Question

To increase the amount of electricity produced in the U.S., we could place what on top of all Wal-Mart stores?

Answer 1

The question will be looking for Solar Panels, as they are flat, non-intrusive, and reasonably low maintenance ie the most sensible choice.

However I mean in theory you could stick windmills on top of big Wal-Mart stores as wind turbines are placed on roofs regularly, so you can tell whoever set you that question that it is vague ;)

Answer 2

That's a great idea !  A wind turbine (windmill) or solar panels
on the roof of every WalMart, but don't stop there.  Let K-Mart,
Target, Sears, Macy's, 7-11, McDonald's, and Subway in on it too. 

In fact, why not place them on the roof of every house and
apartment building too ?

Just one question:  Who's going to pay for them ?
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The biggest single year-round residential use of energy in the
USA is to heat up and keep a ready supply of hot water.  So the
single biggest residential use of energy could be eliminated with
a simple solar water heater on the roof of every house.  From my
own personal observation, I know that Israel is there now, but here
in the USA, it's apparently not worth the time, effort, or expense ! ? !