In a "net zero energy cost building," the purchases and credits from imported and exported energy are a wash. It's a purely financial metric, driven largely by local utility rates.
A "net zero energy emissions building" uses only emissions-free power sources, like wind or hydroelectric, or it buys offsets or credits to compensate for any carbon-emitting power it does buy.
That doesn't make the building carbon neutral, which is a whole other story. For more on that, listen to the Energy:Minute titled "The Meaning of Zero."
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