I used to do a 22 mile round trip commute by person powered bike - took about 45 -50 mins each way. One day tried it with my wife's Panasonic powered Kalkoff. The route was fairly flat and I found it a huge disappointment - power assistance cut out most of the time because I was cycling at around 15 mph, so it finished up being harder work and taking longer than the unpowered version.
Don't forget that you need to be doing say 15 mph most of the time to achieve an average of say 12 mph allowing for junctions, corners etc.
How ever I now have a Kalkoff S Pedelec and although I haven't tried it for the commute route yet, I am confident it would bring the time down significantly. I will let you know once I try it out.
The battery certainly goes down quickly if you ride it fast. I did a 26 mile ride at an average of 17.5 mph recently which exhausted the battery, some hills.
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