From here

http://www.powerdriveinverters.com/choose.html
To determine the size vehicle alternator needed to keep up with a specific inverter, you need to convert AC wattage to DC amperage (the value vehicle alternators are measured in) In the example below, you would need a 91 amp alternator to provide a continuous draw of a 1000 watt inverter:

Convert AC Watts to DC Amps:

AC WATTS ÷ 12 x 1.1 = DC AMPS

Example: 1000 Watts ÷ 12 = 83.33 x 1.1 = 91.66

once you have the DC amps figured out you can take 12 x the DC amps to find your wattage.

course,that would make a 150 watt inverter pull about 165 watts, but I would assume that would be at full 150 watt load. Your first charger pulls about 2 watts and the 2nd pulls about 22 watts (you can read VA or volt amps as watts). So instead of using the max output of the inverter I think I would use the load of the device plugged in plus maybe another 5 watts as a wag for what ever heat is built up in the coils. So 27 watts on the AC would end up being around 30 watts on the DC side assuming my math isn't screwed up