e-ram is crap. I have seen one electric supercharger that actually did something, it worked off a wide open throttle switch. It was a Cedarville College engineering project for an ethanol fuel challenge.
Once vehicles switch over to 42v electrical systems, electric superchargers will become more feasible.
I was looking through the grainger catalog, there was 1 12v blower they had that could flow 150 CFM at 2" back pressure, which would make a little boost at WOT for a 1.5liter engine, the math just isn't in favor of doing it on a bigger engine. Even 240v blowers don't flow enough at much pressure until they are very big and power hungry.
heres an explanation i found that may explain it a little better...
There's no laws-of-physics reason why a forced induction device has to be spun by exhaust gas pressure or a belt from the engine. An electric blower certainly can do the same job as a turbo or belt-drive pump.
The problem is getting enough pressure. The people selling cheap electric superchargers - and there are plenty of them - like to talk about the amazing air flow statistics of their ferocious blowers, but they fail to note that the static pressure capability of those blowers is not nearly as impressive. Let the blower fly around on the end of its cable, or just use it to ventilate your house, and it will indeed shift enough air that you'd think it'd have a fighting chance of doing the job of a supercharger. Actually connect it to an engine, and its flow will plummet. Merely hooking up a fan-based blower that can shift 600 cubic feet per minute (when hanging in space) to an engine that natively inhales, say, 300cfm of air, will definitely not give you one atmosphere of boost.
Forced induction for engines is, by definition, not happening at atmospheric pressure; you want boost, more air being crammed in there than you could get just from a big tube leading to the outside world. Electric blowers don't come anywhere near delivering enough compression power to make a difference to the operation of normal engines, unless the electric motor involved is so inspiringly humungous that it's got similar power performance to a real supercharger or turbocharger - and, then, it can be argued that you might as well have a real supercharger, running directly from the engine via a belt, rather than an electric motor running from the alternator which in turn is running from the engine. There's no way for that electromechanical train to be more efficient than a humble belt.
Most cheap "electric turbos" or "electric superchargers" are actually bilge ventilating fans for boats (they're meant to be preventing dangerous fuel vapour buildup belowdecks), possibly with impressive jet-engine-ish decorations, generally sold at a monstrous markup. The suspiciously cheap eSuperchargers.com proposition is different; they're charging people $US14.95 for the privilege of being told to buy a bilge fan.
The most popular electric supercharger is the eRam. It seems to actually be good for a few per cent more power on many cars ( like this one). But the eRam costs $US300, plus installation time. There are lots of other tweaks you can get for that kind of money. The difference from the eRam is small enough that your engine computer ought not to need any tweaking to deal with it - which is good, but which also indicates firmly that there's not much actually going on. Un-modified ECUs don't work right when you add a proper forced induction device to an engine, because they don't expect to see air coming in at higher than atmospheric pressure.
A more impressive system is Thomas Knight's ESC Electric Supercharger. It's got three big-ass motors driving a conventional Roots blower, and the claims made for it seem plausible enough, though I've little idea how well it really works.
The contraindications for the ESC are considerable, and such that the manufacturer doesn't recommend you buy one if you can get a regular somethingcharger kit for your car. The ESC costs thousands of dollars (around the same price as a regular turbo or even supercharger kit; it's just easier to install), it requires the installation of some extra batteries (which aren't included in the price; you may also need a high-output alternator to recharge them), and it makes a very loud noise (which you may or may not view as a drawback).
You also can't use the ESC all the time, because it drains its batteries much faster than any alternator can charge them. It's more like electric nitrous (or a switchable "Mad Max" supercharger) than like a conventional always-on supercharger. And, unlike regular whatever-chargers, the ESC doesn't run faster when the engine does; its speed is fixed, so it delivers respectable boost at low RPM but fades as you rev higher, because the engine then needs more air per second for the same boost, but is getting the same amount from the ESC.
The new 42 volt automotive electrical standard will indeed make it easier to make an electric supercharger work, because the multi-kilowatt motors required will no longer need to draw (such) horrendous currents through (such) giant cables. 12 kilowatts (an unremarkable small mechanical supercharger power consumption rating) at 12 volts is a thousand amps; you need a cable an inch thick to pass that continuously, though you can get away with mere third-inch 0 or 1 AWG cable for low duty cycle applications. A 42 volt supply would drop the current draw to a mere 286 amps. But the motors will still be big and beefy, and the other electro-charger problems will still apply
91 is right, and I'd like to add too that an electric supercharger (even one that works) is going to create parastic losses, via the aternator. Couple that with current losses in wires, etc., and the "electric supercharger" concept is just not worth it. But hey, people are still going to place their false hopes on these things, "Nismo" air filters, and resistors in fancy black boxes.