  A major ingredient for kilonova lightcurves is the radioactive heating rate
and its dependence on the electron fraction and velocity of the ejecta and, in
principle, on the nuclear mass formula. Heating-rate formulae commonly used as
the basis for kilonova models are, strictly speaking, incorrect for electron
fractions other than $Y_{e} = 0.04$. Here, we introduce new semi-analytical
models for kilonovae with better heating rate prescriptions valid for the full
parameter space of kilonova velocities and electron fractions. This new
prescription produces, on average, dimmer kilonovae at peak that decay more
slowly as compared to previously used prescriptions for otherwise identical
kilonova physics. We show the dangers of using inappropriate heating rate
estimates by simulating realistic observations and inferring the kilonova
parameters via a misspecified heating-rate prescription. While providing great
fits to the photometry, an incorrect heating-rate prescription fails to recover
the input ejecta masses at greater than $5\sigma$. This bias from an incorrect
prescription has disastrous consequences for interpreting kilonovae, their use
as additional components in gamma-ray burst afterglows, and understanding their
role in cosmic chemical evolution or for multi-messenger constraints on the
nuclear equation of state. Given the true heating-rate is uncertain, we
estimate there is a $\approx 5$ and $\approx 10\%$ systematic uncertainty in
the measured ejecta masses and velocities, respectively. For lanthanide-rich
ejecta, the systematic uncertainty could be as high as $\approx 50\%$. This
systematic uncertainty limits the precision of any measurements that rely on
accurate estimates of kilonova ejecta properties.
