Is iPad compatible with adapter from iPhone or just the one it came with?

You can use your iPhone charger, or your iPod touch charger with the iPad. However, it will take about twice as long to fully charge your iPad with those chargers. A better idea would be to use the power charger that came with the iPad to charge both your iPhone and iPad.

You can charge the iPad with the iPhone's power adapter, but charging will take longer than when using the power adapter that shipped with the iPad. You can also charge the iPhone with the iPad's power adaptor.

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http://www.apple.com/batteries/ipad.html

The quickest way to charge your iPad is with the included 10W USB Power Adapter. *iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) _or with an iPhone Power Adapter_.* When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.