Man that SUCKS. what ever you do don't tell them you were running the unit on a light cord. It may void your warranty. reason is light cords = low voltage making your unit draw more amps. Without going into great detail this is all due to OHM's law (P = I X E) where P is watts I is amps and E is voltage
Do yourself a favor and get a heavy cord. this may help you decide what gage cord, you need. If your unit only states watts (take the watts and / by the voltage) example 1200 watts/ 120 volts = 10 amps. Keep your voltage drop to around 3% max.