[One (M7) Q&A, Help & Troubleshooting] Putrid186:
Sorry for replying on an old post guys but because this has been indexed and is high in the search engines, I feel it necessary to reply here.

Voltage is #1 but you'll be hard pressed to find a usb type adapter that isn't 5V out

Amperage, however, is another story. Best rule of thumb: The higher, the safer.

HTC one uses a 1A (1000mA) adapter, as do most phones nowadays. As others have mentioned, using a higher amperage is fine, as the adapter won't have to work as hard and causes zero harm to the phone. You can use a 1250mA, a 2A, a 5A, a 3000000A, it doesn't matter. The charger within the phone will only utilize the amount of amperage coming from the adapter that it needs.

Going lower, however, will often cause problems. I worked the T-Mobile PDA tech call center when the first android device the G1 came out. It used a 1A adapter which wasn't very common, but used a mini USB port which was very common. A famous problem I dealt with was the digitizer going schizo; registering finger presses all over the place. I figured out the common denominator was that it was plugged into a lower amperage adapter (usually for an older phone) when this happened. Sharing this with the team, we decided to ask the details of the adapter they were using whenever this problem was encountered. Sure enough, with 99% accuracy, the adapter was for another phone which was of a lower amperage. That month I did about a dozen followups myself with customers with this issue after sending them a replacement 1A charger, and the issue was resolved.

As more 1A mini-usb style phones came out, we found the trend. Whacky digitizers and drastically low battery life (full life, not charge-length) when used with lower amp adapters.

Going further, also at T-Mobile, we came out with the [MENTION=313982]home[/MENTION] service; a VOIP service through T-Mobile (which was pretty ****ing cool but it never really took off) which used a modified Linksys router: the WRTU54G-TM. The teams involved with tech support for this system were definitely stressed on its release because it required the tech to have not just knowledge on the unit, the service, and the system... but also every single scenario of computer and network configuration out there. I was fine but my team was reprimanded incorrectly... ahh but I digress... Getting to the point: customers would get this new Linksys router and notice that the power adapter port (we called them power puckers on my team) was the exactly same style and size as their last router which was also usually a linksys. So, instead of using the adapter that came with the WRTU54-TM, they just plugged it right into the old adapter.

And then problems.

This took weeks to figure out because the issues were strange, inconsistent, and abundant. Once Clem and I figured out the trend, we blasted emails throughout the callcenter since this, for some reason, didn't reach the bureaucracy threshold of importance: No two adapters are the same (kind of). Just because the plug looks the same doesn't mean it'll work right. The voltage will probably be the same. Your positive and negative DC connections are probably safely correct based on design. BUT THE AMPERAGE

All these old routers used adapters that were a lower amperage than what the WRTU54G-TM needed. The easiest way for us to find out if there was an inappropriate adapter being used was to have the customer unplug the router, plug it back in, and then have them alert us as soon as the power light stopped blinking. The WRTU54G-TM has a very long boot-up time, but it's pretty consistent at about 60 seconds +-5sec. Anything above 70 seconds is when we'd check the adapter amperage, and 100% of the time, it was an old adapter for another product that had a lower amperage than required. We'd get them to dig up their original box, pull the real adapter out, and problem solved. Every. God. Damned. Time.

So, lesson is: NO NO NO NO LOWER AMPERAGE IS

NOT OKAY

. "Charging slower" is NOT the only side effect of a lower amperage adapter. However, higher amperage is totally kosher. As high as your heart desires.

Hope this helps!