A power nap is a nap that lasts for between 15 to 20 minutes.
However some people may take a power nap that lasts 30 minutes.
When you take a power nap you're taking a short sleep so that you can refresh yourself and recharge yourself so you'll be more energized during the rest of the day.
A Power Nap is a short sleep that terminates before deep sleep (slow-wave sleep; SWS); it is intended to quickly revitalize you when you take the power nap.
Cornell University social psychologist James Maas coined the term power nap.
Taking a power nap is a good thing when you need to refresh yourself and are feeling tired.
You can take a longer nap in the day if you want too but if you just need a short power nap 15 to 20 minutes per power nap is usually long enough for most people.