Hi, this is Amy from the altE Store.
Well, what can I say?
Mother Nature seems really mad lately.
Natural disasters like we have never seen before are happening more frequently.
Meanwhile, the electrical grid infrastructure is taking quite a beating.
As a result, more and more people who have installed grid-tied solar to reduce their
electric bills, and their carbon footprint, are finding themselves without power for days,
just like their non-solar neighbors.
Some people didn’t realize that grid-tied solar is required to shut down when the grid
goes down, even during a sunny day, to protect the lineworkers repairing the grid.
Some did realize it, but they have found they are losing the grid more and more frequently,
and for longer periods of time.
So, is it too late for them to add backup to their solar system?
No.
There are options, depending on what type of grid-tied inverter they used.
Most of them require adding a battery bank and a second inverter.
Two of the most popular options are AC and DC Coupling.
Let’s discuss them one at a time.
AC Coupling adds a battery based inverter that connects between the grid-tied inverter
and the grid.
You would add a new critical loads panel, which is a breaker box where you connect everything
you need to remain on during an outage, like your fridge, well pump, furnace blower, and
some outlets for lights and phones, maybe even your laptop and network router, depending
on how widepread the outage is.
The basics for food, water, heat, and communications.
When the grid is up, the new battery-based inverter’s internal transfer switch allows
the grid tied inverter’s solar power to just pass right through and be connected to the
grid, same as usual.
Since a grid tied inverter needs to see good stable power from the grid to stay on,
it is happy, it sees the power, all is well.
Everything in both your main breaker box and your new critical loads panel get power from the grid.
Meanwhile, the new battery based inverter is making sure the battery bank is all nicely
charged, ready to go when needed.
You're making solar power, everyone's happy.
But when the grid is down, the new inverter’s internal transfer switch disconnects the whole
system from the grid as it is supposed to do.
Instead, it draws energy from the new battery bank, and sends nice clean power to the critical
loads panel.
So you have your lights, well, fridge, everything you need running off the battery bank.
But what about your solar?
What happens when the sun comes out before the grid comes up?
Here’s the cool part.
Because the grid-tied inverter is also connected to the output of the battery based inverter,
it sees the power coming out of that new inverter.
It checks it out.
Right voltage?
Check.
Right frequency?
Check.
Up and stable for at least 5 minutes?
Check.
Cool, it says to itself, the grid is up, I can turn on.
So it does.
Which is perfectly fine, because the new inverter has disconnected everything from the grid.
You are now acting as a 100% code compliant off-grid solar system.
Your non-critical loads are off.
Sorry, no hot tubbing for you.
Your appliances in your critical loads panel are using any power generated from the solar panels,
and any extra power is going backwards through the new inverter to charge up the battery bank.
So, let’s go over the pros and cons of this solution.
Pros: it doesn’t require any rewiring of your solar system up to the breaker boxes.
Any changes are made at the output of the inverter on the AC side, all of your DC wiring
remains unchanged.
For a microinverter system, unless the microinverter company has their own battery solution, this
is pretty much your only choice.
You can choose to use all or just some of the microinverters in your AC Coupled system.
Likewise with a system that uses DC Optimizers, like SolarEdge.
Unless you are using their StorEdge solution (which we did another video on), AC Coupling
is your best option, because you can’t disconnect the optimizers from the inverter, you can
only change around the output of the inverter.
Cons: Depending on which battery based inverter you use,
they have different methods for making sure the batteries don’t get overcharged with this setup.
This is a concern, because the solar power is being backfed through the output of the inverter,
not through the input of the charger that is designed to safely manage the battery charging.
Some inverters shift their output frequency when they sense the battery bank is full,
so that when your grid tied inverter is always checking that the grid is within the approved
voltage and frequency window, it will sense something is amiss, and will shut off, giving
your loads time to use up some of the energy from the full battery bank.
Once the battery bank drops to an appropriate level, the frequency goes back to normal.
The grid tied inverter waits for 5 minutes, and then turns back on.
Other inverters simply send a signal to a relay that opens and disconnects the grid
tied inverter until the battery voltage is acceptable.
So while neither is the best method for charging the battery bank, it is better than nothing.
Secondly, the new battery based inverter needs to be at least a little bit bigger than the grid tied inverter.
Sometimes by about 20%.
As a result, this is not a great solution for people with very large grid tied inverters.
Because they need a new inverter that is bigger than you need.
Even with smaller inverters, because you need to send the full output of the grid tied inverter to the batteries,
in order to prevent sending too much current to the battery bank, and damaging the batteries,
you may need a bigger battery bank than what you really need, costing even more money.
For example, if you have a 5000W grid tied inverter, and your battery bank doesn’t
want to be charged at more than a C/6 rate, which is the amp hours divided by the charging amps,
you will need at least 600Ah, 48V battery bank, whether your loads need that much or not.
That’s about $7000 of AGM batteries.
We’ll talk more about this later.
Another con is that not all grid tied inverters are that easy to trick into turning on.
For example, Fronius inverters are known not to work well with AC Coupling.
Take a look at this page to see an AC Coupling solution for a better understanding
of what equipment is needed.
OK, so, if you don’t have micro inverters or SolarEdge, or you have a Fronius or a large
string inverter, what other options do you have?
I’m glad you asked.
DC Coupling might be right for you.
With DC Coupling, you still have to add a critical loads panel, a battery based inverter,
and a battery bank.
But instead of sending the whole output of the grid tied inverter to the battery inverter,
you leave it where it is in the main breaker box.
Instead, with DC Coupling, you take one of the DC strings of the solar array and send
it through a switch to a 600V MPPT charge controller to safely and efficiently charge
your battery bank.
So, when the grid is up, the switch is sending all of the solar power to the grid tied inverter,
same as usual.
The new battery based inverter/charger is using the AC power
to keep the battery bank charged up.
Most likely it’s the AC power you are making with your solar array, but if it is not sunny,
it will use the grid.
Everyone’s happy.
When the grid goes out, the grid tied inverter disconnects itself from the grid,
same as it does now.
The battery based inverter turns on and powers up the critical loads from the battery bank.
You can then physically turn the switch to redirect one string of your solar array to
the charge controller to charge up your battery bank from the panels.
That’s what charge controllers are meant to do.
You get a healthy muiltistage charging of your battery bank that your battery based
inverter can use to keep your lights on.
Ok, same game, let’s go over pros and cons of DC Coupling.
Pros: Because the charge controller is designed to handle a string up to 600VDC, just like
your grid tied inverter, you don’t have to rewire the PV string, leave it as high
voltage to allow for low current from the array.
Although if you are combining the strings at the solar array, you may need to break
them out to 2 strings.
If you have a large grid tied inverter, because it is just using one string of the solar array,
you are sending less current to the battery bank, so you can do a smaller bank if your loads allow.
Using that same example of a 5000W grid tied inverter, if you have 2 strings going to the inverter,
you can switch just one of them, 2500W to the charge controller.
That’s only 52A to the battery bank, so you can safely have a 300Ah battery bank.
That’s half the size that's required by the AC Coupling, or half the money at $3500.
Let’s think about that battery bank size.
300Ah at 48V is 14.4kWh battery bank.
If it is AGM, and you are only using 50% of the energy, for 50% depth of discharge,
that’s around 7kWh of usable stored energy.
You would easily be able to backup a fridge, well pump, lights, laptop, router, and occasionally
run your gas or oil furnace, just enough to keep from freezing, or run a fan to help keep cool.
Also, because you are doing a smaller backup, you can save money with a smaller inverter/charger.
Cons. Because DC Coupling adds a new 600V charge controller to the system,
which can cost well over $1000,
as well as the new inverter and battery bank,
the equipment cost can be higher than AC Coupling that does not require the new MPPT charge controller.
But, as we discussed, that cost can be offset.
by allowing you to have a smaller battery bank and inverter/charger.
So overall, this can be a less expensive option, depending on the battery bank.
So it could potentially be a pro rather than a con, depending on your design.
Also, as we said in the pros of the AC Coupling,
DC Coupling is only an option for grid tied systems systems that have DC running to a string inverter.
It can not be used with micro-inverters or DC Optimizers like SolarEdge.
Check out this example of a DC Coupled kit for more details.
Now note that neither AC nor DC Coupling options would really be enough
to run an electric heater or an air conditioner for long.
But that’s a topic for another video.
I hope this was helpful, if so, give us a like and a share.
And be sure to check out more of our videos.
Also go to our website at altE Store.com, where we’ve been making renewable do-able
since 1999.