Power pack output is usually rated in "VA" or "volt amps", roughly
equivalent to watts (wattage isn't exactly volts*amps for AC). NB that
this is the _total_ power output on both DC and AC circuits. If you
don't use the AC side, you can draw more amps on the DC side.
There is no difference in voltage between HO and N: both are supposed to
be 12V max, and locos are supposed to be designed to reach their
prototypically maximum speed at 12V, but few locos do. In practice, many
(most?) HO and N scale locos reach their maximum prototypical speeds
But there is a difference in power between N and HO. In the Olden Days
(before high-precision gear trains and high-efficiency motors), the
difference could be substantial. A typical DC motor drew 1/2 amp or more
in an HO engine, and about 1/4 amp in an N engine. So p/p for N gauge
could be made at lower power ratings.
Nowadays, the difference is less. I've seen recent O scale locos draw
less than an amp, an unheard of thing 40 years ago. The minimum power
output for any scale in my experience should be 7VA (about 1/2amp at
12VDC), although this will be just barely enough for a small O scale or
G scale loco. Note that if you use an underpowered power pack, it may
run hot, and the circuit breaker may trip.
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