The whole purpose of a solar system is to not just catch the sun’s rays but to convert them into energy that can be used by your electronic appliances at home.
As with most things in life, there are different methods of doing this and one of them is by using inverters (and specifically hybrid inverters, which we will be talking about in this article).
This may be the solution to your solar energy needs, so pay attention class!
Why Do We Need Inverters?
We’ll look at residential systems as an example in this article, so electrical energy from the solar panels will be converted and used to run household appliances.
Without getting too technical, electrical appliances are designed to operate on 220Vrms alternating current (AC). Below is a diagram of what an alternating current output looks like:
Direct Current (DC) is what comes out of your solar panel or battery bank (connect an oscilloscope to the output if you don’t believe us).
Alternating current (AC), as shown in the diagram, changes direction several times per second. In order to use DC electrical energy from batteries or solar panels, you need to convert (invert) it into AC that appliances are designed to use.
Here is some more terminology before we dig deeper into the topic.
The typical phrases you will see on a hybrid inverter are similar to those found on other inverters and we will briefly explain these to help you choose something suitable.
This is the percentage of the solar energy flowing into the inputs of the inverter that will actually be used or fed back into the grid. Inverters can have very high efficiencies because of power electronic switching technology that reduces loss of energy from heating.
Maximum DC Voltage
Whichever solar panels you connect to the inputs of the hybrid inverters cannot under any circumstance exceed this value. The hybrid inverter will have a range of acceptable input voltages and if the input is too low that will result in the inverter turning off. If the input voltage is too high, however, that may result in irreversible damage to the inverter.
This is the minimum voltage required at the inputs of the inverter for it to start up. This will be higher than the minimum voltage required for the inverter to operate.
Minimum DC Voltage
This is the minimum amount of voltage required at the input of the inverter for it to be running. There is a small amount of voltage required to run the controller of the inverter and if this is not met there is no point in having it on in the first place. Sometimes it is more wasteful to have the inverter on than it is to have it turned off.
Maximum Output Current
The output from the inverter has a voltage of 220Vrms and the output current can be found by dividing the output current with this. If current exceeds a certain level, there may be irreversible damage caused to the electronics of the inverter. You may start to smell things burning, which is never a good thing—unless we’re talking barbecues!
Maximum Discharging/Charging Power
The charging and discharging of a battery is controlled by a charge controller which is incorporated into the hybrid inverter.
Power is the rate of flow of energy and there is a limit to how fast energy can be drawn from or put into the battery, determined by the limitations of the electronics of the charge controller. This must be taken into consideration when deciding whether to charge/discharge the battery in the overall control of the inverter.
Entire university papers have been published on this. Without going into too much detail, this is a property of AC and is a measure of the phase difference between the current and voltage.
When connecting to the national electricity grid, the power factor must be above a certain threshold. The electricity has good reasons for this and before choosing an inverter you might want to check whether it can deliver the right power factor.
Total Harmonic Distortion
This is for all inverters that will be connected to the grid. It is a measure of how clean or undistorted the output of the inverter is in relation to a perfect AC waveform.
The output of the inverter has some form of filtration and the better this is, the lower the total harmonic distortion. Every nation has a standard for how low this percentage should be to be allowed to connect to the electricity grid.
Independent MPP Inputs
The Maximum Power Point a pair of current and voltage values where the maximum amount of power output is seen at the output of the inverter. To increase the efficiency of the inverter, the inverter should be running as close to this value as possible because at the maximum power we are making maximum usage of the sun’s solar energy.
Some inverters can track more than one maximum power point, so for example, for a hybrid inverter we can get maximum power from the solar panels and the battery. The national electricity grid does not have this property.
Standard Solar PV Inverters
Standard solar PV inverters convert the DC from solar panels into AC at the required frequency and voltage without storing the electrical energy in batteries or feeding to the grid.
In certain areas in Australia, there is constant sunlight, which is enough to generate electrical energy for household needs during the day.
Advantages and Disadvantages
The solar panels of a solar system will always outlast the batteries. A standard solar PV inverter can run without the need for battery storage and completely eliminate the need for constant battery replacements. This works out cheaper—if you are in the right climate, of course.
In the event that you are generating more electricity than you need, you can make some good money feeding electricity back into the grid. It is extremely important to take advantage of this in Australia because the whole point of having solar is to save money.
There is a limit to how much energy can be fed into the grid and, in the absence of a place to store it, any extra energy generated will be lost. You have to make sure you don’t oversize the solar panels on this one.
In order for the energy needs of the household to be met there would have to be an energy management system to control when to draw energy from the grid, when to feed energy into the grid, and when to do neither. Having such a system on board would cost more and you would probably be better off getting a hybrid inverter.
All inverters convert DC into AC that can be used by electrical appliances. Battery inverters convert the DC flowing from the battery into AC at the correct voltage. The energy from the solar panels, in the form of DC, goes straight into the battery and from there is converted to AC when the battery is being discharged. If the current or voltage at the input exceeds the designed limits… let’s just say anything can happen.
Batteries are typically 12V and 24V at their outputs, but this will rise when the battery is charging and fall when it is discharging. The inverter will have a control system that will deal with these differences, but every battery inverter will have a maximum and minimum voltage that it requires to generate the right kind of AC at the right voltage (220Vrms for a standard Australian household).
Advantages and Disadvantages
The main aim of installing a battery inverter is to capture surplus electrical energy when it exists, which in a residential system is almost always. A battery inverter does not need a complex control system to deal with a grid connection that must abide by strict standards.
This results in a lower overall system cost and most people who use these are looking for something straightforward that will not cost too much to install and maintain.
If you already have a solar system up and running, you could easily add a battery inverter to the mix because there is not as much variation in the operation of these.
Battery output is, for the most part, standard worldwide and there are many options to choose from if you need to insert or replace one in your system.
This is not the most efficient way of using the solar energy available from sunlight during the day. When the batteries are full, unless there is a feed-in option where you can send electrical energy to the grid, energy ends up being lost.
To avoid this, you would need a way to redirect the extra energy to the electrical grid or to the appliances in the home and when you do this you have basically created your own hybrid inverter. If you need a hybrid inverter, it is better to buy one that has been made already instead of turning a battery inverter into a hybrid inverter.
The best way to view one of these is multiple inputs and multiple outputs. In solar systems, the input to the inverter comes from the solar panels and the battery bank. The output of the inverter goes to the electrical load, the battery bank or the national electricity grid. A hybrid inverter manages all these inputs and outputs.
In order to get the most out of the solar system, energy loss needs to be kept to a minimum. When more energy is being generated than is required, the hybrid inverter will direct the energy to the battery storage or to the national electricity grid.
When there is less energy coming from the solar panels than is required, the hybrid inverter will draw extra energy from the batteries or the electricity grid. To better explain this, we will use the simple equation:
Energy from panels= Energy to appliances+Energy to battery+Energy to grid
A hybrid inverter has a controller which determines where the energy from the solar panels flows. Three very important facts determine where the energy goes:
- Electrical appliances are not all turned on all the time, so sometimes during the day some require more energy than others.
- It is more efficient to use energy directly after it comes from the solar panels because energy gets lost in the process of storing the energy in the batteries and retrieving this stored energy.
- Feed-in tariffs paid to solar system owners vary depending on the time of day. In Victoria, for example, feed-in tariffs during peak time (between 3 pm and 9 pm on weekdays) are more than three times higher than off-peak tariffs.
The overall goal of the system would be to store as little energy as possible and make as much money as possible through the feed-in tariffs while making sure that the household energy needs are met.
Advantages and Disadvantages
Hybrid inverters can be configured to take advantage of feed-in tariffs that are available in Australia so you can get paid to feed surplus electricity into the national grid. The demand for electricity in the household will change throughout the day and a hybrid inverter can be used to make the necessary changes to maximise the efficiency of the system.
A hybrid inverter will typically come with the controller, batteries and charge controller as a bundle and it may be cheaper getting these components separately. A hybrid inverter can invert the voltage from the battery bank as well as the voltage from the solar panel, and these two voltages would usually require separate inverters.
Inverters come in three varieties: standard solar PV, battery inverters and hybrid inverters. Standard solar PV inverters are simpler to operate and good for systems where the demand for energy is low or the amount of sunlight is consistently high enough to get solar energy.
Battery inverters have a charge controller that can store any energy that is not being used to a battery bank to use it later. Basically, these are standard solar inverters with batteries and a charge controller added and can be very useful in situations where solar energy is not constantly available.
Some energy will be lost during charging of the battery so be careful not to just add batteries to the mix just for the sake of it because you may be making things worse, not better.
Hybrid inverters have the properties of both standard solar inverters and battery inverters. There has to be a battery bank and connection to the grid in order for this to be worthwhile. These will cost you more money but will, in general, be the most efficient inverter setup, but this is not 100% guaranteed, of course.
Energy gathered from the solar panels will make its way to the electrical appliances, the batteries or the national electricity grid. A controller uses the electricity tariffs to determine when it is best to get from the grid or feed electricity into the grid.
As always, you’ve got to do the math and see if this is worth it. In Australia, where there is a good feed-in tariff and good protection for consumers, this will usually work out provided you are prepared to do your research. Get someone knowledgeable to install for you and configure the controller.
If you want to see how much solar or battery storage could save you over the next 5 years, then take our solar saving calculator quiz below!
Or talk to an Instyle Solar expert about the best solutions for home energy storage or PV-panels.
Otherwise, head back to the solar blog to find even more great educational content.
Photo credit: Depositphotos, All About Circuits