Strava will provide reasonable estimates if the hill is steep enough and the data you enter for you weight is accurate. If you can find a 3-5 min hill then you can calculate the average power based on your time up the hill and the elevation gained. The steeper the hill (and slower the ascent) the more accurate the results will be.

Best would be to borrow someone's powertap. I always thought Vector pedals were better for someone who already had a powermeter.