Every device with a power supply, like camera battery chargers, laptop PSUs, etc all have say how many watts they draw on the PSU itself.
I don't think quality matters much in terms of the inverter because in the end they are all spec-ed to do the same thing. You don't really care about how clean the power is or how much it fluctuates just to charge a camera. The little charger that you plug into the socket cleans the power for you anyway.
The funny thing is that most of the PSUs on that you connect to the device convert the 110V AC to 12V DC to supply power to the device itself. So technically, you could just get a cigarette adapter plug and just splice the wires if you really wanted to get creative. :artist: