What does“low wattage” mean in “A politician not being mocked for low wattage”?

Time magazine (October 25) carries the article titled “The Populist Egghead” with a caption:

“Sen. Cruz isn't being mocked for low wattage the way Palin and Reagan had been. He's being singled out for a lack of common sense born of his rarefied résumé.”

The sentence follows:

The populist claims to possess the horse sense of the electorate and has no need for fancy schools, with their eating clubs, trays of sherry, and debating societies. That was Sarah Palin's (and Reagan’s) posture. http://www.slate.com/articles/news_and_politics/politics/2013/10/ted_cruz_is_a_populist_egghead_the_texas_senator_has_elite_credentials_but.html

I understand ‘low wattage’ is a figurative adaptation of electrical power, but what does it exactly mean in the above quote? Is it a common expression? Were both Sara Palin and Reagan really ‘low wattage’?

Solution 1:

People who are intelligent are said to be bright, and people who aren't are said to be dim. This is sometimes expressed by likening people to light bulbs: if you say someone "isn't the brightest bulb in the box", you're saying that they aren't very smart.

Low wattage is presumably a reference to this metaphor. A low wattage bulb is dim, and someone who is dim is not very smart.

EDIT: As KitFox points out, it's not a common expression. In fact, I'm not sure I've ever heard it before, but the meaning seemed fairly clear to me, particularly in context. Palin and Reagan were both famously portrayed as stupid when they were in the spotlight. Headlines like "Even The Tea Party Doubts Sarah Palin's Intelligence" were commonplace in 2008 and 2010, and Reagan was famously labeled an "amiable dunce" while he was in office. Of course, not everyone agreed with these characterizations, but it's clear that this lack of intelligence is what low wattage refers to.