A lot of people just assume you need a level 2 charger, but I've put 20,000 miles on my SL+ using nothing but a 110-volt outlet. The decision to go with a level 2 charger really depends on how many miles you tend to drive per day. From a standard outlet you will get about 5 miles of range for every hour of charging. If you can charge for 12 hours overnight, you can add 60 miles of range.
When I got my car I looked into our electricity supplier's level 2 charger plan. They offer cheaper rates if you charge overnight using their charger. But they also charge a monthly fee. I did the math based on my average driving, and it turned out it would take 13 years before I broke even for the cost of the charger, installation, and the monthly fee.