Solar Energy Terminologies – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (First Term)

Solar energy terminologies are the words used to describe the power we get from the sun. Irradiance is the sun’s power right now, insolation is the total energy received over a day, efficiency is the share of sunlight a panel turns into electricity, and peak sun hours are the hours of strongest sunshine.

Behavioural objectives

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

1. define irradiance, insolation, efficiency and peak sun hours;
2. state the unit used to measure irradiance;
3. explain the difference between irradiance and insolation;
4. give three reasons why solar panels are only about 15% to 20% efficient;
5. state the number of peak sun hours in most parts of Nigeria.

What are the main solar energy terms?

There are four main solar energy terms: irradiance, insolation, efficiency and peak sun hours.

• Irradiance: the strength of the sunshine at one moment.
• Insolation: the total sun energy received over a period, usually one day.
• Efficiency: the share of sunlight that a panel changes into electricity.
• Peak sun hours: the hours in a day when sunshine is strongest.
• Why they matter: a buyer who does not know them may choose the wrong equipment.
• Common problem: batteries may not charge as expected if the sunshine was wrongly estimated.
• Comparison: generators are rated in “KVA” or “horsepower”, while solar uses these terms.

What is irradiance?

Irradiance is the amount of solar power hitting a given area at one exact moment.

• Unit: watts per square metre (W/m²).
• Rain example: heavy rain falling right now means high intensity, and drizzle means low intensity.
• Highest: around 12:00 noon, when the sun is high overhead.
• Lowest: early morning and late evening, when the sun is low and weak.
• Effect of weather: thick cloud lowers irradiance at any time of day.

What is insolation?

Insolation is the total amount of solar energy that falls on a spot over a period of time, usually one whole day.

• Irradiance: how hard it is raining right now.
• Insolation: how much water is in the bucket after it has rained all day.
• Usual unit: kilowatt-hours per square metre per day (kWh/m²/day).
• Nigeria: our daily “bucket” is usually very full because we have many hours of sun.
• Result: this is why Nigeria is well suited to solar energy.

What is the difference between irradiance and insolation?

Irradiance is the sun’s power at one moment, while insolation is the total sun energy received over a time.

What is efficiency?

Efficiency is the percentage of the sunlight hitting a panel that is turned into electricity.

• Formula: efficiency (%) = (power output ÷ power input) × 100.
• Limit: no solar panel is 100% efficient.
• Market panels: most are only about 15% to 20% efficient.
• Worked example: a panel gets 20 units of power from 100 units of light.
• Working: (20 ÷ 100) × 100 = 20%.

Why is panel efficiency so low?

1. Heat: if a panel gets too hot, it becomes less efficient.
2. Reflectance: some light bounces off the glass like a mirror instead of going inside.
3. Dust: Harmattan dust on the panel stops light from reaching the cells.

What are peak sun hours?

Peak sun hours are the number of hours in a day when sunlight is strong enough to give about 1,000 W/m².

• Daylight: a day may have 12 hours of daylight, for example 6:30 AM to 6:30 PM.
• Weak hours: the sun is too weak in the early morning and late evening to charge big batteries well.
• Nigeria: most parts get about 4 to 6 peak sun hours per day.
• Solar engineers: they count only the peak sun hours, such as 5, when checking if a system will work.

How are peak sun hours used?

1. Find the panel rating, for example 300 W.
2. Find the peak sun hours for the place, for example 5.
3. Multiply: 300 W × 5 h = 1,500 Wh, which is 1.5 kWh.
4. This is the most the panel can give in a day, before losses.

What other basic terms are used with these?

Three other basic terms often appear with solar energy terms: watt, kilowatt-hour and solar panel.

• Watt (W): the unit of power. 1,000 W make 1 kilowatt (kW).
• Kilowatt-hour (kWh): the unit of energy. It is 1 kW used for 1 hour.
• Solar panel: a flat unit of solar cells that changes sunlight into electricity.

How do the four solar terms compare?

The table below puts the four terms side by side with a simple meaning and an example.

Evaluation

1. Define irradiance and state its unit of measurement.
2. Explain the difference between irradiance and insolation, using the rain example.
3. Why is no solar panel 100% efficient? Give three reasons.
4. How many peak sun hours do most parts of Nigeria get per day?

Assignment

Answer both parts. (a) In your own words, explain what peak sun hours are and why a solar engineer counts them instead of all the hours of daylight. (b) A panel gets 20 units of power from 100 units of light. State its efficiency and say whether it is within the usual range for panels sold in the market.