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On-Grid, Off-Grid, and Hybrid- Solar power plant

All solar power systems work on the same basic principles. Solar panels first convert solar energy or sunlight into DC power using what is known as the photovoltaic (PV) effect. The DC power can stored in a battery or converted by solar inverter into AC power which can used to run home appliances. Depending on the type of system, excess solar energy can either be fed into the electricity grid for credits, or stored in a variety of different battery storage systems.

On-Grid System       

On-grid or grid-tie solar systems are by far the most common and widely used by homes and businesses. These systems do not need batteries and use either solar inverters or micro-inverters and connected to the public electricity grid. Any excess solar power that you generate exported to the electricity grid and you usually get paid feed-in-tariff (FiT) or credits for the energy you export.

Unlike hybrid systems, on-grid solar systems are not able to function or generate electricity during a blackout due to safety reasons. Since blackouts usually occur when electricity grid damages; If solar inverter was still feeding electricity into damaged Grid it would risk safety of people repairing fault/s in network. Most hybrid solar systems with battery storage are able to automatically isolate from the grid (known as islanding) and continue to supply some power during a blackout.

In an on-grid system, this is what happens after electricity reaches the switchboard:

  1. The meter– Excess solar energy runs through the meter, which calculates how much power you are either exporting or importing (purchasing).
  2. Metering systems– work differently in many states and countries around the world. In this description I am assuming that meter is only measuring electricity being exported to grid, as this case in most of Australia. In some states, meters measure all solar electricity produced by your system, and therefore your electricity will run through your meter before reaching the switchboard and not after it. In some areas (currently in California), meter measures both production and export, and consumer charged (or credited) for net electricity used over a month or year period. I will explain more about metering in a later blog.
  3. The electricity Grid– Electricity that sent to grid from your solar system can used by other consumers on the grid (your neighbours). When your solar system is not operating, or you are using more electricity than your system is producing, you will start importing or consuming electricity from the grid.

Off-Grid System

An off-grid system not connected to electricity grid and therefore requires battery storage. Off-grid solar systems must designed appropriately so that they will generate enough power throughout the year and have enough battery capacity to meet the home’s requirements, even in the depths of winter when there is generally much less sunlight.

The high cost of batteries and off-grid inverters means off-grid systems are much more expensive than on-grid systems and so usually only needed in more remote areas that are far from electricity grid. However battery costs are reducing rapidly, so there is now a growing market for off-grid solar battery systems even in cities and towns.

AC-coupled off-grid solar systems use a solar inverter together with a multi-mode battery inverter.

There are different types of off-grid systems which we will go into more detail later, but for now I will keep it simple. The above diagram for larger AC coupled system. In smaller scale DC coupled systems, solar charge controller used to manage battery charging, then DC power converted to AC using off-grid inverter and sent to your home appliances.

Simple, affordable, small scale DC-coupled off-grid solar power system use solar charge controllers to manage the battery charging, plus an a simple inverter to supply AC power.

  • Battery bank. In an off-grid system there is no public electricity grid. Once solar power used by appliances in your property, any excess power will sent to your battery bank. Once the battery is full it will stop receiving power from the solar system. When your solar system is not working (night time or cloudy days), your appliances will draw power from the batteries.
  • Backup Generator. For times of the year when the batteries are low on charge and the weather is very cloudy you will generally need a backup power source, such as a backup generator or gen-set. The size of the gen-set (measured in kVA) should to be adequate to supply your house and charge the batteries at the same time.

Hybrid System

Modern hybrid systems combine solar and battery storage in one and are now available in many different forms and configurations. Due to decreasing cost of battery storage, systems that already connected to the electricity grid can start taking advantage of battery storage as well. This means able to store solar energy that generated during day and using it at night. When stored energy depleted, grid is there as back up, allowing consumers to have best of both worlds. Hybrid systems are also able to charge the batteries using cheap off-peak electricity (usually after midnight to 6am).

There are also different ways to design hybrid systems but we will keep it simple for now. To learn more about the different hybrid and off-grid power systems refer to our detailedguide to home solar battery systems.

  • The battery bank. In hybrid system once solar power used by appliances in your property, any excess power will sent to battery bank. Once battery bank is fully charged, it will stop receiving power from solar system. The energy from battery can then discharged and used to power your home, usually during the peak evening period when the cost of electricity is typically at it’s highest.
  • The meter and electricity grid. Depending on how your hybrid system is set up and whether your utility allows it. once your batteries are fully charged excess solar power not required by your appliances can be exported to the grid via your meter. When your solar system is not in use, and if you have drained the usable power in your batteries your appliances will then start drawing power from the grid.

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