First things first, what is AC and what is DC?
AC and DC refer to the type of 'current'.
An electrical current is the flow of electric charge, commonly transferred by electrons passing through a conductor like a wire. This is measured in amperes (A) or 'amps'.
AC stands for Alternating Current, and DC stands for Direct Current.
What are the key differences?
Alternating Current (AC)
- Flow: Electrons rapidly change direction (e.g. 50 times per second in Australia)
- Voltage: Rises and falls in a waveform (sine wave)
- Use: Ideal for power grids, transmission, and large appliances (fridges, washing machines)
- Australian Standard: 230-240 Volts (V), 50 Hertz (Hz)
Direct Current (DC)
- Flow: Electrons flow in a single, constant direction
- Voltage: Steady and unchanging
- Use: Batteries, solar panels, LEDs, computers, phones, and electric vehicles (EVs)
What has this got to do with my battery?
There are two ways in which your battery and inverter can be installed, AC coupled and DC coupled.
To better understand what's going on we need to consider the types of current we're generating and demanding:
- Solar panels - generates electricity as Direct Current (DC)
- Battery - charges and discharges using Direct Current (DC)
- Home - requires Alternating Current (AC)
- Grid - requires Alternating Current (AC)
In a solar only situation, in order to use energy that is generated by the solar panels, the 'inverter' literally converts / inverts the Direct Current to Alternating Current. If it's required in the home, then it's ready to be used. If it's not required, then it'll just be exported to the grid - the Australian grid uses Alternating Current (AC).
However, a battery uses Direct Current to charge and discharge.
A solar inverter isn't designed to charge / discharge a battery, only a hybrid inverter can do this.
So what is a DC coupled system?
In a DC coupled system, it's a straight swap. You replace the solar inverter for a hybrid inverter. The hybrid inverter can deal with both solar and battery charging / discharging - and convert this to usable power.
The solar panels generate electricity in DC, the inverter can send this straight to the battery as DC, or invert it to AC for use in the home / grid.
It only requires 2 circuit spots on the switchboard (1 replaces the existing inverter, 1 for the CT meter).
This is therefore the most efficient system.
However, it requires removal (and often relocation of the inverter - solar inverters are often high on the wall, hybrid inverters need to be closer to the battery and often floor mounted) and rewiring of the solar panels. This is therefore the more costly installation.
What is an AC coupled system?
In an AC coupled system, you leave the solar inverter where it is.
You then add a hybrid inverter to the switchboard, and hook up the battery. You don't need to remove anything from the system, we're only adding to it. You'll have two inverters (one solar, one hybrid) in this scenario, and there is one additional power inversion that needs to happen.
The solar power that's generated is in Direct Current (DC) so it gets inverted to usable Alternating Current (AC) by the solar inverter, and sent back to the home's switchboard - nothing's changed here.
When the home doesn't need the AC, and the battery needs charge, it'll send this power to the battery BUT the battery needs to be charged via Direct Current (DC).
Therefore the hybrid inverter needs to invert the electrical current from AC, back to DC, to charge the battery.
When the electricity is later discharged from the battery, it'll need to be sent to the home or grid in AC, so the reverse then needs to occur: power from the battery is inverted from DC back to AC by the hybrid inverter and is then ready for use.
In this scenario there is one additional power conversion that needs to occur, and each time power is converted some of it is lost. This system is therefore less efficient, but is the quickest, easiest and cheapest way to install a battery.
In this scenario, you require a minimum of 4 spots on the switchboard (1 for the solar inverter, 1 for the hybrid inverter, 1 for the solar CT meter, 1 for the battery CT meter).
What about grid back up?
If you're also considering adding grid back up, it can be done in either scenario, but it does require another slot on the switchboard. Something to bear in mind if space on your switchboard is limited.