While you can charge at 120V (and several people on this forum do this or have done this while waiting for a 240V plug to be installed), it turns out to be quite a bit less efficient in energy usage than charging using any 240V power source.
Tesla has recently updated their charging calculator (click here: http://www.teslamotors.com/charging#/calculator) and by playing with the dials you can see how much actual energy the car needs to charge using different plugs and power sources.
If you use a 120V plug to charge instead of a NEMA 14-50 240V plug, you'll use about 32% more energy (and money) to charge up your car.
Electricity is so cheap though, that that isn't the end of the world - if you drive 1,000 miles a month (12,000 miles a year) at $0.12/kWh, that'll be an extra $12.70 per month in electric expenses.
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