The quantification of the individual 𝐸 𝑎𝑏𝑠 of the catalytic as well as the antenna nanostructure is important before delving into the contributions of 𝐸 ↓ 𝐴𝑐𝑡𝑖𝑣𝑎𝑡𝑖𝑜𝑛 𝑒𝑛𝑒𝑟𝑔𝑦 and 𝐸 ↑ 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 . The benchmark case of Herran et al. uses bi-metallic Au + Pt nanoparticle arrays as the antenna-reactor system (antenna: Au nanoparticle, reactor: Pt nanoparticle) as shown in Figure . In this work, an external cooling system is used for isothermal operation, to exclude the contribution of 𝐸 ↑ 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 . As shown in Figure (a), Pt nanoparticles in the presence of Au nanoparticles have a remarkably high (>44 times at 620 nm) absorptance as compared to Pt nanoparticles without Au nanoparticles. The high absorptance of Pt nanoparticles occurs at wavelengths >500 nm, coinciding with the lattice plasmon resonance of the arrays as indicated by the near-field enhancement spectrum in the bottom part of Figure . Apparently, the lattice plasmon resonance is actually driven by the strong plasmonic, larger Au nanoparticles through plasmonic coupling resulting in more than 120 times near-field enhancement in the inter-particle gaps, Figure . Thus, Pt nanoparticles being exposed to such concentrated energy absorb unusually high amounts of energy. Interestingly, Figure shows that Pt nanoparticles in case A have their own strong near-field enhancement (decoupled from the contribution of Au) which is >2.5 times that of isolated Pt nanoparticles, implying strong electrodynamics that leads to the generation of hot-carriers. It is also important to note that the activities of various Pt nanoparticles are not enhanced equally. As shown in Figure (b), the near-field enhancement is strongly polarization-sensitive, and exclusively dominant for Pt nanoparticles that are in between two neighbouring Au nanoparticles. Thus, the Pt nanoparticles trapped in the interstitial voids of three neighbouring Au nanoparticles remain essentially dormant in the antenna-reactor system. Figure shows that almost all the absorptance of Pt nanoparticles come from those trapped between two Au nanoparticles, with negligible contribution from the ones in the interstitial voids. It is clear from Figure that the Pt nanoparticles in the hot-spots absorb extremely high amount of light energy i.e. > 40 times of that by isolated nanoparticles. The cost to be paid here is the accompanying even larger energy absorption by the Au nanoparticles, which basically thermalizes as antenna Au nanoparticles are not participating in the reaction directly. Figure shows that under AM1.5G illumination, the bimetallic arrays absorb 22.7% of the total incident power, of which 68.6% goes to the Au nanoparticles, leaving the remainder to the hot-spot Pt nanoparticles. When the absorbed electromagnetic energy in Pt is not channelled through hot carrier surface activity, it dissipates as heat. For the heat transfer analysis, it is assumed that all the absorbed energy thermalizes (i.e. the 100% photothermal limit). While both Au and Pt nanoparticles release thermalized energy, the reaction rate depends on the steady-state temperature of the catalyst film, which is determined by both the absorption power as well as the rate of heat loss to the surroundings. Figure demonstrates two limiting cases for heat-transfer analysis where the nanoparticle film on a glass substrate is in an infinite domain. Case 1 represents the limiting case for aqueous phase reactions, while case 2 represents the limit of perfect insulation (details in supporting information). In general, as shown in Figure , much of the film attains at least 90% of the core temperature upon illumination. The steady-state temperature profile is determined by the thermal accumulation effect, which is simply enhanced by a higher heat generation rate relative to the dissipation rate. One way to increase thermal accumulation is by scaling-up the system. In three dimensions, the surface to volume ratio of spheres is inversely proportional the nanoparticle radius, r. This means, for constant heat generation per unit of volume, lesser surface area is available for heat transfer for bigger spheres, thereby increasing the sphere temperature. An analogous spot-size vs. temperature relation is shown in Figure , where a larger illuminated spot size leads to higher core temperatures. In Case 1, which represents liquid phase reactions, more than 12 cm x 12 cm illuminated catalyst area is required to increase the temperature by just 15 o C. Important to note is that the Au + Pt film only absorbs 22.7% of the irradiance. Clearly, a perfectly absorbing surface (case F) can attain much higher temperatures under the same conditions, Figure . As shown by Figure , the second and the more efficient way to increase thermal accumulation is by increasing the intensity, i.e. concentrating the incident light to a smaller area. Clearly, concentrating the same illumination power from an area of 12 cm x 12 cm to 2 cm x 2 cm (equivalent of increasing the irradiance from 1 sun to 36 suns), increases the core temperature by ~55 o C, much higher than what is achieved with 1 sun over 12 cm x 12 cm area.