I bought this primarily to test USB output on a new inverter generator but since getting it I've tested various USB chargers I use as well. I've even tested USB outputs on a few desktop and laptop PC's just because I can. No big surprises.I don't understand electronics but there seems to be built-in circuitry to account for both supply-side current and demand-side draw. For instance testing 3 different phones - an Altel flip, and older Moto android, and my newest Moto android - with two different chargers and my primary desktop PC the results are about what one would expect.I'm reasonably certain that this unit works perfectly! I concluded that the voltage output on the new generator was somewhat "unsteady" depending on other 120 AC demands so I wouldn't trust it to charge my new android phone.