Receptors switch between resting-active conformations (C-O) under the influence of agonists that bind weakly versus strongly. We estimated the two equilibrium dissociation constants, KdC and KdO, from single channel currents of adult neuromuscular nicotinic acetylcholine receptors (AChRs) either by kinetic modeling or from dose-response curves. After removing the chemical potential by normalizing with constants from a standard agonist, the efficiency (η) at which chemical binding energy is converted into mechanical work of gating was calculated as η=1-κ where κ is the free energy ratio logKdC/logKdO. η is also a measure of the correlation between agonist efficacy and affinity. In wt AChRs, ηACh is 50% at the adult α-ε and α-δ neurotransmitter sites, and 56% at fetal α-γ. We measured η for 22 agonists in wt AChRs or for several agonists after a mutation of 8 different binding site amino acids. With both types of perturbation there are 6 η-classes (30, 41, 45, 51, 54 and 60%), with smaller agonists generally having a greater efficiency. Elsewhere we show that for 4 agonists, η values calculated from structures by using MD simulations agree with those obtained experimentally. Efficiency is a universal agonist attribute that offers a new way to classify ligands and a window into the structural basis of receptor activation by agonists.