The truth about battery ready solar systems

Solar Quotes

Most homeowners I speak to are really keen to get batteries.
Just not right now.
They want to wait a few years until the price has come down substantially. And who can blame them when a typical battery system in 2016 has a payback of 20 years but only carries a 10 year warranty.
So clued up homeowners looking to buy solar are really keen to get a system that they can easily add batteries to in the future.
Solar companies have caught on to this and we are starting to see solar systems sold without batteries but advertised as ‘battery ready’.
Here at SolarQuotes we are getting more and more questions about what ‘battery ready’ actually means in these adverts, and the best way to buy a solar system that really is battery ready.
The truth is that ‘battery ready’ can mean lots of things. Many of the ads I have seen are technically correct – in that you can add batteries in the future – but probably not as easily, or with the functionality that many people will be expecting.
In my opinion if you are going to claim a specific solar system as battery ready then you must be more specific and explain the basis on which you are making the claim.
In this post I’ll go through 3 different interpretations of ‘battery ready’  – make sure you understand which one you are being offered.

Battery Ready #1: Ready for ‘Hybrid Without Backup’  by using AC coupling, a battery inverter and batteries.

First let me explain the technical terms in that heading because it was full of jargon (sorry!).
‘Hybrid’ – means a solar system that is both on the grid and has batteries. It is hybrid because it’s kinda half on-grid / half off-grid.
‘Hybrid Without Backup’ – means if the grid goes down, your entire hybrid system will switch off too, despite the batteries. You have no backup. If that sounds ridiculous, please read this.
‘AC Coupling’ – has nothing to do with amorous Air conditioners. It means that the wires connecting the battery part of the system to the solar part of the system are 240V AC (the alternative is DC coupling which means – well – you can work that out for yourself I reckon).
A ‘battery inverter’ is an extra inverter designed only for batteries. It must:
a) convert the batteries’ DC to a grid compatible 240V AC
b) convert the solar inverter’s 240 AC output to DC to charge the battery
c) control the charging so that the battery is not damaged
d) only charge the battery with solar electricity (or possibly off-peak grid electricity)
e) only discharge the battery when the household needs it, don’t discharge it back into the grid – as that would be pointless (unless you have Reposit – but we’ll get to that later).
As you can see the battery inverter must be quite smart. It can only do (d) and (e) if it knows what the house power consumption is and what the solar power output is at any given time.  So the battery inverter will need to talk to a power-monitor (or current transformers which are really cheap meters) that attach to the wires coming both out of the solar inverter and into the house
So to add batteries to any solar system in this fashion – you simply take the battery inverter, and connect it into the home 240V circuit. Connect the power-monitors onto the solar output and household input wires. Then you plug your battery into the battery inverter and Bob’s your mother’s brother: you have a Hybrid System without backup. Twenty four hour solar power – as long as the grid is working.
It looks like this (metering not shown for clarity):

The critical thing to note here is that compatibility between the solar inverter and the battery inverter is not necessary. That is – they do not need to talk to each other, because the output of the solar panels does not need throttling.  Because this is a grid connected system the solar panels can just run at full whack all day with excess energy simply flowing into the grid.
This means that your existing system can have the most obscure solar inverter on the planet and you could still call it ‘battery ready’. It also means that if you have decided that you don’t want backup functionality and don’t mind shelling out for a battery inverter with your batteries, you can buy any solar system – safe in the knowledge that this type of upgrade is possible.
Battery inverters that will allow this kind of upgrade include the super exciting SMA Sunny Boy Storage, and the micro inverters in the Enphase AC Batteries.

But what if you want backup?

Battery Ready #2: Ready for ‘Hybrid With Backup’  using AC coupling, a compatible battery inverter and batteries.

If you think you might want backup when you get round to adding batteries, then the system you will need looks very similar to system #1 but it is different in 2 important ways. Let’s play ‘spot-the difference’.

Did you spot them?
The obvious difference is the off grid switch. This allows you to keep the power on when the grid goes down and not put energy into the grid. Because this could kill the linesmen trying to fix it.
The other, more subtle difference is that the battery inverter needs to talk to the solar inverter. The battery inverter needs a way ask the solar inverter to throttle its output. It needs to do this because when it is operating without the grid, and the battery is full, excess solar power has nowhere to go, except up in smoke in a toxic pile of melted wires.
This has big implications when you are buying your ‘battery ready’ solar system. You need to make sure that your solar inverter can talk to battery inverters.  There a bazillion ways that electronic devices can talk to each other, but the standard that most reputable inverter manufacturers seem to have adopted is called MODBUS. If your inverter can talk MODBUS, then you should be good.
You should also be aware that battery inverters that can go off grid are much more expensive than ones that shut down with the grid. They need to disconnect from the grid and control electricity supply and demand. They also usually need some rewiring in your home to put your essential loads on a separate circuit to the non-essential stuff.
Good off-grid capable battery inverters that talk MODBUS include the all-aussie Selectronic SP PRO and the old faithful SMA Sunny Island

Now I’ve must confess. I haven’t been 100% truthful. You can actually operate the system in the diagram above without a comms link between the inverters. Instead of nicely asking the solar inverter to throttle its output you can just go BANG! And cut the power to the solar inverter. Then you can switch the solar inverter back on when you are ready for more solar power.
You can disable the solar inverter off by cutting its power with a switch controlled by the battery inverter or you can get the battery inverter to shift the frequency of the AC so that the solar inverter craps itself on a low or high frequency trip. Back in my control engineering days we used to call this “Bang Bang” control. Because you bang it on and then bang it off again.
It is a horrible way to control anything – and it will seriously reduce the life of your solar inverter. It may also play havoc with many of your 240V appliances. It will actually invalidate many solar inverters’ warranties too. So I am not going to recommend this style of control. It is just shithouse engineering in my unprofessional opinion.

Battery Ready #3: For real. You just want to plug batteries in and it will work. No additional ‘battery inverter’ thank you very much.

I suspect that this is what most people assume ‘battery ready’ means. They assume that when the time comes that batteries are affordable, they will be able to buy the battery box and get the sparky to quickly wire it into a couple of terminals on their solar system’s inverter. Voila – a fully functioning hybrid system.
If you want a system that is this level of ‘battery ready’ then you need to get a system with a hybrid solar inverter. The hybrid solar inverter converts both the solar panels’ and the batteries’ DC power to AC, and handles all the battery control and switching functions. As you might imagine – these hybrid inverters are much more advanced than a standard solar inverter, and so they are more expensive. Expect to pay $1,500 – $2,500 more for a hybrid inverter over a solar inverter.

Hybrid inverters include the well received Sungrow SH5K and the Redback Smart Hybrid System which comes out of Brisbane and can do just about anything you can think of (including trading electricity with your neighbour – even though that is technically illegal un the Australian Electricity Rules).

If you do specify a hybrid inverter, watch out for these gotchas (make sure they are in the contract you sign so there are no surprises when the day comes to add batteries):
1. Specify if you need backup functionality or not. Not all hybrid inverters allow you to run when the grid is down.
2. Specific if you want ‘true UPS’. UPS means Uninterruptible Power Supply. Many hybrid inverters will power the 240V AC down for 10-30 seconds when they switch from Grid Connect to stand alone. If seamless switching is important to you – then ask for ‘True UPS’.
3. Make sure the consumption monitoring is connected at the initial install. In order to control the battery charging the hybrid inverter needs to know your house consumption. It does that with its own meter. Even before you have batteries, this provides really useful information for you so you can find opportunities to move your loads to daytime and make energy efficiency improvements that will jump out at you when you see exactly how much energy you use and when. You need to specify this as many installers will assume you don;t want monitoring until you have batteries.
4. Check the greatest amount of kWh that the hybrid inverter will handle, and which batteries it can talk to. If you have your heart set on a Powerwall in the future then make sure the hybrid inverter can talk to it (and then sit down and decide if you *really* want a Powerwall when there are better batteries out there!).
5. If you want  to participate in the wholesale market and potentially get a much better price for exported electricity at peak times, then find out if the hybrid inverter is compatible with Reposit Power. I would contact Reposit and ask. If the inverter is compatible then expect to pay an extra $800 for the Reposit controller and software when you buy the batteries.
6. Check if the inverter has energy management relays. These are switches that you can use to switch loads, like hot water heating, on when you have excess solar and off when the solar power drops. If deployed intelligently, this ‘thermal storage control’ can reduce the amount of energy exported to the grid. And it can mean that you need to buy fewer batteries down the line.
7. Check if you need to pay for an extra ‘battery inverter’ card when you get batteries. Some hybrid inverters don’t actually have a battery inverter in them – just space to slide a battery inverter module in – which you have to buy.

So which type of ‘battery ready’ system should you get?

If I was buying a solar system today I’d be happy with option #1. I’m really not worried about grid backup. My local grid goes down for about 2 hours every 2 years. I can live with that. And I’m a picky bugger. I’d enjoy the freedom of choosing any solar inverter I liked.
If I wanted backup, I’d go for option #2. I’d buy a Fronius, SMA or ABB inverter, because I know they talk MODBUS (the language a good battery inverter talks).
What I would not do is Option #3. I think it is unwise to buy a hybrid inverter with the hope of buying batteries in a few years. I don’t think batteries will make financial sense for 3-5 years, and in that time the hybrid inverter you chose could be obsolete, or worse, the manufacturer could be out of business. In 3-5 years time battery inverters will be so cheap that it will almost certainly be cheaper to buy a battery inverter and batteries, and couple them into your non-hybrid inverter than buy an expensive hybrid inverter now.
The only scenario in which I’d buy a hybrid inverter is if I really wanted batteries today. In that case, a hybrid inverter makes for a well-integrated solution. And I’d seriously consider a Redback.
What about you?
Source: Solar Quotes. Reproduced with permission.