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Estimation 2 – Solar systems!

Background info:

Review estimation from previous lesson!


The most basic way of figuring out if a solar system is appropriate for your application is to estimate the power requirements of your application.

Reminder: P = IV = current x voltage. It is also the amount of energy that the system uses per second.

A solar panel will produce a certain amount of power, and a device will consume a certain amount of power. If we want to check whether a system is feasible, we can see what power our system uses, and what power a set of solar panels can reasonably provide. If these match up reasonably well (i.e. have the same order of magnitude), it is a good sign that the system is feasible.

Note: the power requirements are more important than the voltage and current requirements of a system. There are various ways that we can change the voltage and current: for example, a transformer to put the voltage up or down, an inverter to switch from DC to AC. However, since the power is the rate of energy use, it cannot be changed.

Power use of common devices:

single LED: 0.01 Watt

Mobile phone: 1 Watt

Radio: 4 Watt

Laptop: 50 Watt

Ceiling fan: 75 Watt

Lightbulb: 100 Watt

Refridgerator: 100-200 Watt

Car: 10 kiloWatt

Solar systems – other things to consider

Batteries – these are essential if we're assuming we want to use energy after the sun goes done. We need to figure out how much total energy needs to be used after the sun goes down.

1. Figure out the power requirements of your device (how many watts does it use?)

2. Figure out how much energy it needs to store Energy = Power x time

An average car battery (12V lead acid)

- Will need to be charged at slightly higher – 13 or 14V. Make sure panels can provide this!

AC Systems

12V DC is fine for LED lamps, but some standard appliances will require AC (120 or 240V). Running AC is also good if you have long lengths of cables – they lose less energy.

An inverter is put in AFTER the batteries, so we don't need to worry about total energy use, only power.

To design an inverter, we'd need to calculate how much AC power would be used at the one time (if you have several appliances you would need to add these)

Structure/other questions

How much structure/building time you'll need for whatever's supporting the panels? Think about how they attach to the roof, how much modification you would need to do to the building. Think about how much this structure would cost, and how long would it take to build.

Solar panel/system maintenance:

Both the panel and the system will require some time commitment for maintenance.

Maintenance would involve:


keep panels clean and make sure dirt/dust doesn't get on them

Check contacts for corrosion


Make sure they are charged fully at least once a month

Topping up water when needed

Making sure contacts and circuitry are dry and not corroded.

Community impact/Cost estimates:

Before doing this lesson (and indeed, before doing this curriculum!) you should have a pretty good idea of the price of the following:

Solar cells


Tools (if they need to be bought): dremmels, soldering irons, solder etc

Inverter or inverter materials

Rechargeable batteries (most likely 12V lead acid)

Investigate what the local prices are for these materials (or if you have ordered some of them from outside – e.g. donated solar cells)

The solar panel backing/structure is not easy to price, but see if you can get a rough idea of material costs and also labour costs if any machining/welding would be needed.

If students can estimate how much the system will cost, they can compare that to how much money the electricity would save. If they are currently paying for energy (e.g. from a provider, or buying another source like wood or kerosene), they can figure out how much that is per month and see how long it will take for the solar system to start paying itself back.

Another factor to take into account is community impact – if the system does not save money, but instead provides a service that improves life within the community. This obviously will be very dependent on the location!

Lesson Schedule

1. Intro

Now we're going to make a larger solar system!

2. Brainstorming Discussion (5-15mins)

What could we use a solar power system power for? This will be very dependent on the location of the club and their specific needs.

The students might already have a good idea of what project they would like to do, or this might be the first time you've talked about it. If you did Estimation 1 as a separate lesson, the students should have been thinking about ideas for applications since the last lesson.

Either way, at the end of this discussion, they should have AT LEAST TWO projects that they can choose between.

3. More brainstorming: (15-20 mins)

But how do we know how what size our solar panels should be and how they should be designed? Remind about the Fermi problems – they were dependent on making good assumptions. Also remind them about how they figured out the power their lamps used in the last lesson.

Get the students to brainstorm a list of things they need to take into account when evaluating their systems.

These could include:

Power consumption – we have to know if we can provide enough power

How much direct sunlight will a solar panel be able to get in a day?

How large an area of panels do we need to meet our goals

Storing the energy to use it at night (in batteries)

Devices that use different types of power – AC vs. DC. Lots of appliances use AC, and AC is also useful to prevent losses over long power lines. (Talk about AC and DC differences if necessary)

Cost of solar panels + batteries + inverter vs. amount of money electricity would save (is it financially worth it?)

How to mount solar panels on the building – is there enough space?

Where to put the panels on the building

Maintenance of solar panels

How much community impact would the project have?

4. Evaluating the system (30 mins)

Once all the students have a good idea of the considerations they might want to take into account, have them split up into groups and evaluate the different systems. How you do this depends on how many students you have – but make sure that each solar system is being evaluated by at least two different students or groups of students This way we can cross-check answers at the end of the lesson.

Let the students figure out their approaches themselves – if you think they're making an incorrect assumption, question them about it, don't correct them!

A few good approaches:

Estimating power of each of the applications – what power do the applicances use? If they are being used at night how long for → how many batteries do we need?

What area of panels would you need?

How much direct sunlight might a panel get per day

how much energy would 1m2 of panels produce in a day?

How many of the small panels built in the first lesson would we require?

If we're using 12V batteries, what's the smaller number of solar cells we need to get to a 13-14V charging voltage? → how much power would that create

Is the area of panels required feasible? Can it be reasonably installed?

Cost benefit analysis

How many panels would we need – what would they cost?

Batteries – what would they cost?

Do we need an inverter – what does it cost?

Any other costs? (Installation, repair, structure)

What is this replacing?

If paid electricity/other fuel source, how much money per month is it saving?

After how many months will that add up to the initial cost of the system?

Not replacing anything What will be impact on the community?

5. Presentation and decision (10-15 minutes)

After the students have a good idea of the feasibility of each system, have each group present their results to the rest of the class. If there are any huge inconsistencies between groups, examine their assumptions and see if any wrong ones were made. If there is one project that is far less feasible than the other, should probably go for the more feasible one. If both are reasonably close, students could be encouraged to present both to their community choose the one that they/their community is the most exited about!

At the end of the lesson, students should have either one project, with a list of estimates of what will need to be done for that, or a couple projects with the same lists, and a plan for pitching them both and deciding.


Brianna Conrad, 2011/12/30 01:12

Background Info-Power Usage 100W is incandescent light bulb. Esp. for US clubs, this is not necessarily what they will be dealing with for light bulbs. Specify incandescent.

Background Info - Batteries: Many batteries should only be drained 10-20%. This will affect amount of energy they can be used to provide. Though I'm not sure if the capacity they're rated at takes this into account or not? Anyone know?

Background Info - AC Systems: “An inverter is put in AFTER the batteries, so we don't need to worry about total energy use, only power. ”

Not clear…i think what we're saying is that the inverter doesn't affect the energy we can use, but the power. As in, the inverter can run at a certain power but doesn't get depleted the way a battery does, so doesn't have a time limit on how long it can provide that power for. Should remember that inverter is not 100% efficient.

Background Info - Maintenance - Batteries

Topping up water is only for non-sealed flooded lead acid batteries.

Right Before Lesson Structure

“Another factor to take into account is community impact – if the system does not save money, but instead provides a service that improves life within the community. ”

Perhaps would be good to have an example? Like…ability to store vaccines or sommat

A few more notes: Batteries - you'll need more storage capacity if you can't count on it being sunny every day. You'll want to be able to charge your batteries and then provide power for say, 3 days (will differ based on locale) even if your panels cant produce any power for that time.

Support Structure - doesn't HAVE to be on roofs. In fact, is often easier to NOT put on roof (dont have to worry about how much weight the roof can support, if you're going to damage the membrane, when the roof might need to be replaced)

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