documentclass{article}
usepackage{siunitx}
usepackage{fp}
usepackage{booktabs}

% #1: decimal places (default=2), #2: value
newcommand*{KelvinToCelsius}[2][2]{%
    FPevaltempKtoC{round((#2-273.15):#1)}%
    num{tempKtoC}%
}

begin{document}
    begin{tabular}{S S}
        toprule
        si{kelvin} & si{celsius} \
        midrule
           10.3  & {KelvinToCelsius{10.3}}     \
          200.34 & {KelvinToCelsius{200.34}}   \
          500.26 & {KelvinToCelsius{500.26}}   \
         1200.5  & {KelvinToCelsius[1]{1200.5}}\
        12345    & {KelvinToCelsius[0]{12345}} \
        bottomrule
    end{tabular}
end{document}