  White dwarf photospheric parameters are usually obtained by means of
spectroscopic or photometric analysis. These results are not always consistent
with each other, with the published values often including just the statistical
uncertainties. The differences are more dramatic for white dwarfs with
helium-dominated photospheres, so to obtain realistic uncertainties we have
analysed a sample of 13 of these white dwarfs, applying both techniques to up
to three different spectroscopic and photometric data sets for each star. We
found mean standard deviations of < $\sigma T_{\mathrm{eff}}$ > = 524 K, <
$\sigma \log g$ > = 0.27 dex and < $\sigma \log(\mathrm{H/He})$ > = 0.31 dex
for the effective temperature, surface gravity and relative hydrogen abundance,
respectively, when modelling diverse spectroscopic data. The photometric fits
provided mean standard deviations up to < $\sigma T_{\mathrm{eff}}$ > = 1210 K
and < $\sigma \log g$ > = 0.13 dex. We suggest these values to be adopted as
realistic lower limits to the published uncertainties in parameters derived
from spectroscopic and photometric fits for white dwarfs with similar
characteristics. In addition, we investigate the effect of fitting the
observational data adopting three different photospheric chemical compositions.
In general, pure helium model spectra result in larger $T_{\mathrm{eff}}$
compared to those derived from models with traces of hydrogen. The $\log g$
shows opposite trends: smaller spectroscopic values and larger photometric ones
when compared to models with hydrogen. The addition of metals to the models
also affects the derived atmospheric parameters, but a clear trend is not
found.
