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How Much Sunlight Does it Take to Power a Building?

An Exercise Using Data from Black Rock Forest

By Antonia Daly, Friends Seminary

Context (or why does it matter?)

The Sun is the source of all energy that heats the earth and its structures. Solar energy drives wind and water which are sources of renewable energy. Solar energy is also stored in non-renewable fossil fuels, such as coal, oil and natural gas. When fossil fuels are burned to create electrical energy, we know that the process contributes carbon dioxide and other greenhouse gasses to the atmosphere.

What matters is what form our energy comes in and how much of it we use. An energy efficient building employs alternate energy sources as well as energy conservation design strategies to reduce the use of fossil fuels to supply electricity. For example, the Science and Education Center (SEC) at Black Rock Forest (BRF) utilizes a ground source heat pump (GHP) system to heat and cool the building year round. The GHP exchanges underground heat through a fluid circulation system. The GHP extracts heat from the building in summer and places it in the ground, and it transfers heat into the building in winter. Thus, in the heating and cooling process, the sole use of electrical energy is to operate the GHP. Additional electrical power is used in the building to run appliances, such as toasters, lights and computers. The SEC is also designed with many other energy-saving features, such as superior insulation, double-pane windows, and an air circulation system. The net effect is reduced energy consumption for the building, an important environmental matter.

Summary of Exercise.

We start by posing the question, "How much of the energy demand of the building could be met by solar energy?"

To arrive at this answer, we use data from environmental sensors at BRF and at the Science Center. First, data from the Solar Incidence sensor tells us the amount of solar energy that falls on an area of open land at BRF for certain months. Next , we look at data for the total electrical energy usage at the Science Education Center (SEC) for the same months. We compare those amounts to each other to see how much sunlit land area is needed to supply the energy needs of the building. Finally, we calculate how much of the building's energy can be supplied by a solar panel the size of the roof/footprint of the building. We can then draw conclusions about the efficiency of the use of solar panels..

Learning Objectives.




Learning Standards

These standards, taken from the National Science Education Standards and The Principles and Standards for School Mathematics, address the content of this exercise and the most relevant student goals. They are by no means the only possible outcomes.

National Mathematics Standards - from Chapter 7, Gr 7-12

National Science Standards

Time Frame.

In a high school setting, the Instructor should allow 5 to 7 classes for this investigation. One day is given to the introduction and preparation, followed by 2-3 days of data exploration and doing the Worksheet. Days 5-6 can include discussion and debriefing. The extensions can be longer-reaching assignments.

In a college setting, the activity could be begun as an in-class activity in one class period, and then completed by students working on their own.


Before the lesson, the Instructor should be familiar with the following materials:

Students should be guided to explore the BRF website, read history of the forest and its mission, look at both types of data: Real Time and Data Harvester (archival). Know basic Excel functions. Review definitions of watt, kW, kWh; power and energy; Metric and English area conversions.

Materials (as hard copy or from Excel web page link).

  1. Data Lists.
  2. 3 Student Worksheets:


A team of two students is an effective grouping for accurate data reading and transferal of data to Excel. An overhead computer or projector can assist the teacher in guiding the class, but is not necessary.

  1. From the website or as a paper handout, the Instructor shows map of BRF and photo of the Science & Education Center (SEC). Locate SEC building. Note scale in miles. Students can practice converting area into square meters and into acreage using the Conversion Factor List. What is acreage of the Forest? What is the footprint of the Science Center?

  2. As a class, review the meaning of energy: "the capacity to do work by performing mechanical, physical or electrical tasks, or to cause heat transfer, and power: "rate at which energy is used", and units for each. List types of energy: heat, light, electricity, kinetic, potential.

  3. Give out and discuss Fact Sheet on the energy-saving and other green features of the building. Note facts on the ground source heat pump and on solar panel efficiency. Teacher can demonstrate how the ground source heat pump works with an overhead or a board drawing of the diagram.

  4. Use Worksheet #1 to pose some thought questions, either as whole class discussion, or as small group discussion. Students can write their answers first, then discuss them.

    1. Think about the amount of sunlight energy falling on the roof of the SEC. Do you think that amount of sunlight energy is more or less (or about the same as) the amount of electrical energy consumed inside the SEC?

    2. How much land area do you think would be needed to collect sunlight containing the same amount of energy as the electrical energy used by the SEC building? Is it the size of a window? The size of the footprint of the building? 10 times that size? 100 times that size? More? Less? Choose one answer.

    Probably students won't have a good sense for the relative amounts of electrical energy and solar energy involved. That's OK. Points to bring out in subsequent discussion:

  5. As a class, examine the Building Energy Usage Data for February 2001 (Excel File).

    Note that each box represents the kWH of energy used during one day during February 2001.

    Possible points for discussion:

  6. Students examine the Solar Incidence Data for same period.
    1. The BRF environmental sensors measure the rate of arrival of solar radiation per meter squared of the Earth's surface. This is an instantaneous measurement in units of W/m2 (see upper left hand corner of Worksheet #2. Many such instantaneous measurements are averaged to calculate the average solar incidence for a given day. On Worksheet #2, students calculate the average SI for an example day, Feb 4, 2001. (Ans: approx 50W/m2).

    2. Look at the Solar Incidence data for February 2001 (Excel File).

      Column B shows the average solar incidence for each day of the month. Compare the Worksheet #2 result for February 4 with the comparable number on this table. They should agree.

    3. Column C shows the amount of energy that arrived on a square meter of forest during that day. What was done to the numbers in Column B to arrive at the numbers in Column C? (Hint: Column B shows power (Watts) per unit area per day, and Column C shows energy (Watt-hours) per unit area per day). (Answer: multiply the numbers in column B by 24 hours, the amount of time in one day.)

    4. Column D shows the same information as Column C, but in the more common unit of kWH. What was done to the numbers in Column C to arrive at the numbers in column D? (Answer: they were divided by 1000, because there are 1000 watts in a kilowattt.). Column D shows the kWH of energy that arrived as sunshine on a square meter of forest on each day in February, 2001.

    5. Column E shows how much energy fell as sunshine on the roof of the Science & Education Center during each day in February, 2001. To calculate these numbers, you need to combine the area of the SEC (286 m2) with the numbers in Column D. How was this done, and why? (Answer: Column D gives the energy falling on a single square meter of the Forest. To get the amount of energy falling on the many square meters of the SEC, multiple a number in Column D by the area of the SEC to get the numbers in Column E.)

  7. Students work their way through Worksheet #3, referring to the data tables from February 2001 and to the fact sheet. Students can use their calculators or can work directly in Excel and calculate on the spreadsheet. Instructor should familiarize themselves with the reasoning on the answer sheet to be able to anticipate student questions.

Extension Exercises.

Assessment and Resource List.

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