Jakub, welcome! Spend time surfing this site and you will find more information about EVs and charging than you ever thought possible. Good luck, and feel free to ask more questions.
It is not absolutely necessary to "fill" the battery every time. Most of the time (except extreme winter cold) you should be able to make the 40 mi round trip on a single charge from home. In this situation, the Leaf and charging at home is a perfect fit for you.
However, in winter, especially -15 F, you will need to charge both at home and at work to feel comfortable with the range (and to use the heater). If you are hardy mid-westerner, and used to wearing heavy winter clothes, you can probably get away with minimal heat. I typically wear my coat and normal winter cloths, but my biggest problem is cold feet. I don't like to drive in heavy winter boots because braking and accelerating requires more care in heavy boots.
I would suggest that you get a charging station installed at home in addition to the supplied level 1 (L1) 120 V, 12 Amp cord. Although the EVSEupgrade.com product is very good, I would not recommend it as your only method of charging. Imagine that it's -20F outside (hmmm, doesn't require too much imagination does it?), you go out and unplug your EVSEupgrade.com cord, coil up the stiff, cold, dirty cord and place it into the back of the Leaf. Drive to work, get out and uncoil the same cord, plug into a socket at work (with the appropriate adapter) in the snow. Work for 8 hrs, then repeat the process, again with a stiff, cold, dirty cord in the snow/ice and extreme cold. Repeat daily for 3 months every winter. Sorry, that's not my idea of fun.
At a minimum, get a permanent 240 L2 charging station installed at home (or purchase an extra EVSEupgrade.com cord and leave it mounted at home). Also, remember that the EVSEupgrade is not recommended for use outside, in wet conditions, or with extension cords (*Note: Yes, it's possible to use it that way, but you should be careful and not leave it unattended or have the proper gauge extension cord).
EDIT: Do a search for Kubel and review some of his posts. He has a similar commute and temps. I think he is charging with L1 (120 V, 12 Amp) while at work. In this case, you might get 8 hrs = 8 KWH of charge while at work, or about 1/3 of the battery (4/12 bars). In very cold weather, that might give you somewhere between 15-20 miles of range. If you charge at home overnight on L1, you might get another 12 KWH, so you wouldn't be able to completely fill the battery. In such cold areas, some form of L2 (240 V either 16, 20, 25, or 30 amps) is required. Remember, the wall can only supply so much and the car can only accept so much electricity. Try to match the two, but it's not absolutely mandatory, you just need to re-fill the battery for the next day's use. For example, I charge on L1 exclusively but I only have a 8 mi RT commute so I only need a couple of hours (even on the coldest days) to re-fill the battery. However, it does take a full 20-21 hours to fill the battery from completely empty (which I've only done twice just as an experiment).
