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My motorcycle of course has a 12V battery.

I have been informed that the OEM Honda cigarette lighter is wired with an "inline 1A fuse".

I'm considering ordering a phone charging USB adapter to insert into this cigarette lighter. The USB device has 2 ports; one is rated 5V/1A and the other 5V/2.1A.

Because the charger is stepping down the battery's 12V -> 5V, does this mean I can still use the 2.1A port to charge my phone without blowing the inline 1A fuse (because 2.1 x 5 W < 12 x 1 W)? Or is it as straightforward as: I may safely use the 1A port only and not the 2.1A port?

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It depends on the efficiency of the electronics inside the charger. Presumably it will be some kind of switching regulator that will be fairly efficient.

Your calculations are correct for 100% efficiency.

Power requirements of charger = 5V x 2.1A = 10.5W (just using the 2.1A port)

Power available via 1A fuse = 12V x 1A = 12W

So in theory you should be OK, because there is room for some conversion losses. The 1A fuse will also not blow immediately after exceeding 1A, so there will be room there too.

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    Not all 5V USB sockets use a switching converter - I've used a 7805 out of one (linear regulator so draws as much current as it supplies). Some truly evil very cheap designs just use a big resistor, which means the voltage isn't even controlled
    – Chris H
    May 18, 2022 at 15:30

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