Inverter: Meaning, Types & Ratings: (Components of PV System IV) – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (First Term)

An inverter is the part of a solar PV system that changes DC (direct current) from solar panels and batteries into AC (alternating current), which home appliances use.

Behavioural objectives

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

1. explain what an inverter is and why a solar PV system needs one;
2. state the difference between AC and DC;
3. compare pure sine wave and modified sine wave inverters;
4. name the types of inverter by use;
5. explain what VA and KVA mean in an inverter rating;
6. add up the load of appliances and choose an inverter size that allows for surge.

What is an inverter?

An inverter is an electronic device that changes DC power into AC power.

• Input: DC from the battery bank or solar panels.
• Output: AC for fans, TVs, fridges and light bulbs.
• Process: changing DC to AC is called inversion.

What is the difference between AC and DC?

Direct current (DC) flows in one direction only, while alternating current (AC) changes direction many times every second.

• DC (Direct Current): like a one-way street. It is produced by solar panels and batteries.
• AC (Alternating Current): like a two-way street. It comes from the national grid (“NEPA”) and runs most home appliances.

Why does a solar PV system need an inverter?

A solar PV system needs an inverter because panels and batteries give DC, but most appliances need AC.

• Appliances: without an inverter, solar panels cannot power AC appliances such as a TV.
• The bridge: the inverter takes the “one-way” DC and flips it back and forth very fast to make “two-way” AC.

How does an inverter work?

An inverter switches DC on and off in alternating directions very quickly to make AC.

1. DC enters the inverter from the battery or panels.
2. Electronic switches reverse the direction of the current many times every second.
3. The voltage is raised to the level the appliances need.
4. AC leaves the inverter and goes to the sockets and appliances.

What are the types of inverter by wave shape?

There are two main types of inverter by wave shape: pure sine wave and modified sine wave.

Pure sine wave inverters (the “smooth” one)

This inverter makes a smooth wave that looks like the power from the national grid (or even better).

• Best for: sensitive electronics like laptops, modern LED TVs, refrigerators and medical equipment.
• Benefit: appliances run quietly and last longer.
• Drawback: they are more expensive.

Modified sine wave inverters (the “choppy” one)

This inverter makes a “staircase” or “square” wave, which is rougher than a sine wave.

• Best for: simple loads like old-fashioned light bulbs, heaters and basic phone chargers.
• Drawback with fans: a fan may hum or buzz and get very hot.
• Drawback with electronics: it can damage expensive electronics over time.

How do pure sine and modified sine inverters compare?

Pure sine wave is the better standard for protecting home appliances, while modified sine wave is the cheaper choice for simple loads.

• Pure sine: like drawing a perfect, smooth circle.
• Modified sine: like drawing a circle with straight lines only (a hexagon). It looks round from far away but is bumpy up close.

What are the types of inverter by use?

There are three types of inverter by use: stand-alone (off-grid), grid-tie and hybrid.

• Stand-alone (off-grid) inverter: works with batteries and has no link to the grid. It suits homes without grid supply.
• Grid-tie inverter: feeds solar power into the grid. It shuts down when the grid fails.
• Hybrid inverter: works with solar panels, batteries and the grid together.

What do inverter ratings mean?

An inverter rating, such as 1KVA, 2.5KVA or 5KVA, shows how much load the inverter can carry at once.

What is VA/KVA?

VA (Volt-Ampere) is the unit of apparent power, and it is very similar to Watts.

• KVA: 1 KVA = 1,000 VA.
• Conversion used in this note: Watts are roughly 0.8 of the VA rating.

What other ratings are on an inverter?

• Input DC voltage: the battery voltage it needs, such as 12V, 24V or 48V.
• Output voltage and frequency: the AC it supplies. Nigeria uses 230V and 50Hz.
• Efficiency: the share of the DC power that comes out as useful AC power.

How do you choose the right inverter size?

Add up the Watts of everything you will turn on at the same time, then allow for surge.

1. List every appliance you will use together.
2. Write the Watts of each appliance.
3. Multiply the Watts by the number of units.
4. Add all the results to get the total running load.
5. Add the extra surge power for motor appliances.
6. Pick an inverter whose Watts are higher than this total.

Worked example

• 5 bulbs (10W each): 50W.
• 1 TV: 100W.
• 1 fridge: 300W (it also needs a surge).
• Total running load: 50W + 100W + 300W = 450W.

What is the surge factor?

The surge factor is the extra power that motor appliances need for a moment when they start.

• Appliances affected: fridges, pumps and air conditioners.
• Size of the surge: about 3 to 5 times their normal power.
• Example: a fridge that uses 300W normally may need 1,500W for a moment to start.
• Rule: the inverter must be big enough to handle that “kick”.

Evaluation

A school staff room needs to run 10 ceiling fans (70W each), 15 LED bulbs (12W each) and 1 small refrigerator (200W running, 1,000W starting surge).

1. What is the total running wattage?
2. Which type should the school buy, pure sine or modified sine? (Hint: think about the fans.)
3. The total starting power needed is about 1,880W. Is a 2KVA (1,600W) inverter enough, or should the school buy a 3KVA (2,400W)?
4. Name the three types of inverter by use.

Assignment

Answer all three parts in your exercise book.

1. Explain the difference between AC and DC, and say why a solar PV system needs an inverter.
2. State three differences between a pure sine wave inverter and a modified sine wave inverter.
3. What is the surge factor? Use a 300W fridge as your example.