It takes twice the power to raise volume 3db...So take your efficiency at 1 watt...2 watts makes 3 more.4 watts makes 6 more.8 watts makes 9 more.16 watts makes 12 more.32 watts makes 15 more. 64 watts makes 18 more.128 makes 21 more.256 makes 24 more.512 makes 27 more.