In the book Bicycling Science by David Gordon Wilson, it says the following:
When new, clean, and well-lubricated, and when sprockets with a
minimum of 21 teeth are used, a chain transmission is highly efficient
(at a level of maybe 98.5 percent or even higher)
(Third Edition, chapter 9: Mechanics and Mechanisms: Power Transmission, page 318)
It goes on to describe that efficiency inevitably suffers due to wear, dirt, and poor maintenance and adds "it is not known by how much the efficiency of chain drives decreases in adverse conditions,".
Later, on page 342 it provides some other factors which influence efficiency:
Transmission efficiency decreases as the size of the rear sprocket
Efficiency diminishes as the amount of torque transferred (or chain tension) is decreased.
The maximum efficiency attained is at relatively high power (175 W) and low pedal rpm (60) and in the lowest gear (meaning the
largest- diameter rear sprocket, with twenty-one teeth) and is just
over 98 percent.
The additional losses because of chain offset (the two sprockets not being in line) are negligible.
The type of lubrication, or even whether there is lubricant present, has almost no effect on efficiency
And finally some typical gearing ratios are matched against some other factors to estimate efficiency:
Based on personal riding experience I would find it hard to believe a 20% loss was not an overestimate, but I have no other sources with quantified data to back that up.