  Let $K$ be a perfect field and let $E$ be a homotopy commutative ring
spectrum in the Morel-Voevodsky stable motivic homotopy category
$\mathcal{SH}(K)$. In this work we investigate the relation between the
$E$-homology localization and $E$-nilpotent completion of a spectrum X. Under
reasonable assumptions on $E$ and $X$ we show that these two operations
coincide and can be expressed in terms of formal completions or localizations
in the usual sense of commutative algebra. We deduce convergence criteria for
the $E$-based motivic Adams-Novikov spectral sequence.
