Just speculating here, because I've not done the substitution in baking, and couldn't find info on white chocolate powder online. If the label on your white chocolate powder is basically similar to regular chocolate (cocoa solids, cocoa butter, sugar), use the same weight of that as the chocolate chips. (Measure the weight by scale or figure by using the nutrition information on its package.) If the white chocolate powder is a mix for something else (like frosting, or hot cocoa) and has other, maybe artificial ingredients, I'd be wary of using it to replace the chips - difficult to estimate equivalents and to predict how it would work in baking.
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If the label on your white chocolate powder is basically similar to regular chocolate (cocoa solids, cocoa butter, sugar), use the same weight of that as the chocolate chips. (Measure the weight by scale or figure by using the nutrition information on its package.)
If the white chocolate powder is a mix for something else (like frosting, or hot cocoa) and has other, maybe artificial ingredients, I'd be wary of using it to replace the chips - difficult to estimate equivalents and to predict how it would work in baking.