What is the difference between Grid-Tied, Hybrid and Off-Grid solar systems?
What would be the best in your situation? Let’s look closer at the benefits and downsides of the various options.
Grid-Tied is a term used to describe a solar system that is connected to the utility power grid.
Advantages of Grid-Tied Systems
1. It Cost Less
Batteries and other equipment are required for a fully functional Off-Grid solar system, which add to costs as well as maintenance. Grid-Tied solar systems are therefore generally cheaper to install.
Solar panels will often generate more electricity than is needed for consumption. Homeowners can feed this excess electricity back into the utility grid instead of storing it in batteries.
Feed-in tariff schemes play an important role in how solar power is incentivized. Without it, residential solar systems would be much less feasible from a financial point of view. In South Africa municipalities are currently buying electricity from homeowners at a lower rate than selling it to them.
2. The Utility Grid is a Virtual Battery
Electricity has to be spent in real time. However, it can be temporarily stored as other forms of energy (e.g. chemical energy in batteries). The electric power grid is in many ways also a battery, without the need for maintenance or replacements.
Additional perks of being Grid-Tied include access to backup power from the utility grid in case your solar system goes faulty. At the same time you help to mitigate the utility company`s peak load. As a result, the efficiency of the electrical system as a whole goes up.
Equipment for Grid-Tied Solar Systems
There are a few key differences between the equipment needed for Grid-Tied, Hybrid and Off-Grid PV solar systems. Standard Grid-Tied solar systems include following:
- Grid-Tie Inverter (GTI) or Micro-Inverters
- Power Meter
Grid-Tie Inverter (GTI)
Solar Inverters regulate the voltage and current received from solar panels. Direct current (DC) from the solar panels is converted into alternating current (AC), which is the type of current that is utilized by the majority of electrical appliances.
In addition to this, grid-tie inverters, also known as grid-interactive or synchronous inverters, synchronize the phase and frequency of the current to fit the utility grid (50Hz in South Africa). The output voltage is also adjusted slightly higher than the grid voltage in order for excess electricity to flow back to the grid.
Micro-inverters go on the back of each solar panel as opposed to one central inverter that typically takes on the entire solar array.
Micro-inverters are more expensive, but in many cases yield higher efficiency rates. Buildings with shading issues should definitely look into micro-inverters to better the situation.
Most homeowners will need to replace their current power meter with one that is compatible with net metering. This device, often called a net meter or a two-way meter, is capable of measuring power going in both directions, from the grid to the house and vice versa.
Local utility companies / municipalities should be consulted to see which net metering options are available. In some countries, the utility company issues a power meter for free and pay full price for the electricity you generate; however, this is not always the case.
Off-Grid Solar Systems
Off-Grid solar systems (off-the-grid, standalone) are an alternative to Grid-Tied PV systems. For homeowners that have access to the grid, Off-Grid solar systems are usually not a good option. Here`s why:
To ensure access to electricity at all times, Off-Grid solar systems require battery storage and a backup generator (if you live off-the-grid). A battery bank typically needs to be replaced after 10 years and are expensive.
Advantages of Off-Grid Solar Systems
1. No need for a utility grid
Off-Grid solar systems can be cheaper than providing power lines in certain remote areas.
Consider Off-Grid if you’re more than 100 meter from the grid. Erecting overhead transmission lines are very costly.
2. Become energy self-sufficient
Living off the grid and being self-sufficient feels good. For some people, this is worth more than saving money. Energy self-sufficiency is also a form of security. Power failures on the utility grid do not affect Off-Grid solar systems.
On the flip side, batteries can only store a certain amount of energy and during cloudy times, being connected to the grid is actually where the security is. You can however install a backup generator to be prepared for these kinds of situations.
Equipment for Off-Grid Solar Systems
Typical Off-Grid solar systems require the following:
- Solar Charge Controller
- Battery Bank
- DC Disconnect (additional)
- Off-Grid Inverter
- Backup Generator (optional)
Solar Charge Controller
Solar charge controllers are also known as charge regulators or just battery regulators. Solar battery chargers limit the rate of current being delivered to the battery bank and protect the batteries from overcharging.
Good charge controllers are crucial for keeping the batteries healthy, which ensures the lifetime of a battery bank is maximized.
Without a battery bank (or a generator) it’ll be lights out by sunset. A battery bank is a group of batteries wired together.
Lithium Ion batteries are a better option than lead acid or gel batteries. The initial cost is more but in the long run it cost lest because of a longer lifespan and better DOD (depth of discharge) – 80% compared to 50% of lead acid batteries.
DC Disconnect Switch
AC and DC safety disconnects are required for all solar systems. For Off-Grid solar systems, one additional DC disconnect is installed between the battery bank and the Off-Grid inverter. It is used to switch off the current flowing between these components. This is important for maintenance, troubleshooting and protection against electrical fires.
Off-Grid inverters do not have to match phase with the utility grid as opposed to grid-tie inverters. Electrical current flows from the solar panels through the solar charge controller and the bank battery bank before it is converted into AC by the Off-Grid-inverter.
It takes a lot of money and big batteries to prepare for several consecutive days without the sun shining (or access to the grid). This is where backup generators come in.
In most cases, installing a backup generator that runs on diesel is a better choice than investing in an oversized battery bank that seldom gets to operate at its full potential. Generators can run on propane, petroleum, gasoline and many other fuel types.
Backup generators typically output AC, which can be sent through the inverter for direct use, or it can be converted into DC for battery storage.
Hybrid Solar Systems
Hybrid solar systems combine the best from Grid-Tied and Off-Grid solar systems. These systems can either be described as Off-Grid solar with utility backup power, or Grid-Tied solar with extra battery storage.
If you own a Grid-Tied solar system and drive a vehicle that runs on electricity, you already kind of have a hybrid setup. The electrical vehicle is really just a battery with wheels.
Advantage of Hybrid Solar Systems
Hybrid solar systems are less expensive than Off-Grid solar systems. You don’t really need a backup generator and the capacity of your battery bank can be downsized. Off-peak electricity from the utility company is cheaper than diesel.
Equipment for Hybrid Solar Systems
Typical hybrid solar systems are based on the following:
- Charge Controller
- Battery Bank
- DC Disconnect (additional)
- Battery-Based Grid-Tie Inverter
- Power Meter
Battery-Based Grid-Tie Inverter
Hybrid solar systems utilize battery-based grid-tie inverters. These devices can draw electrical power to and from battery banks, as well as synchronize with the utility grid.
The bottom line is this: Tapping the utility grid for electricity and energy storage, providing it is reliable, is significantly cheaper and more practical than using battery banks and/or backup generators.