By using manual ELISA method, you can detect about 80 single total IgE measurements (96-well). The rest of wells are for blanks and standards.

In our facility we use both methods (Immunocap and ELISA), we detected about 20-30 % in coefficient of variation (CV) between both methods. Immunocap is more sensitive than ELISA. The number of tests you can performed in immunocap is higher, because is an automated method, for that reason is faster than ELISA.

The greatest variation is probably among the test providers, as no uniform standard exist for the antigens. The platform is less important, but automatic systems usually have lower CV than manual ELISA´s. Furthermore, ELISA is a batch platform that are not suitable for bigger rutine labs.

Also see:

Williams, P. Brock, et al. "Analytic precision and accuracy of commercial immunoassays for specific IgE: establishing a standard." Journal of allergy and clinical immunology 105.6 (2000): 1221-1230.

Szecsi, Pal B., and Steen Stender. "Comparison of immunoglobulin E measurements on IMMULITE and ImmunoCAP in samples consisting of allergen-specific mouse-human chimeric monoclonal antibodies towards allergen extracts and four recombinant allergens." International archives of allergy and immunology 162.2 (2013): 131-134.

Szecsi, P., and L. Vennemalm. "Detection of IgE antibodies to penicillin allergens: comparison of commercial systems of ImmunoCAP and Immulite 2500." ALLERGY. Vol. 63. COMMERCE PLACE, 350 MAIN ST, MALDEN 02148, MA USA: WILEY-BLACKWELL, 2008.

An automated method will give you the possibility to perform more test under optimizied test conditions in comparison to an ELISA, were test conditions is due to technicians and lab conditions.  This is will normally give you lower CV% for automated systems.  However, there are also differences between different systems, both in variations and in units. 

See also:

Wood, R A, Segall N, Ahlstedt S and Williams P B, Accuracy of IgE antibody laboratory results, Ann Allergy Asthma Immunol. 2007;99;34-41