There is nothing wrong with the "prejudice plus power" definition. It is the best definition of racism as a systemic effect that exists.

As the US shifts towards being a majority-minority system, any power element of perceived racism against whites will no doubt have to be examined. I'm yet to be convinced (from transatlantic distance) that it exists.

What I do think is economically disadvantaged white people ought to educate themselves about how their situation is some ways interlocked with the racist treatment of economically disadvantaged minorities in the same area; the systems that established racism now disadvantage them too.

(This is also true in the UK; if white people in poverty understood what they had in common with black and asian people in poverty, Britain First and the EDL would not exist, and Brexit might not have happened; as always, their fears are exploited by populists when they share the same circumstances as those they are asked to fear.)

But racism isn't simply the same as racial prejudice; it's prejudice in action, and that is what the definition is about.