Milwaukee Public Library Rooftop Goes Solar

By Niels Wolter with Focus on Energy


In the 1890's, the expansive rooftop of Milwaukee Public Library was designed simply to keep out the weather. Over a century later, Milwaukee's visionaries decided to turn 30,000 sq. ft. of the library’s roof into a green roof full of rainwater-loving plants—and a solar power plant.

When the 31.5 kilowatt system was installed in November 2009, the Milwaukee Public Library joined a select group of Milwaukee-area organizations that also installed solar electric modules on their rooftops: Kohls, General Electric, Johnson Controls and the Urban Ecology Center. 


Milwaukee Public Library’s large, sunny roof is an ideal place for a solar power plant. (Photo:


The library's solar electric system is projected to generate 40,000 kilowatt hours (kWh) of power a year for the next 40 to 50 years. This is enough electricity for eight energy-efficient homes or 12 GM Volt cars. But because the library uses a lot of electricity to operate, the new system will produce less than 10% of its electricity needs per year.


The 31.5 kilowatt system during installation. There is plenty of open, sunny space on the roof for future expansion. (Photo: Milwaukee Central Library)


The Ideal Solar Roof

To generate solar electric power, the ideal commercial rooftop site should have:

  • Year-round sunshine
  • A newer roof
  • Sufficient load-bearing capacity
  • Significant daytime power usage
  • Plenty of space

The Milwaukee Public Library’s roof was a near-perfect spot for solar, especially once its older roof (built in 1986) was replaced.


Replacing the library’s 1986 roof in preparation for the green roof. The poles in the background will soon hold the solar electric system. (Photo: Milwaukee Public Library)


Solar Power Production

The electricity generated will power the systems for lighting, heating, and cooling, helping to offset the library’s electric bills. And as electric rates increase in the future, so will the value of the solar electric system's output.

The new solar electric system may also reduce the library’s electric bills, through reduced “demand” charges. Utilities charge their large customers according to the customer’s peak power demands each month. Since there’s a good chance that the library will use the most power on sunny days, the solar electric system will be generating electricity, reducing the library’s peak power needs, and therefore offsetting the utility’s demand charges.

Project Price

The price tag for the installed system was $279,000. The City of Milwaukee paid $89,000, and incentives from We Energies and Focus on Energy covered the rest. We Energies provided a $100,000 nonprofit grant plus a $30,000 performance-based buy-down incentive. Focus on Energy contributed $60,000 in the form of a nonprofit grant. (The library benefited from larger “nonprofit” incentives from We Energies and Focus on Energy because it’s a tax-exempt organization and not eligible for federal tax incentives.)

As for the price of solar, costs will drop as the industry grows and technology is developed. For example, solar electric prices dropped 15–20% between the time the library contracted for the system's installation and the writing of this case study (March 2010).

The library’s solar system will pay for itself over about 15 years, which is a third of its projected lifespan. About 75% of the savings will be from electricity generation and 25% from demand reduction. 


There are no moving parts on this solar electric system, so it should require very little maintenance. Its core components, the solar modules and the inverters, have warranties of 25 years and 10 years, respectively, so any replacement will be a decade or more in the future. In fact, the equipment needs basically no care, so the library’s buildings and grounds staff can pretty much ignore it.

More Rooftops, Please!

The next time you fly over Milwaukee, note the hundreds of large, empty rooftops baking in the sun. Imagine a future in which they’re all covered with solar! Roofs are a significant untapped market for renewable energy. A study by Navigant Consulting for the National Renewable Energy Laboratory found that solar electric could meet 25% of our country’s electric needs if all suitable rooftops in the country had solar.

When URS Corporation completed a preliminary survey of Milwaukee's rooftops, it found that 35 million square feet of rooftops, or 73% of the roof area, was ideal for solar. Which means there’s potential for more than 5,500 solar electric systems in Milwaukee the size of the Milwaukee Public Library's.

Leasing Rooftops

Today some forward-thinking electric utilities and new startup companies—such as Portland Electric, Pacific Gas and Electric, and Recurrent Energy—are leasing large sunny roofs and installing solar power plants.

In Milwaukee, owners of large rooftops can either own their own solar electric system and offset their power usage and demand, or rent their roof area to someone else who will generate solar power.

Three Kohl’s department stores in southeastern Wisconsin host large solar electric systems owned by SunEdison. In turn, SunEdison sells the electricity to Kohl’s for less than the utility's cost of power.

Our Solar Future

A 2008 study by Navigant Consulting, completed for We Energies, predicts that the cost of a solar electric system will equal the cost of utility power in the next two to ten years. If this forecast is correct, by 2020 you may see solar on many Milwaukee rooftops—large and small.

About the author:
Niels Wolter is the senior solar electric program manager at Wisconsin Energy Conservation Corporation and Focus on Energy. Niels has been managing Wisconsin's solar electric promotion programs for over a decade.

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