A typical inverter uses MOSFET or IGBT transistors to switch a DC voltage which is then fed through a capacitor as a form of isolation. Typical inverters are use on Solar panel systems to convert PV cells energy into AC, battery backup to AC (Uniinterruptible power supplies), or single phase to multiphase AC to drive AC motors efficiently.
One of the reasons they are "big" is because typically they operate at 60 or 50 hz, and at those frequencies if you are using magnetic fields for isolation (like you would if you drove one side of a transformer) the transformers are annoyingly large and hard to make efficient. [1] Many modern inverter start with 280 - 480v DC and use a series of transistors to create an approximation of a sine wave (this is how the cheesy plug into your car lighter inverters usually work). Once you get above a 100W it starts to get a bit more difficult to do cheaply and with reasonable efficiency.
Efficiency drains are also present in the upscaling the voltage (whether your using a boost switching circuit or a simple diode/capacitor pump). So getting these things to be efficient is hard, and they are of course generally fairly large per watt.
I suspect Google is looking for something to invert PV solar arrays, but high density power conversion is always valuable.
[1] That said, a lot of people made high voltage supplies out of using a 555 to switch a transistor on and off which fed the 'low' side of a power supply transformer. I had a Xenon Strobe circuit that did that, made a nice little 600V supply.
BTW.... Dart and others (like the iPad recharger [1]) typically convert AC->HV DC (rectifier)->Flyback (at 10's of kHz)->low voltage DC. The intermediate conversion to DC followed by "chopping" at a higher frequency on the flyback transformer allows designers to use smaller magnetics than what would be required of "classic" 50-60Hz wallwarts. I'm sure you're already well aware of all of this given your comment. But so am I (despite the comments suggesting I'm misunderstanding converters-vs-inverters). ;-)
If the Google call is for energy generation, then there's also the added difficulty of maximum power-point tracking as well...
[1] http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pr...
Works fine except for the output which by spec has to be 50 or 60hz. I believe even existing designs use a boost switcher to convert x DC to ~ 200V DC before shaping it into something that looks nominally like a 110V sine wave.
So if you break the problem in two (input to source DC) and (source DC to 110V AC sine wave) then I completely agree that advances in SMPS design components and techniques can really help the first part, but I think we're still searching for a low loss power amplifier for the second part.
A transistor generates no loss if it is in the full on or full off state (saturated). But every time a transistor switches from the on to off state (or back) it goes through its linear region. While in the linear region the transistor acts as a resistor and generates heat. If you increase the switching frequency the transistor switches more often and thus generates more heat.
The solution to this problem is to use more efficient transistors or decrease the switching time (the time it takes to switch from high to low, or back).
Of course higher switching frequencies also have lots of other problems such as radiation, skin effect, etc.
Not entirely true. They make a lot LESS loss when fully on than when linear, but there's still some loss.
Even with a highly efficient transistor you can still get losses while in the linear region if your gate drive circuit can't push enough current. When designing a switching power supply you don't just hook the microcontroller output to the gate of the transistor. To do it right you might need one or two or three intermediate stages of power amplification so that you can switch the main transistor's gate very quickly.