Afaik the only Essentials hosts which may be managed by vCenter Server Standard are the ones in ther ROBO Kits.
Other than this, see http://www.vmware.com/files/pdf/vsphere_pricing.pdf (page 7)
VMware vSphere Essentials Kits
VMware vSphere Essentials Kits are all-in-one solutions for small environments (up to three hosts with two CPUs each) available in two editions— Essentials and Essentials Plus (see Figure 3). Both editions include vSphere processor licenses and vCenter Server for Essentials for an environment of up to three hosts (up to 2 CPUs each). Scalability limits for the Essentials Kits are product-enforced and cannot be extended other than by upgrading the whole kit to an Acceleration Kit (see Paid Edition Upgrades section below). vSphere Essentials and Essentials Plus Kits are self-contained solutions and may not be decoupled, or combined with other vSphere editions.
André