Why are Philadelphia office towers all lit up at night? We shed some light.

Carl Watson is a Wynnewood electrical engineer who knows building codes inside and out. He also knows a thing or two about lighting.

“When I drive into Center City at night, I just marvel at all these skyscrapers, especially the new ones, and they’re lit from ground to sky," he said. "It’s unbelievable.”

Watson isn’t the only person wondering why many Philadelphia office buildings keep their interior lights on in the evenings. That was the question a reader asked Curious Philly, the forum through which Inquirer reporters answer queries about the city and the region.

Philadelphia and most cities have adopted modern building codes that require new office buildings, or ones that have undergone major retrofits, to adopt energy-efficiency measures, which include motion sensors and timers to manage interior lights in unoccupied offices. The energy-efficiency code first went into effect in 2004, and it has been updated several times since, most recently in 2018.

Building codes only govern the way a building is designed. City Council this fall will consider a law requiring building owners to regularly “tune up” their buildings to make sure the motion sensors and other energy-efficiency measures have not been overridden.

“The bill requires a tune-up of existing systems; it wouldn’t be a requirement to install anything new,” said Richard Freeh, senior program manager in the Office of Sustainability. “The goal here is to make the existing equipment run the way it’s designed to run.”

You can’t judge every office building on outward appearance alone. Some office buildings have converted floors to residential use — apartments or hotels — which are not governed by the code aimed at nonresidential spaces.

And many offices remain occupied after 5 p.m. — law firms, customer-service call centers, or financial companies trading in markets in different time zones.

A building’s janitorial and maintenance workers often work after hours, said Stephen Resinski, the regional managing director of the real estate firm Colliers International. Motion sensors — either infrared or ultrasonic — will detect the presence of humans and keep the lights on, usually for 30 minutes.

“If it’s lit up, that means there’s somebody in there, moving around on the floor,” said Resinski, who is also the sustainability chair for the Building Owners and Managers Association of Philadelphia.

By midnight, most cleaning crews have called it a day,

“Traditionally, by 1 o’clock, you should see all the buildings going dark, particularly in the tenants’ space, where the tenants are paying for the electricity,” he said.

Resinski said building owners, as well as tenants, are acutely aware of reducing costs and have strong financial incentives to embrace energy efficiency. “If your building has $2 more in operating costs per square foot than your neighbor next door, you’re going to lose tenants to the building next door,” he said.

Most of the buildings Resinski manages have been retrofitted to the new code. “Everybody’s going to LED, using a lot less electricity, pushing out a lot cleaner, brighter light. That’s all driven to the bottom line. Even when you see the lights on, we’re using a lot less energy than we were 10 or 15 years ago in the same fixture.”

Watson, the electrical engineer who casually uses terms like “lumens” and “candlepower” in conversation, is fluent in the differences between the standards adopted by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) and the IECC (International Energy Conservation Code).

And he is also familiar with human nature.

The best building designs, and the strongest energy-efficiency codes, he said, are only as good as the people managing the buildings.

“The guy who runs the building — it’s usually a guy — determines that building’s performance,” said Watson. “He can run it automatic mode and all these sophisticated systems do their job, or he can say, ‘I do things manually — I always ran it this way.’ ”

City government is wise to this behavior.

In June, Councilwoman Blondell Reynolds Brown — sponsor of the 2012 benchmarking law that requires owners of office buildings greater than 50,000 square feet to report their energy usage to the city — introduced Council Bill 1906, which would require building owners to “tune up” their buildings every five years to make sure they are operating according to design. The “Building Energy Performance Policy” is now before Council’s Environment Committee.

With some exceptions for buildings that can demonstrate stellar performance, the bill would require an inspection of a building’s energy and water systems, including its heating and air-conditioning units. The inspections would involve checks of sensors, as well as identifying outdated lighting technologies, over-lit spaces, and areas needing lighting controls.

Building owners who fail to get an inspection, or to make needed repairs, are subject to fines of $2,000, plus additional fines of $500 a day starting at 30 days after a missed deadline for compliance.

The city’s Office of Sustainability, which would be responsible for enforcing the law, supports its passage as a means to achieve Mayor Jim Kenney’s goal of reducing the city’s carbon emissions by 80% by 2050, said Freeh, of the Office of Sustainability.

Though energy efficiency has improved 5% since the city began requiring larger buildings to report energy usage in 2013, the city still finds that building managers have overridden some sensors or automatic systems, said Freeh. “This program would fix that,” he said.

“There’s a lot of reasons for buildings to be lit outside of traditional 9-to-5 working hours,” said Freeh. “So I don’t want to anticipate we would have a dark skyline. Our goal really is to cut energy waste.”

Watson said that upgrading lighting is one of the most cost-effective ways for building owners to improve efficiency and achieve a quicker payback than upgrading the building envelop — windows, doors, insulation — or replacing the heating and air-conditioning system.

“Unless you’ve got a really leaky building, new windows will maybe achieve a 10-year payback,” said Watson. But he said that financial planners, who consider the cost of capital, “will tell you that anything longer than seven or 10 years is beyond the financial horizon — even if it pays for itself, it isn’t worth it, financially.”