It all depends on the vehicle, my car weighs 800kg so a 100kg load increase in town may well hurt the fuel economy noticably especially if your area is mountainous.
Modern car AC has improved out of sight regarding motor load compare to earlier ones, I had a Commodore wagon that the AC would easily add 1l/100, but then I was only getting ~18l/100klm out of it anyway
come to think of it, it was a work car and always had a couple of hundred kilo of gear in the back, may help explain the appalling fuel economy.
