The three main types of solar power systems:
- On-grid – also known as a grid-tie or grid-feed solar system
- Off-grid – also known as a stand-alone power system (SAPS)
- Hybrid – grid-connected solar system with battery storage
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 do not need batteries and use either solar inverters or micro-inverters and are connected to the public electricity grid. Any excess solar power that you generate is exported to the electricity grid and you usually get paid a 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 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. 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.
Batteries are able to be added to on-grid systems at a later stage if required. The Tesla Powerwall 2 is a popular AC battery system 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.
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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. Off-grid solar systems must be 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 are usually only needed in more remote areas that are far from the 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. The above diagram is for a larger AC coupled system. In smaller scale DC coupled systems, a solar charge controller is used to manage the battery charging, then the DC power is converted to AC using an 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.
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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 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.
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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
Modern hybrid systems combine solar and battery storage in one and are now available in many different forms and configurations. 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. To learn more about the different hybrid and off-grid power systems refer to our detailed guide to home solar battery systems.
- The battery bank. In a hybrid system once the solar power is used by the appliances in your property, any excess power will be sent to the battery bank. Once the battery bank is fully charged, it will stop receiving power from the solar system. The energy from the battery can then be 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.
Best Off-Grid Solar System
Choosing the best off-grid solar system is not easy as off-grid systems are many times more complicated than common grid-connected solar systems. In this article, we highlight some of the many considerations which must be taken into account and discuss the various off-grid system types available. We also explain why a good quality inverter-charger is vital to building a reliable off-grid system. Finally, we will dive into the different brands available and determine which systems are best suited to different applications.
Many people believe off-grid solar systems can be easily put together. This may be true in the case of a small caravan or cabin, but in reality, larger off-grid systems used for homes and businesses need to be carefully designed by an experienced solar installer or system designer.
Off-grid system types – AC or DC coupled
Off-grid systems are built using either AC or DC coupled power sources. AC-coupled generation sources include common solar inverters, wind turbines or backup generators (gen-sets), while DC-coupled sources include MPPT solar charge controllers or micro-hydro systems.
Whether a system is AC or DC coupled is generally based on the size of the system. Most small-scale systems less than 6kW are DC coupled and use efficient MPPT solar charge controllers. Larger off-grid systems can be either AC or DC coupled depending on the type of off-grid inverter-charger used, and compatibility with different solar inverters (AC) or solar charge controllers (DC). Most modern multi-mode inverters can be both AC and DC coupled, which creates a very secure, flexible power system with multiple charging options and can enable black-start operation if the main inverter shuts down, the backup generator fails or the batteries are depleted.
