  We study universal aspects of fluctuations in an ensemble of noninteracting
continuous quantum thermal machines in the steady state limit. Considering an
individual machine, such as a refrigerator, in which relative fluctuations (and
high order cumulants) of the cooling heat current to the absorbed heat current,
$\eta^{(n)}$, are upper-bounded, $\eta^{(n)}\leq \eta_C^n$ with $n\geq 2$ and
$\eta_C$ the Carnot efficiency, we prove that an {\it ensemble} of $N$ distinct
machines similarly satisfies this upper bound on the relative fluctuations of
the ensemble, $\eta_N^{(n)}\leq \eta_C^n$. For an ensemble of distinct quantum
{\it refrigerators} with components operating in the tight coupling limit we
further prove the existence of a {\it lower bound} on $\eta_N^{(n)}$ in
specific cases, exemplified on three-level quantum absorption refrigerators and
resonant-energy thermoelectric junctions. Beyond special cases, the existence
of a lower bound on $\eta_N^{(2)}$ for an ensemble of quantum refrigerators is
demonstrated by numerical simulations.

Statistical Mechanics;Mesoscale and Nanoscale Physics