The efficiency of trombe wall can be calculated using this equation:

efficiency = solar gain / incident solar energy

solar gain = transmitted energy - lossess

transmitted energy = Incident solar radiation x Area (glass) x SHGC

losses = losses by radiation of wall to window + conduction losses of moving air to glass

note that calculating these losses is rather complicated. They are functions of several values such as emmitance of wall surface and glass, wall surface temperature, width of air gap, area of wall facing glass panels.

There are some papers giving equations for these losses you can find in journals.

Adding to Gerfried point,  Designbuilder can simulate Trombe walls. Actually one of their templates is a trombe wall

Tromb wall eff. = extracted energy from wall until it reaches initial temp./ solar falling energy on the all surfaces of the wall.

You should be able to calculate both sides of the equation right side.

This study of Trombe wall in the Canadian Climate highlights situations in which this approach is not necessarily beneficial.

http://www.cmhc-schl.gc.ca/odpub/pdf/65861.pdf