15 amps is needed for 1500w?
Power = volts * amps.
So 1500W = 120V * 12.5A
A standard USA 120V outlet is rated at 15A normal use, 80% of that for continuous use so 12A.
I just read that one of these converters suggested that a 1500w converter works for 800w continuous use. 3000w+ for 1500w continuous use.
OK, so their nameplate rating assumes a very low duty cycle - might be appropriate for running power tools on a construction site or similar.
Anyhow, the bottom line here is that you have only 1500W available from your outlet without setting the house on fire (or, hopefully, tripping the breaker first).
A quick google search suggests the Ford Focus EV is capable of charging at 120V, so it will achieve that 1500W without any help from a voltage converter; in fact the voltage converter will just waste energy (by getting hot).
Another consideration is that most EVs charge slightly more than twice as fast at 240V compared to 120V even at the same current - there's a certain amount of fixed overhead (energy wasted) per hour of charging, so at very low rates of charging this dominates the (in-)efficiency. Therefore your 120V charging - even if it is just about enough overnight to make your commute - will end up costing you slightly more in electricity than 240V charging. Maybe not enough to cover the cost of the 240V wiring, but another reason to justify it.