In this tutorial, I will guide you on how to choose from varying sizes and types of inverter for your PV system. I will specifically show you how to calculate the capacity of inverter that will work best for your PV system. This tutorial is the fourth step in six-step tutorials on how to design a solar PV system.
If you jumped into this article from a google search or somewhere like that, I strongly recommend you to visit the first step, Determining your Power Consumption Requirements, and the second step, How Many Panels Do I need. The third article is How Many Batteries Do I need. The information in those articles is very critical to understanding this fourth step. More so, if you’re a complete solar energy beginner, I recommend Fundamentals of Solar Energy and Everything You Need to Know About Solar Electricity, those articles are written for you in mind.
Before you bring out your calculator, I want to begin the tutorial by explaining what we mean by “Inverter” and its function in the solar PV world. After that, I will make you understand different classifications and types of inverters in the Nigeria market. I will briefly discuss how to cater for appliances that cause surge during start-up when choosing the capacity of inverter for your project. After that, I will show you how to calculate the capacity of the inverter you need in your PV system.
Let’s get going!
What is an inverter
An inverter is an electronic device that converts the DC power produced by the panels (or stored in batteries) to AC power that can be used in your home. One other important function of an inverter is to maximize the power output of the panels or batteries by varying the current and voltage.
I’m sure you understand why we need an inverter in PV systems. Now, let’s talk about the different classifications and types of inverters that you can find in the Nigeria market and discuss the pros and cons of each type.
Classification of Inverters
Inverters could be classified in so many ways. But for this tutorial, we are going to focus on two main classifications. Inverters are classified based on their output and configuration topology.
We have three types of Inverter based on this classification. They are Sine Wave Inverter, Modified Sine Wave Inverter, and Square Wave Inverter.
Sine Wave Inverter
A sine wave is the kind of output you get from your energy distribution company (NEPA) and a fuel generator. This is because it is generated by rotating AC machinery and sine waves are a natural product of rotating AC machinery.
The major advantage of this inverter is that most of our household appliances are design to use the sine wave output. So, this is the perfect output and gives the guarantee that our household appliances will work efficiently. This type of inverters is more expensive but widely used in residential and commercial applications.
Modified Sine Wave Inverter
The construction of this type of inverter is complex than simple square wave inverter but easier compared to the pure sine wave inverter. The output of this inverter is neither pure sine wave nor the square wave.
A modified sine wave inverter will work fine with most appliances, although the efficiency or power will be reduced with some. Compressor, such as refrigerator; motor like water pumps, will use more power from the inverter due to lower efficiency. Compressors and motors will lose about 20% of their operating power. The advantage of this inverter is that they are cheaper than sine wave inverter.
Square Wave Inverter
The output waveform for this inverter is a square wave. This type of inverter is least used among all other types of inverter because all appliances are designed for sine wave supply. If we supply square wave to sine wave-based appliance, it may get damaged or losses too much energy. The cost of this inverter is very low but you can barely use it in any household appliance I can remember. However, in industrial application, It can be used in simple tools with a universal motor.
We have three types of Inverters based on this classification. They are Central inverters, String inverter and Micro-inverter.
Central Inverter –
A central inverter is a standalone inverter with a higher power rating, starting from 250kw. Because of their high-power ratings, Central inverters reduce the need for more inverters in a PV system, making it a more unified inverter system.
1. One advantage of a central inverter is that you only need one of them, making fault-tracing easy to detect.
2. It’s cheaper to install one central inverter than multiple string or micro-inverters. For example, if your installation demands a 250Kw inverter, buying one central inverter of 250kw will be cheaper than buying 25 units of 10Kw of string inverters.
3. Central inverters have more warrantee period than string inverters.
1. If the central inverter broke down, it will affect the whole system
2. In case of central inverter breakdown, you will need to invite advanced technician of the inverter manufacturer to come to the installation
A string inverter is a type most commonly used in home and commercial solar power systems. Depending on the size of the installation, there may be more than one string inverter installed in the PV system.
1. where there’s more than one string inverter, you don’t need to shut the whole system down because of one faulty string inverter.
2. They are cheaper than micro-inverter
1. They have less warranty period than central and
2. they require more regular maintenance than a central inverter
A micro-inverter consists of a small box (size of TV decoder) attached to the back of or installed very close to a solar panel. Its role is to convert the DC electricity produced by a single solar panel to a more friendly AC current.
1. One faulty inverter will not affect the whole system
2. panel mismatch or partial shading of the panel will
not have a big impact on the system
3. It’s cheaper to maintain Micro inverters than central
or string inverter
1. You have as many inverters on your roof as you have solar panels.
2. They are more expensive than both central and string inverter
Power output is usually the main factor to consider when buying an inverter for your system. This is because:
- The input rating of the inverter should never be lower than the total watt of appliances. If the freezer you want to power is rated 300W, the inverter rating should be more than 300W.
- The inverter must have the same nominal voltage as your battery. If the nominal voltage of your inverter is 24V, you have to run the wire of your battery to output 24V as well.
- The voltage of your array (panels) should not exceed the inverter’s maximum (that’s for a system that doesn’t require battery system. If a battery bank is required, the charge controller will be used to do the dirty job). The production will be limited by what the inverter can output (and depending on the extent, the inverter’s lifetime may be reduced).
- The array voltage is too low for the inverter you’ve chosen (that’s again for a system that doesn’t require a battery system). The system will also under produce because the inverter will not operate until its “start voltage” has been reached.
For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances as a rule of thumb.
How to Cater for Appliances that Cause Surge
In case of appliances that operate with motor or compressor (like refrigerator, air-conditioner, water pump, etc), the inverter size should be minimum 3 times the capacity of those appliances. This is generally done to cater for surge current during start-up of those appliances.
Recall that the Total Watts used by our appliances in step 1: determining power consumption requirements, is 1,130W. You may like to go back to step 1 to find out how we arrived at that.
In table 1 below, we multiplied the capacity of appliances that use compressor by 3. For appliances that don’t use a compressor, we added 25% of their capacity.
At the end of the exercise, the capacity of Inverter we are going to use is 3,000W or 3KVA