SMA Sunny Boy Storage: Easily retrofit battery storage to your existing solar

The Sunny Boy Storage. A low(ish) cost way to add a Powerwall and other batteries to your existing solar system. Image: SMA

The SMA Sunny Boy Storage is a battery-inverter that you can use to add battery storage to your existing rooftop solar system.

It does not store energy itself, but integrates modern battery packs, such as the Tesla Powerwall, into your home’s electricity supply. It is exciting because it is the cheapest and simplest way to add a battery such as the Powerwall to your existing system. This is because it can re-use your existing inverter – no need to replace it.

It is compatible with any inverter because it is AC coupled. This means it simply connects into your existing solar inverter‘s 240V AC output.

It takes this AC power and converts it into DC power to charge your batteries:

The Sunny Boy Storage charging

And when the batteries discharge it converts their DC into the AC used by the home:

And discharging.

The Sunny Boy Storage is the best priced battery-inverter on the market that I can find. It comes with a 10 year warranty. It is able to output a maximum of 2.5 kilowatts of AC power, below the maximum output of most battery storage systems. These systems could still be used with the Sunny Boy Storage, but not at their full power. The average efficiency of the Sunny Boy Storage at converting current is around 96.5%, which is very good.

It is for on-grid use only and cannot currently be used to give backup power from your batteries during a black out. A retrofit kit making this possible will be available later this year, but it is likely to be a substantial extra cost and will not be compatible with some inverters.  The Sunny Boy Storage is due to be released in Australia in August 2016.

The Sunny Boy Storage’s Vital Statistics

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Are You Sure You Want Batteries?

Let’s say you have an existing, on-grid, rooftop solar system and you want to add battery storage to it. Well, the very first thing you need to do is have your head examined, because at the moment battery storage is still very expensive and cannot pay for itself anywhere in Australia. Not even in Canberra and the city of Adelaide which have battery subsidies.

However, if after your head has been thoroughly examined, you decide you want to install batteries for non-economic reasons, or, you think the hole in your head makes you look stylishly attractive, then read on.

Batteries Don’t Help The Environment

People can want to install batteries for a variety of reasons, but at the moment, helping the environment is not a good one. This is because when you charge and discharge a battery, some power is always lost . So sending one kilowatt-hour of solar electricity directly into the grid will cut fossil fuel use by more than using one kilowatt-hour of stored solar electricity.

In the future, as renewable generating capacity expands, home energy storage could help cut greenhouse gas emissions, but at the moment this is not the case. Not even in South Australia which has the most wind and solar power in the country. When the situation changes, I’ll let you know.

The exception to this is if your rooftop solar system is export limited, which means you are not allowed to export all the surplus solar electricity you generate and so some goes to waste. This most often happens in rural areas. In this case, using battery storage can definitely be an environmental plus.

How AC Coupling Works

Solar panels on your roof will generate DC current during the day. Even if it is cloudy they will still produce some power. This gets sent to your solar inverter which transforms the DC current from the panels into the AC current your home uses. If you have microinverters instead, the situation is similar, except you have many small inverters instead of one large one.

The Sunny Boy Storage comes with its own mini power meter, placed on the wire that connects your home to the grid. This meter tells the Sunny Boy Storage if your home is exporting or importing electricity at any moment in time. ¹

The easiest way to add storage to this is to attach a battery-inverter, such as the Sunny Boy Storage, to the output of your solar inverter. The battery system is connected to the Sunny Boy Storage and it should look like this:

How the Sunny Boy Storage is retrofitted to your existing system via AC Coupling

This setup is called AC coupling because the Sunny Boy Storage is coupled, or connected, to a cable that carries AC power. When the household produces more solar electricity than it is using, the Sunny Boy Storage will take AC power from this cable and convert it into the DC batteries need to charge. And when the household is using more power than its rooftop solar provides, the Sunny Boy Storage will take DC from the batteries and convert it to AC for use in the home.

The advantage of this system is that the Sunny Boy Storage doesn’t need to communicate with the inverter. They have nothing directly to do with each other, so it doesn’t matter what kind of inverter the rooftop solar system has. As long as the inverter works, you can add batteries with a Sunny Boy Storage.

The Sunny Boy Storage Doesn’t Do DC Coupling

There is another way to attach batteries to a rooftop solar system. That is to use the DC power produced by solar to charge the batteries. Then wire the battery directly into the solar inverter for your DC-to-AC conversion. There is no place for the Sunny Boy Storage in this sort of arrangement. DC coupling is not its bag, baby.

The disadvantage of DC coupling is it needs a special hybrid solar inverter which costs more than a conventional one. And it  also must be compatible with your battery system and able to communicate with it. While this can be arranged when installing a completely new rooftop solar system, for an existing system it would require replacing the solar inverter it has now, which might still have many years of life left in it, and replacing it with an expensive hybrid solar inverter.

The advantage of DC coupling is the overall efficiency will be higher than with AC coupling.

The Sunny Boy Storage And Efficiency

Unfortunately, some energy is lost when the Sunny Boy Storage converts AC into DC to charge the battery and when it converts DC from the battery into AC. Fortunately, the Sunny Boy Storage’s efficiency at converting is quite high and averages 96.5%. This loss will apply when it both charges and discharges the battery.

This loss of energy will be on top of what is lost by the battery. So if the Sunny Boy Storage is used with a Tesla Powerwall, with a claimed efficiency of 92.5%, then the system efficiency would be around 86.1%. If used with the LG Chem RESU battery system with an efficiency of 95%, the overall efficiency would be 88.5%.

The Sunny Boy Storage’s Power Is Not High

The amount of electrical energy stored in batteries is measured in kilowatt-hours. The rate at which batteries can supply stored energy is called power and is measured in kilowatts. No matter how much power the battery system has, the amount of power the Sunny Boy Storage can supply is quite limited.

The Sunny Boy Storage can give a maximum 2.5 kilowatts of AC power. That’s not a lot. There are oven elements that draw 2.4 kilowatts. And that’s just for using the oven and not the grill or any hotplates. So with one of those oven elements you couldn’t even roast a turkey and play Xbox at the same time without exceeding the Sunny Boy Storage’s power limit. You could probably watch TV, provided the show was slow paced and not too exciting. (That’s how TV energy consumption works, right?)

This means when the solar panels aren’t producing power, any electricity consumption over 2.5 kilowatts will have to come from the grid. This is regardless of how many kilowatt-hours of energy you may have socked away in your batteries or how much power they could supply if only the Sunny Boy Storage wasn’t electron-blocking them.

The 2.5 kilowatts maximum the Sunny Boy Storage supplies is less than many battery systems can offer. The Tesla Powerwall can give a continuous 3.3 kilowatts of power, while the LG Chem RESU provides 3.6 kilowatts. Not being able to reach their full potential won’t harm your batteries and will actually help to extend their life. But it is still a major pain in the economic butt. It can make it difficult to use all your stored energy overnight. It can also make it very difficult to use it all during peak pricing periods on time of use tariffs.

Making The Sunny Boy Storage Smarter With The Sunny Boy Home Manager

When I described how the Sunny Boy Storage integrates into an existing rooftop solar system and how it must communicate with an extra mini electricity meter, I actually described a situation that is pretty dumb. Not dumb as in it won’t work, or there is a better way to arrange the basic set up, but dumb as in not very smart. As in the system can’t know you may want to save your stored energy for peak electricity price periods, or that you want to charge the batteries with off-peak grid electricity.

But this is fixed by installing the Sunny Boy Home Manager which will communicate with the Sunny Boy Storage and SMA mini Energy Meter and allow you to intelligently monitor, control, and plan your energy use. It has a one year warranty and by itself is likely to cost over $600.

Operating Temperatures

The Sunny Boy Storage’s operating temperature is from -40 to 60 degrees Celsius. So unless you take it down to the Australian Antarctic Territory, you can be confident it will never conk out due to being too cold. And provided it is kept out of direct sunshine it should never shut down on account of excess heat.

Warranty

The Sunny Boy Storage has a 10 year warranty which is the same as the warranty of many battery systems.  However, because of the nature of the beast, battery storage rarely lasts a great deal longer than its warranty, but quality inverters often do. So I don’t think it is unreasonable to expect your Sunny Boy Storage to easily outlive its battery storage.

Compatible Battery Systems

Just which battery systems the Sunny Boy Storage is compatible with is a very important question, and so far  SMA  have answered that it is compatible with the Tesla Powerwall and that is it. But other compatible systems will be announced. Obviously, it is in their interest to make it compatible with as many systems as possible, and they have indicated it could potentially work with any lithium-ion battery system, but just whether it will work with the non-Tesla energy storage system you may happen to have your eye on, I cannot say. At this point all that can be done is wait for clarification.

Retrofitting The Sunny Boy Storage To Provide Back Up

Currently the Sunny Boy Storage will not let you use your batteries during a black out. If the grid fails then you will be left sitting in the dark no matter how many kilowatt-hours you may have stored.

However, towards the end of the year, SMA will make a retrofit kit available for the Sunny Boy Storage that will allow your batteries to give backup power during a grid outage.

This might sound like a great idea, but it is important to remember that at a maximum output of 2.5 kilowatts, the Sunny Boy Storage will not let you live as though the grid is still working. In addition to getting the retrofit kit installed, you are likely to also need your switchboard rewired, so only certain loads such as lights and refrigeration will run off battery power. But the good news is if you have the Sunny Boy Home Manager it should make managing your power use easier.

At What Price?

The Sunny Boy Storage is likely to cost under $2,000.  But exactly what it will cost to install is very difficult to judge. This is because very few people are likely to say to an installer, “I just want a Sunny Boy Storage. I don’t want any batteries or anything. I just want a battery-transformer installed that is completely useless to me.” Rather, it will be part of a package deal including a battery system, an SMA Energy Meter, and an optional Sunny Boy Home Manger.

If you are very fortunate and get an excellent deal, you may get a Sunny Boy Storage and a battery system such as a Tesla Powerwall installed for as little as $10,000.  But at the moment, you are likely to have to pay considerably more, so be ready for your wallet to feel a lot lighter after this process.

The Sunny Boy Storage Is One of The Best Ways To Add Battery Storage

For those want to install energy storage to an existing rooftop solar system, the Sunny Boy Storage should definitely be one of the simpler ways to do it. However, be aware that its low power output can make the economics of installing on-grid battery storage worse than they already are.