Grid-Tie PV System VS Off-Grid PV System

There are basically three types of solar PV systems existing today for solar power generation at home, including grid-tie, off-grid and hybrid PV systems. However, as expense and efficiency are concerned, grid-tie and off-grid systems are more popular and most installed around the world. Now let’s compare them with each other and help you decide which system should be suitable for your house.

grid-tie solar PV system
grid-tie solar PV system

Grid-Tie System

1. This installation requires your system is connected to the local national (utility) grid.
2. After the connection, the system can feed the electricity (usually extra electricity) it generates to the national grid (this helps it earns money from the grid), and your home can get the electricity from the national grid once your system stops generating electricity at night or in special conditions.
3. The current form your solar panels generates is direct current (DC), which must be converted into alternating current (AC) for home use by going through a PV inverter. The current converted by the inverter must be compatible with the voltage of the local grid.
4. The inverter also needs to be a grid-tie type. An off-grid one cannot be used here.

Off-Grid System

off-grid PV system
off-grid PV system

1. Off-grid means the system is disconnected from the local national grid and cannot feed extra electricity to earn money.
2. This installation needs storage batteries, which usually cost a lot of budget.
3. This system is suitable for installation in areas where houses cannot be connected to the national grid or where residents are far away from grids and cities.
4. It needs a charge controller which is used to charge the batteries of the system.
5. The DC from solar panels usually goes to a battery and then to an inverter.
6. This inverter is an off-grid inverter.

Grid-Tie Advantages

1. It can constantly power your house unless the national grid is down and the system meets its special conditions.
2. It can help earn money from the local government by selling extra electricity.
3. It costs less compared with an off-grid system because it doesn’t need expensive batteries.
4. It works more efficiently because its DC doesn’t to go into a battery. This process causes energy loss.

Off-Grid Advantages

1. It can be used in remote areas where national grids cannot reach.
2. It can store extra energy and use it whenever your house needs.
3. It doesn’t need the grid-connecting process.
4. It is totally environmentally friendly.

Current Condition

In comparison, residents around the world prefer a grid-tie system more, and many countries like the UK, the US, the Netherlands and China are encouraging their residents to install this system. But In India, an off-grid system seems to enjoy more welcome because many regions in the country cannot reach national grids.


Working principle of a grid-tied PV inverter

Before learning the working principle of a grid-tied PV inverter, you need to learn the basic function of a PV inverter. What is it? It functions to convert direct current into alternating current. Where does the direct current come from? It comes from the solar panels that absorb sunlight and turns it to direct current which a common family using alternating current cannot use. Thus, an inverter comes into use. The principle of a grid-tied inverter is similar to that of a common inverter.

The direct current generated by these solar panels is fed to an inverter. In daytime the solar modules generate solar electricity and keep feeding the inverter. As this happens the solar inverter directs the generated electricity to the household electrical loads or is fed to the grid.

The amount of electricity generated depends on the quality of sunlight. If your system produces more electricity than you need, the remained electricity will be directed to the utility grid. If you require more power than the power produced, the extra electricity will be drawn from the utility grid. Thus the utility grid serves as an infinite battery back-up for your system.

At the DC side of the system the power output of a module depends on the voltage. So by varying the voltage and the current the maximum power point can be found. Therefore, most inverters grid- tied or off-grid include a Maximum Power Point Tracking system.

At the AC side the grid interactive inverter supplies power in the sinusoidal form. The frequency of the power generated must be synchronized with the grid frequency. The feed in voltage must not be greater than that of the grid voltage.

On the occurrence of a power outage in the electricity grid, the grid-tied inverter is shut down. This shut-down ensures the safety of the service personnel who is responsible for the problem of power outage. Each grid interactive inverter must have this feature as mentioned in the National Electricity Code (NEC). This feature of autonomy is a key differentiating factor between solar inverters and grid tied PV inverters.

The quality of an inverter is essentially decided by its efficiency, which guarantees how much electricity can be finally generated and how much money one can finally earn by using the solar power system. The highest conversion efficiency is 98.5%, and the highest conversion efficiency of a micro grid-tied inverter is 96.5% or so.

The Installation Process for a Residential PV System

Solar energy has been used to generate electricity in large solar power plants. As solar technology advances, even a family can generate electricity on its own by installing a small PV solar system at home. It is a new thing and so how to install it in a house still remains a mystery for many who wish to go solar. Now it is time to know how this solar power system is installed at home.

1.Search the web and learn needed information.
No mater whether you search Google, Bing or Yahoo, you had better make clear of two things: the advantages and disadvantages of generating electricity with solar energy and the requirements of installing such a home use power plant. Usually solar power generation doesn’t need biofuels like coal and gas and its costs are relatively low. But in the beginning it requires some amounts of investment. If you wish to install such a system at home, you should at least consider the amount of electricity your family consumes, where its components should be installed, function tests and so on.

2.Investigation on the spot
In common conditions the company you want to hire to do this business for you will send some of its installers to investigate your house conditions. For example, they will need to know the area of your rooftop, the shading area on it, the style of it (some are slopes and others flat), the direction your house faces and so on.

3.Making an installation plan.
By investigating your house conditions, the experts will probably need to make a plan of how to do the job for your family. Different families have different physical conditions, and so the plans will tell the house owner how the work can be done smoothly.

4.Signing a contract with the installer.
After getting the customer’s permission, the two sides need to sign an agreement to protect the profits of each other.

5.Applying for connecting to the utility grid.
You need to wait for the PV devices (including solar panels, a PV inverter, racks, wires and other components) to arrive and apply for installation and inspection by the county or the utility company. The installation usually won’t take long, and then the inspectors will inspect your system and confirm it. After everything is done, the utility company will install a new two-way meter and connects your house’s system to the grid.

6.After installation maintenance.
Usually the system is not easy to meet trouble. If something troublesome occurs, you need to call the contractor and let him do the job for you. For a common installer, this warranty will last from 5 to 25 years.