The 5W, 10W and 12W chargers are all capable of charging an iPhone. The wattage is made up of two variables - voltage and ampage (or amperage if you're American). The wattage also states the MAXIMUM output of the power block.
All three chargers have the same voltage output (5V), the difference comes with the amp output (1A, 2A and 2.4A). For the sake of argument, let's ignore the voltage because it's not really relevant to this discussion (seeing as all the chargers give out 5V).
When using a 5W power supply to charge an iPhone, you will be drawing the maximum current available (1A). If you use a 10W, you'll pull the SAME amount of amps (still 1A), and with a 12W....do you get the point yet?
Two things limit the charging time of your device:
The MAXIMUM output of the charger
The MAXIMUM power draw of the device
If you are using the EXACT right charger (same voltage and ampage as the device) then it will draw the maximum amps/watts available. If you use a higher rated power supply you will STILL pull the same amount of amps/watts but it will no longer be the maximum amount the charger can put out. You will NOT charge your device any faster. A higher-rated power supply will NOT damage your device, because the device will only draw as much current (amps) as it needs. The charger will not force the little electrons into the device any faster than it can handle.
On the other hand, if you put a higher-rated device on a lower-rated charger you will get the following results:
Your device will take a LOT longer to charge. In fact, it may appear not to be charging at all. Experiment for yourself: plug a new iPad 4 into an old USB 1.0 or 2.0 port overnight and see how much charge you've gained in the morning.
You've heard of "trickle charge" right? This is where the current being fed into the internal battery isn't actually enough to properly charge the battery. The battery will be discharging power at almost the same rate at which it is charging. Yes, this will damage your battery over time. You saw it a lot in the original Nokia USB-charging phones - the batteries died after a few years and would no longer maintain charge. There's nothing wrong with doing this as a once-off, for emergencies, or on an irregular basis, but you will do progressive damage to your device if you keep doing this.
At home I have an iPhone 4, 4s, 5 and iPad 4. They all get charged off the same power pack - a 12W (5V, 2.4A) power block that came with my iPad 4. All of my devices charge, none of them have been damaged.
- Using a higher-rated power supply on an iPhone won't damage it or charge it any faster.
- Using a lower-rated power supply on an iPad will charge it very slowly and do progressive damage to the battery.
Moral to the story: buy a 12W charger and use it for all your devices to save carrying multiple chargers.