Sizing Solar Panels: The Solar Balance Scale – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Third Term)

Sizing solar panels means working out how many panels a home needs to cover the energy its appliances use each day.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Explain the idea of the solar balance scale (energy in against energy out).
2. Calculate the daily energy requirement of a set of appliances in watt-hours.
3. Calculate the daily energy output of one solar panel using peak sun hours.
4. Work out the number of panels needed and round up correctly.
5. Explain why an extra allowance of 20% to 30% is added in Nigeria.

What is the solar balance scale?

The solar balance scale is the rule that the energy your panels produce must be at least equal to the energy your appliances use.

• Panels: like a “salary”, they bring energy in.
• Appliances: bulbs, fans and TV are like “expenses”, they take energy out.
• Battery: like a “bank account”, it stores what is left over.
• Balanced system: energy in is greater than or equal to energy out.
• Unbalanced system: energy out is greater than energy in, so the battery runs flat and the lights go off.

Example of an unbalanced system

• Energy used at night: 1,000 units.
• Energy collected by the panels in the day: 800 units.
• Shortfall: 1,000 − 800 = 200 units, so the battery will slowly go dry.

What is the difference between a watt and a watt-hour?

A watt (W) measures how fast an appliance uses energy, while a watt-hour (Wh) measures the total energy used over time.

How do you find your daily energy requirement?

The daily energy requirement is the total energy, in watt-hours, that all your appliances use in one day.

Formula: Watts × Hours = Watt-hours (Wh)

1. List every appliance to be powered.
2. Write the power rating (W) and the number of each appliance.
3. Multiply the total watts by the hours of use per day.
4. Add the watt-hours of all appliances together.

Example calculation

• Target: the 2,000Wh is the energy the panels must supply every day.

What are peak sun hours?

Peak sun hours are the number of hours in a day when sunlight is strong enough for a panel to give its full rated power.

• Strong sunlight period: a panel gives its full watts only when the sun is very bright, around midday.
• Not the same as daylight hours: the sun may shine for 12 hours, but only a few hours count as “peak”.
• Value used in this lesson: about 5 peak sun hours per day for Nigeria.

How much energy does one panel give in a day?

The daily output of a panel is its rated watts multiplied by the peak sun hours.

Formula: Panel watts × Peak sun hours = Daily energy output (Wh)

• Example: one 300W panel in Lagos or Abuja gives 300W × 5 hours = 1,500Wh per day.
• Reason: a 300W panel does not give 300W all day, only when the sun is strong.

How do you work out the number of panels needed?

The number of panels is the daily energy needed divided by the daily output of one panel, rounded up.

Formula: Number of panels = Total energy needed ÷ Output of one panel

1. Find the total daily energy needed (2,000Wh in our example).
2. Find the daily output of one panel (1,500Wh for a 300W panel).
3. Divide: 2,000 ÷ 1,500 = 1.33 panels.
4. Round up to the next whole number: 2 panels.

What is the Nigeria factor (top-up allowance)?

The Nigeria factor is an extra allowance of about 20% to 30% added to the energy need to cover losses.

Why is a top-up needed?

• Dust: dirt on the glass blocks some sunlight, especially in the harmattan.
• Clouds: cloudy days reduce the sunlight reaching the panel.
• Heat: very hot panels give slightly less power.
• System losses: energy is lost in wires, the charge controller, the battery and the inverter.

Applying the top-up to our example

• Result: two 300W panels give 3,000Wh in a day, which covers the 2,000Wh need with a safe margin.

What happens in the rainy season?

In the rainy season, peak sun hours can fall to about 2 hours, so each panel gives much less energy.

• One 300W panel: 300W × 2 hours = 600Wh per day.
• Panels needed for 2,000Wh: 2,000 ÷ 600 = 3.33, which rounds up to 4 panels.
• Meaning: two panels leave a margin on normal days but may not cover the full need in heavy rain.
• Options: add another panel, or use fewer appliances on cloudy days.

What are the key rules for sizing?

1. Always round up: you cannot buy half a panel. If the answer is 3.2, buy 4.
2. Plan for rain: an extra panel helps keep the batteries from going flat.
3. Check the rating: read the label on the back of the panel for the “Pmax” (maximum power in watts).
4. Use real running hours: count the hours an appliance actually draws power, not the hours it is plugged in.

Class practical exercise

Imagine you are a solar engineer. A customer wants to power the appliances below.

• One fridge (150W): the compressor switches on and off, so assume it runs for 10 hours in total. 150W × 10 = 1,500Wh.
• 3 bulbs (10W each): used 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?

Teacher’s answers

1. Total energy needed: 1,500 + 300 = 1,800Wh.
2. One panel gives 250 × 5 = 1,250Wh. Then 1,800 ÷ 1,250 = 1.44, which rounds up to 2 panels.
3. With a 30% top-up, 1,800 × 1.3 = 2,340Wh. Two panels give 2,500Wh, which already covers this. A third panel is only needed if dust or rain is severe.

Evaluation

1. Write the formula for energy in watt-hours.
2. What are peak sun hours, and how many do we assume for Nigeria on average?
3. How much energy does one 300W panel give in a day with 5 peak sun hours?
4. Why do we always round up the number of panels?
5. Give two reasons for adding a 20% to 30% top-up when sizing panels.

Assignment

A house uses 4 bulbs (10W each) for 6 hours and one TV (100W) for 5 hours every day.

1. Calculate the total energy needed in watt-hours.
2. Using 300W panels and 5 peak sun hours, calculate how many panels are needed before any top-up.
3. State how many panels you would recommend, and why.