewhere (often for USB power bricks it’s on the side that faces up against the outlet when plugged in) that says something like 100V-240V @ 50-60 Hz. The important part here is that the lower voltage is 100V or less, and that the frequency covers both 50 Hz and 60 Hz. If the device does not meet those requirements, or does not list this information, do not use it without an active adapter (best case it simply won’t work, worst case it will fail destructively and possibly dangerously).

Older power adapters that include an AC transformer or devices that run entirely on AC power are very likely to not work at all, and are also pretty likely to fail in very dangerous ways (think catching fire or exploding). It’s unlikely you will have anything that fits this unless you’re dealing with devices from before 2000.

I want to know if in Japan I need an adapter for my USB Wall Bricks.

The answer is

No.

(One interesting point – no such ‘adaptor’ exists! Because there is nothing to adapt.)

The whole issue can be instantly resolved by, obviously, glancing at the specs printed on power plugs, where one will instantly see they (of course) operate perfectly in all of Japan.

Just as the OP mentions, it’s one of those curiosities where the interweb is filled with misinfo.

I’m adding here the two most important comments on this page, which say it all:

"That answer is very misleading. Can you point to ANY device that works in the US but not in Japan ? I’ve worked in electronic product development for 30+ years and I have never encountered something that wasn’t compatible"

Key point highlighted.

"You absolutely do not need any adapters. I go back and fore between the two countries (with literally cases of equipment) all the time. Any other information you hear, is wrong."

OBSCURE CAVEAT !

It occurred to me that. Let’s say you are, perhaps, an audio or video engineer and or indeed a person who manufactures electrical equipment or such. It’s possible that part of your life is using things like this …

of course, obviously, such things that literally manipulate the electrical supply and are used by electrical supply engineers are irrelevant to this discussion, and some models only work with one very specific power source, or whatever.

Of course obviously you can’t casually use such stuff unless you know what you’re doing, and it’s inconceivable any power engineer, etc, would be asking the question on this page.

(Note that, as a curiosity, the thing in the photo if you ! merely plug two of them in series ! you’ve just done an incredibly dangerous thing; the issue is completely outside the question "Are Japan/US power blocks interchangeable, answer Yes".)

Excuse my incredibly ugly carpet BTW.

Japan’s power grid is a bit special, in that the eastern/western sides are 50Hz/60Hz respectively.

If we’re not talking about USB power bricks, then the frequency may matter (especially for motors) because that 50/60 difference is about 15-20%, far more than typical tolerances of motors. Japan’s voltage of 100 is also quite a bit below the expected 120v for US devices.

However, USB power bricks are not motors. In fact, they are typically rated for 100v-240v by default because they are something called Switched-mode power supplies. These power supplies are universally chosen for USB bricks due to their small size. It also means that they are usually made to support 100v-240v @ 50Hz-60Hz by default because there is no advantage of limiting it otherwise (check the device label).

There’s zero chance that your USB brick is anything but this, because other transformer types are bulky and large.

I would imagine that the frequency/voltage difference shouldn’t matter as long as the prong fits in.

You would imagine wrong. That’s how fires start.

You’ll need to check the expected voltage for the USB bricks, it should be written on them. Most mobile adapters are supposed to work with the full range of voltages/frequencies, so you’d see something like 100-240V ~ 50/60Hz (See an example here). However it may be that for some reason your specific brick is only rated for a sub-set of the worldwide power specs, and you should check that before plugging it in.

The prongs themselves in Japan are the same as in the US.