For more details, see the Glossary entry on Power.
In order for Strava to calculate your watts, you'll need three things:
A bike attached to your ride. If you have no bikes on Strava, or do not attach a bike to your ride a default road bike weight of 9kg, or approximately 20lbs will be used instead. See Gear for more help.
Trusted elevation numbers - from either a device with a barometric altimeter, like a Garmin Edge 500, or from our internal elevation correction. For troubleshooting, see Checking your Elevation Correction, and for more information about elevation correction, go here.
To ensure power estimates are as accurate as possible, make sure you have accurate weight data for your bike, and yourself. Make sure you include weight for clothes (helmet, shoes etc) tools and other gear and supplies.