to comment, i would point out that these
"charger for cells, laptop, handheld radios"
things are already low voltage like the solar cells
and the doubble conversion is going to be very high in losses, compared even to simple regulation for various voltages.
course AC converters have come MILES since i got one.
the one i have takes like 4-10A 12v of total waste , just to START converting
when i wanted a simple 250W out of it in 110V power, it took 28Amps from the battery 336W total, that is a whole light bulb of waste. and its waste was in the form of heat.
so that is something that you might want to add to your thought process.
what the waste is, for the consumption
i would think that even in the GOOD NEW quality EFFICIENT ones, that the bigger it is, the more it takes to fire it up.
when you read the SPECS on Efficiency, they should have specs on LOAD efficiency, or you should KNOW what the initial load is.
if your using up 150W to charge at 30W a battery.
and Using 300W to charge a 150W battery, use of the device could be tailored.