Power Supplies and Wires For some time I’ve been wondering how
the wire size for power supplies limits the power. So I’ve done
some quick calculations to determine if it’s a problem. The
first type that is of interest are the “Inverters” that are used
to convert 12VDC to 240VAC (mains power) to allow electric
devices to be operated in a car. I’ve seen some reports from
dissatisfied user about Inverters not supplying as much power as
expected and I’ve had problems with my 150W Inverter not always
supplying my Thinkpad (which definitely doesn’t draw 150W). The
second type is phone chargers as charging a phone in a
reasonable amount of time is always a problem. Inverter Rating
Fine Print vs Laptop PSU My Thinkpad Power Supply claims
“Efficiency Level IV” which according to the US EPA document
describing the efficiency marking protocol for external power
supplies  means that it is at least 85% efficient when supplying
50W+. The peak output of the PSU is 4.5A at 20V which is 90W
peak output, 90/0.85 == 106W power