Safari High Avg Energy Impact
I'd simply ignore it. Energy Impact is just an arbitrary number showing how much energy an app is consuming (each instant and on average).

Activity Monitor is simply telling you that the app which uses the most energy is Safari.

To make sure that Safari is not eating up on CPU, you can check the CPU usage over time, for example using a tool like 'uptime' from the command line (simply launch the Terminal an type uptime), this will tell you the uptime of the system and the load average over the past 1, 5 and 15 minutes, this number should be close to (or less than) 1 on Yosemite. A load average constantly above 2 (or more) could indicate some process sucking up on CPU, which can found using Activity Monitor.