.

At low temperature ($\sim$100 mK), calorimetry based on Transition Edge Sensor (TES) technology is quite mature and has been used for energy measurement at virtually all energy scales from infrared-photon ($\sim$100 meV) [@NIR_TES_08] to alpha detection ($\sim$10 MeV) [@alphaTES_08]. The measured energy resolution of well designed devices throughout this regime roughly matches the theoretical expectation of $$\sigma_{E}^{2} = \frac{4 k_{b} T_{c}^{2} C}{\alpha} \sqrt{\frac{n {\mathcal{F}}(T_{c},T_{b}) \xi(I)}{1-\frac{T_{b}^n}{T_{c}^n}}}  
\label{eq:Eres_lim}$$ where $T_{c}$ is the superconducting transition temperature of the TES, $\alpha$ ($\frac{T}{R}\frac{\partial R}{\partial T}$) is the unitless sensitivity parameter, and the unitless terms within the square root all combine to usually be of order 2 [@Irwin_TES].

![[]{data-label="fig:IrwinEres_T"}](figs/Gen/IdealEres_Tc_SimData.pdf){width="2.9in"}

In Fig. \[fig:IrwinEres\_T\], we benchmark the measured performance of the current generation of massive calorimeters operating near 10 mK (stars) and a detailed proposal for a 50 g Si coherent neutrino scattering detector [@Tali_11_CNS] (black circle) against the theoretical scaling law of Eq. \[eq:Eres\_lim\] assuming that the total heat capacity of the detector is 2$\times$ that of the absorber and that $\alpha = 20$ (values of 10–500 are common). Unfortunately we find that the energy-resolution performance of all current experiments is 2–3 orders of magnitude worse than expected from Eq. \[eq:Eres\_lim\]. Consequently, either there are substantial 