Sizing Solar Panels – The Solar Balance Scale SS1 Solar Photovoltaic Installation & Maintenance Lesson Note

The Solar Balance Scale

Think of your solar system like a bank account.

• The Panels are your “Salary” (money coming in).
• The Appliances (Bulbs, Fans, TV) are your “Expenses” (money going out).

To be “wealthy” in electricity, your panels must bring in more energy than your appliances take out. If you spend 1,000 units of energy at night but your panels only catch 800 units during the day, your “bank account” (the battery) will eventually go dry, and your lights will go off.

Finding Your Daily Energy Requirement

Before we size the panels, we need to know the total energy we want to use in a day. We measure this in Watt-hours (Wh).

The Simple Formula:

Watts×Hours=Watt−hours(Wh)

Example Calculation:

• 2 Fans (60W each): Running for 10 hours = 120W×10=1,200Wh
• 5 Bulbs (10W each): Running for 8 hours = 50W×8=400Wh
• 1 TV (100W): Running for 4 hours = 100W×4=400Wh
• TOTAL ENERGY NEEDED = 2,000Wh

This 2,000Wh is the “target” our solar panels must hit every single day.

How Much Does One Panel Actually Give?

If you buy a 300W Solar Panel, does it give you 300W all day? No. It only gives its full 300W when the sun is very bright (usually between 11:00 AM and 3:00 PM).

In Nigeria, we use the “Peak Sun Hours” rule. On average, we get about 5 hours of strong, “effective” sunlight per day.

The Panel Output Formula:

PanelWatts×PeakSunHours=DailyEnergyOutput(Wh)

Example: One 300W panel in Lagos or Abuja:

300W×5 hours=1,500Wh per day.

The Final Math (Sizing the Array)

Now we know what we need (2,000Wh) and what one panel gives (1,500Wh). To find out how many panels to buy, we divide the need by the output.

The Calculation:

Total Panels=One Panel’s OutputTotal Energy Needed​

From our example:

1,500Wh2,000Wh​=1.33 panels

The “Nigeria Factor” (Inefficiency): Because of dust, clouds, and heat, we always add a “top-up” of about 20% to 30%. Instead of 1.33 panels, we round up to 2 panels. Two 300W panels will give us 3,000Wh, which is plenty to cover our 2,000Wh need and leave some extra for rainy days!

Summary and Class Activity

Key Rules for Sizing

1. Always Round Up: You can’t buy “half” a solar panel. If the math says 3.2, buy 4.
2. Plan for Rain: In the rainy season, you might only get 2 Peak Sun Hours instead of 5. Having an extra panel helps keep the batteries alive.
3. Check the Rating: Always look at the sticker on the back of the panel to see the “Pmax” (Watts).

Class Practical Exercise

Imagine you are a Solar Engineer. A customer wants to power:

• One Fridge (150W) that runs for 24 hours. (Wait! A fridge compressor turns on and off, so let’s say it only “eats” for 10 hours total). 150W×10=1,500Wh
• 3 Bulbs (10W) for 10 hours. 30W×10=300Wh

1. What is the Total Energy Needed? (1,500+300=?)
2. If you use 250W panels and assume 5 Peak Sun Hours, how many panels do you need?
3. If the house is in a very dusty area, would you recommend adding one more panel? Why?