A Nottingham MP has uncovered the full costs of "keeping the lights on" at the 10-storey government-owned building.
It comes as the HM Revenue and Customs (HMRC) Unity Square building in Queensbridge Road faced criticism after members of the public saw its lights had been left on overnight.
After questioning the Treasury about it, she shared a monthly breakdown of the bill, spanning from April to September.
The total cost for all six months was £557,805.20.
HMRC, who owns the building, says work is now underway to save money and electricity.
After posting her finding, Ms Greenwood said she was surprised to see HMRC take action to pay the bill so quickly.
Speaking to ITV News Central, the Labour MP for Nottingham South, said: "It does seem a little bit of a coincidence that suddenly the answer we are getting from HMRC is much more positive than it was when I wrote to them just a couple of months ago.
"It does sound like they have got plans to get their electricity bill under control - they are going to switch the lights off and only have them activated for 5 minutes rather than 30 minutes every one time someone trips that.
"And we know there are only four HMRC staff in the building overnight on average.
"So it is clear that they need to do more. Perhaps they are starting to do more, but my constituents will know if it's made a difference because they can all see for themselves."
A HMRC spokeswoman told Nottingham's Local Democracy Reporting Service that, by moving to regional centres, the department will save around £300 million cumulatively up to the year 2025 to 2026 and will deliver annual cash savings of £74 million in the year 2025 to 2026, before rising to around £90m from 2028 to 2029.
They also said Unity Square has been designed and built to a BREEAM (Building Research Establishment Environmental Assessment Method) 'excellent' standard.
It claims the building uses its energy and water efficiently, alongside, its waste disposal.
However, in response to the problems raised said they are "still being fine-tuned".