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An e-charger will be on a few seconds every few minutes... all it'll be good for is short bursts of acceleration.
But the heavy batteries needed to run it... or the ultra-high voltage alternator or not-so-high voltage second alternator needed in their stead will still be there weighing your car down and sucking accessory power out of the engine when it's off. The only successful aftermarket e-charger of previous decades, the Thomas Knight, came with 15 hp worth of hand-wound electric motors and required a few extra lead-acid batteries to give it a minute's worth of around 6 psi boost.
Newer systems with lithium batteries and more efficient motors can do better, but they're still $$$. Audi is using the e-charger to remove lag on an already heavy and powerful system, where the size of the turbos make lag a real issue... and it can be tied into a mild hybrid set-up.
For small cars, it makes more sense to simply have a single scroll light boost turbo to supplement power at low rpm without causing too much extra fuel consumption at cruise.
If you still want e-assist over that, the simplest and most cost-effective method might simply be to replace the starter motor with an assist motor that can move the car at low speeds, to be used in conjunction with an auto-stop system. An Indian group was looking at something like that... but ultra-cheap, it was to attach to the motor via accessory belts. Supposedly saved fuel, but it's been years and it hasn't gone commercial.
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