Sunny Boy Storage Review

sunny boy storage
A lonely SB Storage at the Melbourne Solar Expo, 2016

SMA are back!
The inverter giant SMA is just about to be the third inverter manufacturer to harness the momentum created by the coveted Tesla Powerwall. The Sunny Boy Storage will position itself uniquely as the only battery inverter that will link to the Tesla Powerwall by utilizing your existing solar inverter. It’s a huge boost for SMA, who have been struggling in recent years in solar inverter market. The recent release of their new Sunny Boy Inverter was underwhelming, in fact, it’s been an eternity since SMA released an innovative product. Of late, SMA hasn’t even attended solar trade shows, evidently because there has been nothing to show-off. But what of the soon to be released Sunny Boy Storage? Will aligning itself with the Tesla PowerWall via this little white box of tricks help redeem the reputation that SMA have so deservingly earned?

What different about the Sunny Boy Storage?

Until now SolarEdge and Fronius have been the only inverters compatible with the Tesla Powerwall. SMA however, is not just offering the third inverter to link to the Powerwall. SMA is releasing a battery inverter that will give the ability for any and every inverter to connect to the Powerwall. For existing systems, you will not need to go the expense of replacing your inverter, but you can link your existing inverter to Tesla’s battery via the Sunny Boy Storage. For new systems, you are no longer limited with your inverter choice if you want to install the Powerwall. Seriously, this is huge step for the energy storage industry.

How does the Sunny Boy Storage do it?

It’s called AC coupling. There is nothing new in the concept; SMA was just the first to bring an AC coupled battery inverter that is compatible with the Tesla Powerwall.  The Fronius Symo Hybrid and SolarEdge/SorEdge inverter use DC coupling. This is how the two methods differ.

DDC Coupled solutionC coupling

Fronius and Solar Edge use the DC Power from the solar panels (via the inverter-charger) to charge the battery. It’s only when the battery discharges that it will send the DC power back to the inverter then “invert” the DC power to AC power, which the house then uses. Inversion only happens once.


AC  coupling

The Sunny Boy solution, on the other hand, waits for the DC power to be inverter into AC power by your existing inverter. The SMA then inverts the AC power back to DC power so it can charge the Powerwall battery. When it’s time for the battery to discharge,  the Sunny Boy Storage inverts the DC power from the battery back to AC power, where the power is then used by the home.
So much AC/DC.  You’ve been, thunderstruck.


So AC Coupling uses three steps of inversion. DC coupling only inverts the solar power from DC to AC once.

Is AC coupling less efficient than DC coupling?

Yes, you will have more losses in an AC coupled system. When you invert power from DC to AC to DC to AC, you would expect your system to run about as efficiently as a mixed up rock and roll band from the 70’s. Is it really that bad?

I won’t take into account the efficiency of the Tesla Powerwall Battery, as this is the common denominator. The figures I’ll use are the inverters’ “European weighted efficiency” rather than “Maximum inverter efficiency” figures. If maximum efficiency is like stating how efficient your car is when you are driving on an empty highway at 100km/hr with a gentle tail breeze, European weighted efficiency attempts to take into account real-world conditions – it throws in a little bit of city driving.

AC coupled efficiency

A typical string inverter today has a European weighted efficiency of around 97 percent (comparing SMA, Sungrow, and Solar Edge). The Sunny Boy Storage runs on a max efficiency of 96.5 percent. Combine the two and you end up with a complete battery inverter efficiency of 93.6 percent. (0.97 x 0.965 = .936).

DC coupled Efficiency

Compare this to the Fronius Symo Hybrid with a European weighted efficiency of 96 percent and the StorEdge inverter that operates with 97.4 percent efficiency.

So a European weighted efficiency comparison shows the efficiency difference between the  AC coupled SunnyBoy Storage and the DC Coupled Fronius or SolarEdge solution is less than 4 percent.  If you clean your panels a little more often you will just about be able to make up that loss. Throw another panel on the roof and you’ll overcompensate.  It’s not worth getting too upset about. The more significant efficiency loss lies in the battery.

Can the Sunny Boy storage use other batteries?

Yes, in SMA’s own words:

 The Sunny Boy Storage is the battery inverter for high-voltage batteries from all reputable manufacturers.

There three different ways to read that one little sentence.

  1. If I were to interpret that practically, I would assume LG’s soon to be release “high voltage” battery and the Fronius solar battery would be the reputable manufacturers they speak of.
  2. If I were to interpret that politically, I would assume the Fronius battery won’t even get a look in. Fronius and SMA are fierce competition. Neither SMA or Fronius were willing to confirm or deny this allegation when I spoke with them both today.
  3. When I interpret the sentence technically, it’s technically wrong! The Sunny Boy storage is designed to work at with 360v batteries. 360 volts is indeed higher than 48 volts, but it’s not “high voltage”. By the definitions in Australia standards:
  • Extra-low voltage d.c is between 0 and 120V ripple free
  • Low voltage d.c. is between 120 volts and 1500v d.c
  • High voltage d.c. is over 1500 volts d.c.

So the Sunny Boy storage is actually a low-voltage rather than an extra-low voltage inverter charger. Without getting too caught up in semantics, the energy storage industry has incorrectly adopted the terms “high-voltage and low-voltage” rather than the slightly less obvious “low voltage and extra-low voltage”.
May I digress a little further?

Low voltage vs. extra-low voltage batteries

Part of the reason the Tesla Powerwall has had a limited selection of inverters is because it’s a low voltage (360vdc) battery. Traditionally batteries that have been used to run homes “off-the-grid” have been 12 to 48 volts. This was because off grid homes naturally adopted the same batteries that were used for camping and caravanning.  As off grid systems morphed into grid support systems, new technologies continued to adopt the old 48-volt convention.When industry leaders like Tesla and Fronius sat down to design a home storage solution, they naturally challenged the status quo. There are two clear advantages of using higher voltage batteries.

Firstly, as power demands continue to increase on 48-volt batteries, so do current demands (power = voltage × current). As current demands increase, so must the cable sizes and connections. If instead we increase the battery voltage, the cables can be kept to a reasonable size.
Secondly, for various reasons, domestic solar systems are designed to work somewhere between 200 and 500 volts. Stepping that low voltage down to extra-low voltage and back up again reduces system efficiency. It also involves designing and manufacturing inverters to perform an unnecessary step.

Low voltage batteries (call it “high voltage” if you must) seem to be a common sense approach to domestic energy storage solutions. I’ll place my bet that 360 volts d.c will become the new norm for home energy storage.

The Pro’s and Con’s of the Sunny Boy Storage

Getting back to the main point, how does the Sunny Boy Storage stack up? It’s not released yet, and details are somewhat limited. But here’s my take on the specification sheet that we do have. I’ll update my post as SMA releases more information.

Sunny Boy Storage Specifications

The Pro’s of the Sunny Boy Storage

  • It’s SMA and it’s German made – that translates as “quality”.
  • You can retrofit it to any inverter. Customers today sometimes agonize over the decision of purchasing a standard string inverter or investing more on a hybrid inverter. Now, it’s not so crucial. If you purchase a quality non-hybrid inverter, you will be able to retro-fit a Powerwall or any compatible low voltage battery.
  • 10-year warranty. I’m glad to see SMA are stepping up their standard warranty.
  • It’s IP65 so you can mount it outdoors. You would expect no less.
  • It has WiFi connectivity so it seems you’ll easily be able to connect it to Sunny Portal to see your usage patterns. If you wanted to see that usage overlaid with your solar production, you’d probably have to settle for the un-inspiring SMA Sunny Boy inverter, or for the Sunny Home Manager.
  • It’s light. I’m not really sure when “light” became a selling point, but there you have it.
  • Prices have not been released yet, but if I am correctly ballparking the price of this little unit, the Sunny Boy storage will be a very cost competitive way to achieve a quality AC coupled solution. At least that is SMA’s published aim. However, consumption monitoring will no doubt be an add-on, as there is no mention of an SMA energy meter included.

The Con’s of the SunnyBoy storage

  • At 2500 watts of charge and 2650w of discharge, the potential to utilize excess solar is just a little bit wanting. The latest spec on the PowerWall will allow 3.3kW of charge and discharge and the Fronius will allow a healthy 4kW.
  • Because it is AC coupled, you will still clip your power in the middle of the day on solar system with an oversized array. For example: if you have a 5kw inverter with 6.5kW of panels, you will never be able to use more than 5kw of power. Compare this for to a 5kW Fronius hybrid system with the same 6.5kW of panels. At lunchtime in perfect conditions, you could be producing 6.5kW of power. Your battery could charge at 4kW, and your inverter could send the remaining 2.5kW through your inverter to your appliances. Admittedly the real world value extra production would not be astronomical.
  • AC coupling is inherently less efficient. As described above, AC coupling needs to invert power three times. Every time this occurs there is a power loss through heat. However, efficiency is most important when expensive resources cannot meet demand. With the current government incentives, adding extra panels is extremely cost effective. If you have the ability to install a solar array larger than your demand, the small loss in efficiency becomes a mute point.
  • With no full release of the installation guidelines, we don’t exactly know what we are in for. One thing for certain is the Sunny Boy Storage will also need an SMA energy meter. Ka-ching! To compare, Redback and Sungrow include the energy meter with their hybrid inverters, Fronius does not. Furthermore, it seems the Sunny Boy Storage may also need the Sunny Home Manager if you want to fully monitor all the data available on Sunny Portal. If this is the case, then… Ka-ching, Ka-ching!


SMA have officially rejoined the game by creating an AC coupled system that connects to comparatively higher voltage batteries like the low voltage Tesla Powerwall. No that wasn’t a typo. By creating an AC coupled battery inverter, SMA has cleverly positioned themselves as the only solution that will allow you to connect the Tesla Powerwall to any existing string inverter. It may or may not also connect to Fronius’s Solar Battery, and it is likely it will connect to the soon to be released LG low voltage battery. If my prediction that Low voltage batteries is where battery technology for home storage is rapidly moving, Sma’s concept is even smarter. The German built battery inverter with a 10-year warranty, IP 56 rating, and WiFi is really attractive at first glance. However it’s charge and discharge capabilities are unfortunately a little lower than that of the Powerwall. If efficiency is King, the Sunny Boy Storage is not: clipping and efficiency losses dictate that. We’re still waiting on pricing, but SMA is advertising it as the most cost effective AC solution on the market. Hopefully, they have taken into account the additional costs of the SMA Energy Meter and if required, the Sunny Home Manager. If the price is right and the efficiency does not rule your rooftop, the Sunny Boy Storage may be the solution you have been waiting for.

Mark Cavanagh

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Mark Cavanagh

Mark is the Owner and Manager of MC Solar & Electrical. He’s an Electrician, accredited solar installer/designer and an electrical contractor.

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