poor battery performance on MacBook Air

I have owned several Airs and have always been satisfied with their performance. My current model is

abut 18 months old - the last model produced before the big refresh. The battery life has never been great, but recently it's down to ~2 hrs of use from a full charge. I've checked my stats and they are decent. 280 charge cycles and the battery charges to 89% of it's original capacity. So I looked at Activity Monitor to see if any processes were sucking energy. Under average energy impact everything was less than one, except Safari which is at 30. I have no idea what that number means. My previous Air ran the same software and it did not have this problem. My first thought is to replace the battery anyway, but if there is something going on in the software, it might kill a replacement battery.