PV System Design Principle – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Third Term)

PV system design is the process of sizing the panels, batteries and inverter of a solar system so that they match the electricity the user needs.

Behavioural objectives

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

1. Explain what a load and load assessment are.
2. Find the wattage of an appliance, or calculate it from volts and amps.
3. Calculate energy demand in watt-hours.
4. Complete a load table and read the total watts and total energy from it.
5. Explain why a 20% to 30% safety margin is added to a design.
6. List the main steps in designing a PV system in the correct order.

What is load assessment?

Load assessment is making a list of everything you want to power with a solar PV system, with the quantity and power rating (watts) of each item.

• Load: anything that uses electricity, such as a fan, a phone charger or a fridge.
• Light load: an appliance that uses little power, such as an LED bulb or a phone charger.
• Heavy load: an appliance that uses a lot of power, such as a fridge, an electric iron or a kettle.
• Why it comes first: we cannot size panels, batteries or an inverter until we know what they must supply.

Planning a party is a good comparison: you must know how many guests are coming before you buy food.

What two things do we need to know about each load?

1. Quantity: how many of each item there are, for example 5 bulbs.
2. Power rating (watts): how much power each item uses at one time, for example 10 W per bulb.

What are the main parts of a PV system?

A standalone PV system has four main parts: solar panels, a charge controller, batteries and an inverter.

• Solar panels: change sunlight into direct current (DC) electricity.
• Charge controller: protects the batteries from overcharging and deep discharge.
• Batteries: store energy for use at night and on cloudy days.
• Inverter: changes DC from the batteries into alternating current (AC) for household appliances.

How do we find the wattage of an appliance?

The wattage of an appliance is printed on its rating plate, a small sticker or metal plate at the back or bottom.

Typical values are given below. Always read the actual plate, because brands differ.

• LED bulbs: usually 5 W to 15 W.
• Standing fan: 50 W to 70 W.
• Plasma TV: 80 W to 150 W.
• Laptops: 45 W to 90 W.
• Electric iron: 1,000 W to 2,000 W. This is why we say ironing and solar are not best friends.

What if only volts and amps are shown?

Power in watts is found by multiplying volts by amps.

Watts = Volts × Amps

• Example: an appliance marked 24 V and 2 A uses 24 × 2 = 48 W.

What is energy demand?

Energy demand is the total electrical energy a load uses over a period of time, measured in watt-hours (Wh).

Watts only tell us the power at one moment. We also need to know how long each appliance runs.

Watts × Hours = Watt-hours (Wh)

• Unit conversion: 1,000 Wh = 1 kilowatt-hour (kWh).

Worked examples

• Fan: a 60 W fan runs for 10 hours at night. Energy = 60 × 10 = 600 Wh.
• Bulbs: 5 bulbs of 10 W each stay on for 5 hours. Total power = 5 × 10 = 50 W. Energy = 50 × 5 = 250 Wh.

How do we create a load table?

A load table is a table that records every load, its power, its hours of use and its energy, so that nothing is forgotten.

Steps for filling the table

1. List each appliance and its quantity.
2. Write the watts of one item from its rating plate.
3. Multiply quantity by watts to get Total Watts.
4. Estimate the hours of use per day.
5. Multiply Total Watts by hours to get Total Energy (Wh).
6. Add up the Total Watts column and the Total Energy column.

Sample load table for a small room

What do the two totals tell us?

What is the safety margin (the 20% rule)?

The safety margin is an extra 20% to 30% added to the calculated load to cover system losses.

• System loss: some electricity is lost as heat in the wires, the charge controller and the inverter.
• Result of no margin: the batteries work too hard and wear out faster.
• Everyday comparison: it is like carrying extra fuel in case of a traffic jam.

Worked example

• Calculated energy: 1,300 Wh.
• With 20% added: 1,300 × 1.2 = 1,560 Wh.
• With 30% added: 1,300 × 1.3 = 1,690 Wh.
• Design value: about 1,600 Wh, which lies within this range.

What are the steps in designing a PV system?

PV system design follows seven steps, starting with the load and ending with the choice of the other components.

1. List all loads and their quantities (load assessment).
2. Find the watts of each load from its rating plate.
3. Work out the hours of use and calculate the daily energy in Wh.
4. Add the safety margin of 20% to 30%.
5. Size the inverter from the total watts.
6. Size the battery bank and the solar panels from the total energy.
7. Choose a suitable charge controller and system voltage to match the panels and batteries.

Evaluation

1. What is a load? Give two examples.
2. Calculate the energy demand of an 80W TV used for 5 hours.
3. An appliance shows 12V and 5A. What is its power in watts?
4. In the sample load table, which total tells us the size of the inverter, and which tells us how many batteries and panels we need?
5. Why do we add a 20% to 30% safety margin to the calculated energy?

Assignment

The “Home Audit”

1. Tonight, take a notebook and look at 3 appliances in your house. Find the Wattage sticker on each.
2. Estimate how many hours your family uses them in a day.
3. Calculate the Watt-hours for those 3 items.
4. Discussion: If your father wants to add an “Electric Kettle” (2,000W) to a small 1 kVA solar system, what do you think will happen?