Energy Efficiency in Solar System – SS2 Solar Photovoltaic Installation & Maintenance Lesson Note (First Term)

Energy efficiency in a solar system means using appliances that need less electricity to do the same job.

Behavioural objectives

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

1. Distinguish between energy efficiency and energy conservation.
2. Compare the power used by old bulbs and LED bulbs, and calculate the saving from swapping them.
3. Name appliances that suit a solar home, such as inverter fridges and DC inverter air conditioners.
4. Explain standby (phantom) load and how to avoid it.
5. Describe three passive cooling methods.
6. Explain how an energy budget and the “Daytime Rule” reduce battery use.

Why does efficiency come first in solar design?

Efficiency comes first because every watt saved reduces the number of panels and batteries the owner must buy.

• Grid homes: People are often careless with power from the grid (PHCN/NEPA) because it is relatively cheap when available.
• Solar homes: Panels and batteries cost money, so every watt counts.
• Smaller system: A lower load means a smaller and cheaper set of panels, batteries and inverter.

The Golden Rule: It is much cheaper to buy a low-energy lightbulb than to buy a bigger solar panel to power an old, hungry lightbulb.

What is the difference between energy efficiency and energy conservation?

Energy efficiency is using technology that needs less electricity for the same job, while energy conservation is changing habits to use less electricity.

• Energy efficiency: buying a device that does the same work with less power, such as an LED bulb.
• Energy conservation: using less energy by behaviour, such as turning off a TV when nobody is watching.

Both reduce the load on the solar system.

How can lighting save energy?

Lighting is the easiest place to save energy, so it is called the “low hanging fruit”.

• Old (incandescent) bulb: uses about 60 W and turns most of that into heat, not light.
• Fluorescent tube: common in many Nigerian homes, but less efficient than LED.
• LED bulb: uses only 7 to 9 W to give about the same brightness as a 60 W old bulb.

Worked example: saving from swapping bulbs

1. Find the power saved by one bulb: 60 W − 9 W = 51 W.
2. Multiply by the number of bulbs: 10 × 51 W = 510 W.
3. Find the energy saved per hour: 510 W × 1 hour = 510 Wh (watt-hours).

• Meaning: the system carries 510 W less load whenever all the bulbs are on.
• Use of the saving: this saved energy can help run other loads, such as a refrigerator.

How do cooling and appliances affect energy use?

In a tropical climate like ours, fans, fridges and air conditioners are the “heavy lifters” of household energy use.

Refrigerators

• Old fridge: starts with a huge “surge” of power that can trip a solar inverter.
• Inverter fridge: has an “inverter” compressor that starts slowly and uses much less battery at night.
• Advice: look for the “inverter” compressor when choosing a fridge for a solar home.

Air conditioners

• Standard AC: uses so much power that it is called a “solar killer”.
• DC inverter AC: the type a client must use if an AC is wanted on solar.
• Saving: a DC inverter AC uses much less energy than a regular one.

Standby (phantom) load

A standby load is the power an appliance draws when it is “off” but still plugged in.

• Examples: TV, decoder, phone chargers and microwave left plugged in.
• Effect: the small loads add up and drain the battery all day and night.
• Solution: switch off the socket entirely, or unplug the device.
• Tip: a multi-socket extension with a switch turns off many devices at once.

What is passive design?

Passive design is building a house so that it stays cool without using electricity.

• Cross ventilation: placing windows opposite each other so the breeze flows through.
• Shading: planting trees or using wide roof overhangs to keep the sun off the windows.
• Insulation: using ceiling materials such as POP or glass wool to block heat from the roof.
• Light colours: light-coloured walls and roofs reflect more heat than dark ones.

Discussion point

• Cool room (28°C): the fan can stay on speed 1.
• Hot room (35°C): the fan stays on speed 5 and uses more power.
• Result: a cooler house saves battery charge for the morning.

How do you create an energy budget?

An energy budget is a plan of what electrical energy to use and when, just like a money budget.

Steps to make an energy budget

1. List every appliance in the house.
2. Write the power rating of each in watts (W).
3. Estimate the hours each one is used per day.
4. Multiply power by hours to get watt-hours (Wh) for each appliance.
5. Add all the watt-hours to get the daily energy need.
6. Remove or reduce the biggest and least necessary loads.

Example budget (values chosen for practice)

The “Daytime Rule”

The Daytime Rule says to do heavy work during the day, when the sun is out.

• Do: pump water, iron clothes or grind pepper at 1:00 PM, using direct sun.
• Don’t: wait until 8:00 PM to pump water, because it uses battery power.
• Reason: daytime loads run from the panels, so the battery stays charged for the night.

What should a designer check at a client’s house?

A designer should check the existing loads and advise the owner before sizing the system.

1. Count the old bulbs and tell the owner to change them to LEDs.
2. Check the fridge. Is it an old “power guzzler”?
3. Check for air conditioners and advise a DC inverter type if one is needed.
4. Look for appliances left on standby.
5. Explain that solar power is managed power, so the owner must plan how it is used.

Evaluation

1. State the difference between energy efficiency and energy conservation.
2. Why is it cheaper to replace an old bulb with an LED than to buy a bigger solar panel?
3. What is a standby (phantom) load, and what should a solar home owner do about it?
4. List three passive design methods that keep a house cool.
5. Why should a pump be run at 1:00 PM instead of 8:00 PM?

Assignment

1. A house has 8 old bulbs of 60 W each. Calculate the power saved each hour if all are replaced with 9 W LED bulbs.
2. Explain why a standard AC is called a “solar killer”, and name the type of AC a client should use on solar.