If you own both an iPad and iPhone, you likely have two similar looking Apple USB charging adapters. Although the iPad’s charging adapter works fine when connected to an iPhone (or iPod Touch), the iPhone’s charging adapter should not be used with the iPad. This is because the iPad’s charging adapter is 10 watt, and the iPhone’s is 5 watt. The iPad wasn’t designed to charge with only 5 watts, and attempting to use a 5 watt charger will result in an extremely slow charge time.
Unfortunately there are several variations of these Apple charging adapters, and many don’t list the watts on them. The rule of thumb is if the label mentions iPhone/iPod or 1.1 Amp, the charger is 5 watt. If the label mentions iPad or 2.2 Amp, the charger is 10 watt. This is also true for third-party USB charging adapters.