AC or DC coupled battery?
An AC coupled battery is charged by an inverter from electricity from the grid.
A DC coupled battery is charged by an inverter from electricity from solar panels.
AC Coupled batteries.
Any house, whether it has solar or not, can AC couple a battery.
Tesla Powerwall 2 is a well known example of an AC coupled battery.
It has an inverter inside the battery box.
Hybrid solar inverters (e.g. Huawei L1 and M1, Fronius GEN24 Plus, Sungrow SH models)
can also be used as the inverter for an AC coupled battery.
If the house didn't have solar, then the battery can be charged from the grid when power prices are low.
It's usually cheaper to charge a battery from solar power(see DC coupling below) and if the house has
three phase power, then DC coupling provides much greater charge/discharge rates than AC coupling which
is restricted to just 3kW.
DC Coupled batteries
Imagine a regular solar installation.
Panels on the roof and the inverter converting that solar power into AC power for the house loads.
Now add in a battery, plugged straight into that solar inverter.
That's all DC coupling is. The inverter is managing both the panels and the battery.
That's why those type of inverters are called 'hybrids'.
Some excellent hybrid inverters are:
Fronius Gen 24 Plus models
Huawei L1 and M1 models
Sungrow SH models
Goodwe ES, EH and ET models
Growatt SPH and XH models
The batteries that work with these inverters are:
Fronius with BYD HVS and HVM batteries
Huawei with Huawei Luna batteries
Sungrow with Sungrow and BYD batteries
Goodwe with Goodwe, BYD and some LG batteries
Growatt with Goodwe Ark batteries.
Battery costs are about $1,000 per kWh of storage, installed.
Some are a little less than that, some a little more, but as a rule of thumb, $1,000/kWh is close.
The sophisticated battery management system in solar batteries is designed to protect the battery and extend it's life.
We have been told that 5,000 to 7,000 cycles is to be expected, so if there were one cycle a day that's 13-19 years.
Most manufacturers give a warranty that the battery will still be 60% effective after 10 years.
Not surprisingly, that's a conservative warranty that means they won't have to be paying for too many replacements.
Payback on a battery
Payback, or ROI, depends on what happens to power prices over the years to come.
The more they go up, the faster the payback, but at the moment, using Synergy's A1 tariff, it typically takes 9 years to recover
the cost of the battery, assuming power prices stayed unchanged for the next nine years.
Back up power during grid outages.
When the power goes out, people with batteries can keep the lights on.
Most inverters require an external 'back-up' box to provide this feature
Even if they don't have a battery, most of these hybrid inverters with a backup box, can provide some backup power directly from the solar panels.
An AC coupled inverter on three phase can only charge/discharge the battery at 3kW.
The same inverter and battery, DC coupled, charges at whatever it is rated to..5kW/10kW
Oversizing means adding more solar panels that the inverter capacity.
For instance 6.6kW of panels on a 5kW inverter would be a 133.33% oversize.
When you add a battery, most inverter manufacturers allow more oversizing.
Some allow 150% (e.g Fronius) whereas others allow 200% (e.g. Huawei)
That means that a 5kW inverter might be managing 10kW of panels.
The extra panel power is used to charge the battery whilst still sending power to the house.
I installed a Huawei battery last year at my house.
I love seeing my solar entirely power the house, day and night.
It makes me feel good and when I get an EV, I have plenty of panel power to charge that as well.
We have had a few blackouts and the backup kicked in and we were the only house on the street with power.
I did not install the battery to "make money" although, eventually, it will break-even and start to make a profit.
The inverter/batteries I recommend highly to you are:-
Huawei and Huawei battery
Sungrow and Sungrow SBR battery
Fronius and GEN24 Plus and BYD HVM battery