Part 1: Estimating Energy Demand and Solar Array Size

Driving Question: 
Can we design a photovoltaic (PV) array that can charge all of our school’s laptop computers?

Step 1: Estimate Energy Use for One Laptop
To design your solar system, you first need to estimate how much energy is required to charge a single laptop. Research estimates for the following: 

• Typical power draw of the most commonly used school laptop (Watts)
• Average time required to charge (hours)

You may use: 

• Manufacturer specifications.
• Reputable online sources.

Use the formula below to calculate the energy required to charge one laptop: 

Energy (kWh) = [Power (W) x Time (hours)] ÷ 1,000

Now, determine how many laptops your PV system will need to charge at once. Consider the following questions:

• How many student laptop computers will be charged at school? 
• Should staff laptop computers be included? 
• When will laptop computers be charged? All at once, or at different times of the day? 

Next, determine how long laptops will need to charge. Consider the following questions:

• Will laptops charge to 100%, or only to a certain percentage?  
• On average, how long will one laptop take to sufficiently charge? 

Step 2: Calculate the Total Energy Demand
Use your decisions from Step 1 to calculate the total energy demand your PV array must meet. Use the formula: 

Total Energy Demand (kWh) = [Power (W) x Time (hours) x Number of Laptops ] ÷ 1,000

Step 3: Estimate the Number of PV Panels Needed
A commercial PV panel typically produces about 300 watts under full sunlight. The total energy generated depends on panel power and sunlight hours.
Note: In this investigation, your solar system adds energy to the electrical grid. Your goal is to generate enough energy over time to offset the energy needed to charge all laptops—not to power them directly at every moment. 

Consider the following question and use a reputable source to make an accurate estimate.

• On average, how many hours of effective sunlight does your location receive per day?

Then, use this formula:

Number of Panels = Total Energy Demand (kWh) ÷ {[Panel Power (W) x Sunlight Hours] ÷ 1,000}

Step 4: Adjust for Real-World Conditions
Solar panels do not always produce their maximum rated power. The 300 W rating assumes ideal conditions. In real-world conditions, factors such as changing sunlight, panel angle, temperature, shading, and system losses reduce output to about 75% of rated power.

To account for this, increase your number of panels by 25%.

Use this formula, and round up to the next whole panel:

Adjusted Number of Panels = Ideal Number of Panels x 1.25

Step 5: Calculate the Area Needed for the Solar Array
A typical commercial PV panel is about1.6m² (0.9m x 1.8m) or 17.5ft² (39” x 65”) and weighs approximately 18 kg (40 lbs). Consider both the total area and mass when thinking about where the array could be installed. 

Use this formula:

Total Area = Number of Panels x 1.6m²

Step 6: Reflect and Plan for Site Selection