This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed firstname.lastname@example.org with numerous contributions by others.
Motors are traditionally specified by a system which attempts to equate
them to wet engines. There are significant problems with this, but they
probably aren't of concern to beginners. An "05" motor takes a six or
seven cell battery and puts out 75 to 120 watts, and so on up to a "60"
which takes 28 cells and puts out 1200 watts. Incidentally, there are
about 750 watts in a horsepower.
The actual power output for a given voltage (number of cells) depends on
the load. Unlike wet engines, electric motors put out more power with more
load. If you don't like the performance you get from your plane, you can
try a bigger propellor---up to a point. More power, of course, means less
In the ideal world, motors would be specified by the total power they are
capable of supporting and by the number of cells (or voltage) with which
that power is produced.