Approximately 1 teaspoon of onion powder is equivalent to 1/3 of an onion. So 3 teaspoons of onion powder would be necessary to substitute for an entire onion.
Onion powder is made from dehydrated onions, so it is much more concentrated than a regular onion, which contains moisture. This is why it doesn't require very much onion powder to equal an entire onion. There are important differences between onion powder and fresh onion, however; onion powder has a less potent flavor than a fresh onion, so if a heavy onion flavor is desired, it may be necessary to add more than the recommended amount of onion powder.