The concept of charge efficacy is introduced as a measure of the effectiveness of incident charge for producing single-event upsets. Efficacy is a measure of the single-event upset (SEU) sensitivity within a cell. It is illustrated how the efficacy curve can be determined from standard heavy-ion or pulsed laser SEU cross-section data, and discussed how it can be calculated from combined charge collection and circuit analysis. Upset rates can be determined using the figure of merit approach, and values determined from the laser cross-sections or from the mixed-mode simulations. The standard integral rectangular parallel-piped (IRPP) method for upset rate calculation is re-examined assuming that the probability of upset depends on the location of the hit on the surface. It is concluded that it is unnecessary to reformulate the IRPP approach.