Well considering that P=E*E/R then 10*10/20 equals 5 watts in my book.
Audio amps generally have 10% THD as a rule of thumb, at 8 ohms. And
an amp that is rated for 5 watts at 8 ohms won't deliver 5 watts into
20 ohms. And that's because the Vcc doesn't go up when you put 20 ohms
on. The Vcc stays the same.So if you put 20 ohms on a 12.5 watt amp
rated for 8 ohms, it will draw only 5 watts of power at full blast.

Believe me I've subbed out 16 ohm speakers for 8 ohm, not only does
the power drop but the distortion goes way up to the point that it
would give you a headache.