Solar-Powered Classroom

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

This lesson is inspired by Global Problem Solvers: The Series. In the second season of the show, the team designs solar-powered mobile classrooms that can be used after a hurricane destroys a school in Florida. Watch the trailer for the second season here:

In this lesson, your students will be challenged to design their own solar-powered mobile classroom. They will decide what electrical devices (like lighting and computers) the classroom needs, and how many solar panels will be required to power the classroom. This section will provide you with the background information you need to help guide your students through this lesson.

Electrical power is measured in watts (W). A watt is not very big, so power measurements are frequently expressed in kilowatts (kW) instead. One kilowatt is equal to one thousand watts:

Equation 1:

Solar panels are rated in terms of how many watts they can provide. You can buy small, portable solar panels that produce a few dozen watts, while larger rooftop solar panels can produce hundreds of watts, and be combined to produce thousands of watts (Figure 1).

In order to run a classroom on solar power, the total wattage of the solar panels needs to be greater than the combined wattage of all the electrical appliances (this is for the best-case scenario in full sunlight; for ideas about how to address cloudy days or nighttime power usage, see the variations section). This means your students will need to figure out the power consumption of everything they want to include in their portable emergency classroom. There are several different ways to do this, outlined below and in a student handout, along with some other useful information. Your class will probably need to use a combination of these methods.

Check the Product Packaging or Label

Sometimes, the packaging or label for a product will directly tell you how many watts it consumes. This is very common with light bulbs (Figure 2). However, be careful with LED and CFL bulbs. They will frequently advertise an "equivalent" or "replacement" wattage in addition to their actual wattage. This indicates the wattage of the (older, less efficient) incandescent bulb they are intended to replace. For example, the LED bulbs in Figure 2 can replace 60 W incandescent bulbs, but only use 8.5 W.

Internet Search

If you can't find the wattage listed directly on the appliance, try an internet search. Searching for the exact product name or model number along with a phrase like "power watts" might give the best results. If you don't know the exact product name/model number, try a more generic search like "refrigerator power watts." Many websites have information about power consumption for common electrical appliances.

Direct Measurements

You can use an electricty usage monitor (Figure 3) to directly measure the power consumption of any plug-in appliance.

Calculate Power Consumption (Advanced)

Sometimes, if you look at the labels for an electronic device (like a cell phone or laptop charger), it might list volts (V) and amps (A) instead of watts (Figure 4). The values might be expressed in millivolts (mV) or milliamps (mA). One millivolt is one thousandth of a volt, and one milliamp is one thousandth of an amp. They will probably also list an "input" and an "output."

That might seem like a lot of information, but don't worry! Volts and amps are both units used to measure electricity, but they are not the same as watts. Volts measure voltage, or how hard the electricity is being "pushed." Amps measure current, or how much electricity is flowing. You can use the input volts and amps to calculate electrical power using this equation:

Equation 2:

But wait—the input voltage on the charger in Figure 4 is a range, from 100–240 V. How do you know what voltage to use? This depends on what country you are in. In the United States, use 120 V. If you are in a different country, you will need to look up the voltage supplied at your electrical outlets. So, for example, the power consumption of the charger in Figure 4 is (remember to be careful with units—the charger lists the input current as 2,000 mA, which is equal to 2 A):

Equation 3:

With some guidance and help interpreting the labels, your students can do this calculation to find the power consumption of an appliance.

Heating and Cooling

Depending on the climate where you live, heating and cooling can be some of the biggest uses of energy in a building. However, the electricity used to heat or cool your classroom might not be easy for your students to measure. For example, your school might have central air conditioning (AC) instead of window AC units; or it might be heated by oil or natural gas instead of electricity.

Your students can approximate the power required to heat or cool your classroom based on its square footage. Electrical space heaters and window AC units frequently list the square footage they are intended to cover (e.g. 400 square feet, or a 20×20 foot room). This is easy for space heaters as they usually list their wattage directly (Figure 5).

Unfortunately, in the US, air conditioners are rated in British Thermal Units (BTU) per hour instead of watts (Figure 6). You can convert between BTU/hour and watts using this equation:

Equation 4:

So, for example, the 5,000 BTU air conditioner in Figure 6 consumes 5,000×0.293=1,465 W, or about 1.5 kW.

Kilowatts vs Kilowatt-hours

If you read your electric bill, you might notice that amounts are expressed in kilowatt-hours (kWh), not kilowatts. A kilowatt-hour is a unit of energy, not power. Power is the amount of energy used per unit time:

Equation 5:

One kilowatt-hour is the amount of energy used when you use 1 kW of power for one hour. If needed, you can convert between energy and power using Equation 5.

Additional Background Links

• Global Problem Solvers: The Series, Cisco

Here are all seven episodes of the second season of Global Problem Solvers: The Series:

Prep Work (10 minutes)

• Make sure you understand the different methods to find/calculate the electrical power used by a device.
• Print student worksheets and fact sheets.
• Consider assigning the videos below as homework in any LMS.

Engage (1 hour)

Watch season 2 of Global Problem Solvers: The Series as a class or assign the episodes for students to view individually as homework. The season consists of seven episodes, each 3–5 minutes long. You can start watching online on the series website or individual episodes from season 2.

After watching, discuss how the show relates to your classroom, particularly electricity usage. What things do students see that use electricity in the classroom? Are there any things that use electricity that you can't see? Hint: think about what things have stopped working if you ever lost power at school or at home. If you had to use a mobile classroom like a trailer, converted bus, or shipping container after a natural disaster, could you bring those things with you? Would you need all of them? How would you power them?

Explain that in this lesson, the class will work together to decide what electrical devices they would need for a mobile classroom. They will then break into small groups to find out how much power each electrical device uses, and combine their results as a class to figure out how many solar panels they would need to power their classroom.

Explore (1 hour)

1. Optional: depending on what you have already covered in class, you might need to explain the units of electrical power (watts) to your students. See the teacher background section and student fact sheet for more details.
2. Ask students to make a list (individually) on the student worksheet of things that use electricity in the classroom.
3. Discuss the lists as a class, and make a master list. Are there any items that everyone included? Is there anything that only a few people thought of? You might need to give students hints about things that are not directly visible in the classroom (e.g., if your school has central air conditioning instead of a window AC unit).
4. Now ask students to (individually) pick which devices from their list they think they would need in a mobile classroom. They should think about whether there are things in their regular classroom they could do without in an emergency (for example, switching to manual pencil sharpeners instead of an electric one).
5. Again, discuss the lists as a class. Come to an agreement about which items are/are not needed for the mobile emergency classroom, and how many of each item are needed (for example, you might only need one projector, but will probably need multiple light bulbs). Make a master list.
6. Break the students up into small groups. Assign at least two groups to each item on the list (this will allow them to compare their answers and check for errors). It will be their job to find out how much electrical power that item uses in watts. There are several ways to do this, described in the student fact sheet (see the teacher background section for more details and examples). Each group should show their work and/or write down where they got their information.
   • Read the package or label
   • Do an internet search
   • Use a plug-in electricity usage monitor
   • Calculate the power use in watts by multiplying amps times volts (power = current × voltage).
7. Collect all the results as a class. Each student should fill in the results on their own worksheet so they have all the data. If there are disagreements between groups for individual items, try to find the source of the disagreement. For example, did they get their information from different websites? Did one group make a mistake in a calculation? Agree on a single number you will use to calculate total power usage.
8. Each group should now calculate the total electrical power required for all the devices in the mobile classroom. Everyone should get the same answer since they are now all using the same numbers.
9. Now, each group should design a solar power system for their mobile classroom, and figure out how much the system would cost. The student worksheet guides them through this process.
   1. Look up information about at least three different solar panels online, including their wattage, size (length × width), and price.
   2. Figure out how many of each type of solar panel you would need to power your mobile classroom. For example, if your mobile classroom requires 2,050 W, you would need twenty one 100 W panels, or nine 250 W panels.
   3. Calculate the total cost for each type of panel, based on the number of panels you would need and the price per panel.
   4. Figure out how much space each option will take up, based on the number of panels required and the dimensions of each panel. A standard shipping container is 8 feet wide and 40 feet long. Will the panels fit on the roof of a shipping container?
   5. Based on your results, choose a type of solar panel. Write an explanation for why you chose that one.

Reflect (20 minutes)

Ask each group to briefly tell the class about the solar power system they designed (number of panels, wattage, cost, etc.). Compare systems between different groups. Did any groups design identical systems with the exact same number and type of solar panels? Did any group design a unique system with a solar panel that no one else found?

Optional: discuss some of the potential limitations or problems with your solar power system. See the variations section for more details.

Assess

Each student can be assessed individually using the student worksheet.

Lesson Plan Variations

• Solar power might be clean and renewable, but it isn't available at night or on cloudy days. For a solar-powered system to deliver consistent power when the sun isn't out, it will also need batteries for backup storage. The batteries can store extra power on sunny days for use at night or on cloudy days. Have your students look into battery storage technologies that are available for use with solar power systems. How many and what size batteries would they need to power their entire classroom on a cloudy day? How much do the batteries cost?
• A real solar power system also contains other electrical components, like an inverter that converts the direct current (DC) produced by the solar panels to alternating current (AC) used by plug-in electrical devices. Your students can also look up prices for these components, to develop a more realistic price for their entire solar power system.