I just purchased a new 2013 Fusion Hybrid in September 2014. Until the cold weather hit, I was averaging 56 mpg UK (47 mpg U.S. or 5L/100km) over the course of 6000km (3700 miles). I noticed that the last 2 fill-ups (the only two I have entered) averaged 49mpg UK (41 mpg US, 5.7l/100km) as the weathered dipped below freezing. I would like to know from other hybrid owners how bad a hit their fuel economy takes when more frigid temperatures hit.
I can't say how bad a hit it was when I had the Prius. It's been awhile since then and the winters were also mild. Do you have data for the previous car? The percent drop it experienced for winter should be close for what the Fusion will experience.
I think the hybrid has active grill shutters. Do they close up in cold temperatures to help with warm up time?
About a 10% decrease seems normal for any car, not just hybrids, when the temp drops. Although in midwinter, December, Jan/Feb I expect it to be closer to 15% maybe even 20% if you start the car for defrosting etc.
Yes, the battery is affected by the cold. It reportedly warms up quickly though. Plus they are mounted within the trunk or cabin space. So they won't lose heat as quickly as the engine out in the cold.