The three main types of solar power systems
1. On-grid - also known as a grid-tie or grid-feed solar system
2. Off-grid - also known as a stand-alone power system (SAPS)
3. Hybrid - Solar plus battery storage with grid-connection
First we will describe the common components of the three types before going into more detail about the different systems and how they work.
All solar systems include:
Solar panels or solar modules are installed together in what is known as a solar array. Modern solar panels are made up of many solar cells or photovoltaic (PV) cells which generate direct current (DC) electricity from sunlight or energy from the sun. Note: It is light energy or irradiance, not heat, which produces electricity in photovoltaic cells.
Solar panels generate DC electricity which needs to be converted to alternating current (AC) electricity for use in our homes and businesses. This is the role of the solar inverter. In a string inverter system, solar panels are linked together in series and the DC electricity is brought to a single inverter which converts the DC to AC. In a micro inverter system, each panel, or every two panels, has it’s own micro inverter attached to the back side of the panel. The panel still produces DC, but is converted to AC on the roof, and is fed straight to the electrical switchboard.
(electricity consumption.) AC electricity is sent to the switchboard where it is directed to the various circuits and appliances in your house that are using electricity at the time. Any excess electricity is sent to either a battery storage system if you have a off-grid or hybrid system, or to the electricity grid if you have an on-grid system.
1. 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 are connected to the public electricity grid and do not require battery storage. Any solar power that you generate from an on-grid system (which is not used directly in your home) is exported onto the electricity grid and you usually get paid a feed-in-tariff (FiT) for the energy that you export.
Unlike hybrid systems, grid-tie solar systems are not able to function or generate electricity during a blackout or power outage due to safety reasons; since blackouts usually occur when the electricity grid is damaged. If the solar inverter was still feeding electricity into a damaged grid it would risk the safety of the people repairing the fault/s in the network. However most hybrid solar systems with battery storage are able to automatically isolate from the grid (known as islanding) and continue to operate during a blackout.
Batteries are able to be added to an on-grid solar system at a later stage if required. The popular Tesla Powerwall 2 is an AC battery which can be added to an existing solar system.
In an on-grid system, this is what happens after electricity reaches the switchboard:
- The meter. Excess solar energy runs through the meter, which calculates how much power you are either exporting or importing (purchasing).
- Metering systems work differently in many states and countries around the world. In this description I am assuming that the meter is only measuring the electricity being exported to the grid, as is the 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), the meter measures both production and export, and the consumer is charged (or credited) for net electricity used over a month or year period. I will explain more about metering in a later blog.
- The electricity grid. Electricity that is sent to the grid from your solar system can then be 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.
2. Off-Grid System
An off-grid system is not connected to the electricity grid and therefore requires battery storage. An off-grid solar system must be designed appropriately so that it 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 less sunlight. The high cost of batteries and inverters means off-grid systems are much more expensive than on-grid systems and so are usually only needed in more remote areas that are far from any 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.
There are different types of off-grid systems which we will go into more detail later, but for now I will keep it simple. This description is for an AC coupled system, in a DC coupled system power is first sent to the battery bank, then sent to your appliances. To understand more about building and setting up an efficient off-grid home see our sister site go off-grid/hybrid
- The battery bank. In an off-grid system there is no public electricity grid. Once solar power is used by the appliances in your property, any excess power will be sent to your battery bank. Once the battery bank 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.
3. Hybrid System
Due to the decreasing cost of battery storage, systems that are already connected to the electricity grid can start taking advantage of battery storage as well. This means being able to store solar energy that is generated during the day and using it at night. When the stored energy is depleted, the grid is there as a back up, allowing consumers to have the 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.
- The battery bank. In hybrid system once solar power is used by the appliances in your property, any excess power will be sent to your battery bank. Once the battery bank is full, it will stop receiving power from the solar system.
- 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.
Choosing off-grid or hybrid?
For many people the dream of becoming completely energy independent may be very difficult and expensive. To help you understand whether it is cost effective and what you should know before you decide to build or go off-grid refer to our related off-grid design guide.
For a more detailed explanation of the different hybrid systems and battery options now available see the hybrid/off-grid energy storage review here