It would certainly be nice to have 120VAC from our vehicles without having to go through our DC/DC converters then to a 12VDC to 120VAC inverter.
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Why is the cost so much higher to go from 120VDC than 12VDC? Is it simply the volume of 12VDC inverters that keeps the cost so low? I'd think it would be easier dealing with the lower currents of 120VDC to 120VAC.
Dexion is correct in that the typical 12VDC to 120VAC inverter these days has two stages: the first boosts the incoming 12VDC to a regulated 165VDC and the second is an H-bridge to chop that into a 60Hz bipolar square wave. A "pure sine wave" version will have an LC filter on the output that rounds off the square wave into something closer to a sine wave (with maybe 10% THD rather than 40%).
Obviously, if you skip the first stage you will need to supply the H-bridge with close to 165VDC. Some devices - mainly those with switching power supplies - will tolerate a lot of variation in the "line voltage", but products with induction motors (like refrigerators and air conditioners) really need the V/Hz relationship to be maintained so they don't overheat. So, the upshot is that you need a regulated SMPS stage no matter the input voltage. A 2kW inverter for $450 doesn't seem too out of line. Can you get something cheaper from Harbor Freight that runs off 12VDC? Sure, but it's also something cheap from Harbor Freight...
Otherwise, yeah, the reason 12VDC inverters are so cheap is simply volume...
That said, I do kinda like this idea, but it would be really nice to make the charger bidirectional so that one box can perform both functions.