If your car is the only device on the circuit you are charging from, set it to "max". Really, if you are using a Juice Box 40, it should be on a 50 amp NEMA 14-50 junction box /plug, and the circuit should be on a dedicated circuit line for that use only, with a 50 amp circuit breaker in the nearest panel, be it main, or sub panel.
Max for your car is 30 amps. Varying voltage will affect the charging rate and amount of kwh charge rate it will take. 240V will give you 7.2kwh, commerical 208 about 6.5 to 6.6 kwh , and the POS 120V charger they give you will give you 1.3kwh if your home has 120V and 12 amps at a regular plug. You should really only charge off of a 20 amp cicuit at 120V with these chargers as it's continuous draw at high amperage for the wiring in your home if only on a 15 amp breaker. The plug that your washer and dryer are on are almost always 20 amp breakers. Don't run the other devices at the same time as you are using the 120v recharger to charge your car, it you use it on occaision.