Questions and Answers on Solar Power Storage Systems

[Updated August, 25, 2021] “High-voltage, DC coupled, lithium iron phosphate” – the new business field of battery storage for PV systems has brought with it many new technical terms. pv magazine, together with SMA, has held two webinars with the SMA storage system experts Martin Rothert and Michael Ebel and has presented the questions and answers in pv magazine.

We’ve put together a selection of the topics for you here.

Is it possible to connect several storage systems with the Sunny Boy Storage? What genreally determins whether storage systems can be connected into parallel?

The ability to connect up to three Sunny Boy Storage in parallel on a single line conductor or across three line conductors is planned and will be made available for all our customers as a free firmware update. Ultimately, the ability to connect AC coupled storage systems in parallel is simply a question of higher-level PV system managment and the applicable connection standards. By contrast, connecting DC coupled storage systems in prallel is significantly more complex.

[Updated August, 25, 2021] Unfortunately, we can’t say when the ability to connect up to three Sunny Boy Storage in parallel on a single line conductor or across three line conductors will be available.

What kind of personal protection is needed for high-voltage systems?

Martin Rothert

The battery must have protection class II (double insulation). Thanks to the all-pole sensitive residual-current monitoring unit already integrated in the inverter, no other special protective devices (e.g., type B residual-current devices) are required in the installation.

The question is always asked whether storage systems should be connected using a single-phase or a three-phase system. Are there disadvantages to the three-phase connection?

Firstly, the three-phase inverter has manufacturing costs that are approximately 20% higher, which is usually also reflected in a 20% higher selling price. Secondly, efficiency is poorer when photovoltaic voltage and battery voltage are the same, as the DC link voltage is higher on three-phase inverters. Thirdly, the system’s no-load consumption is higher, as more semiconductors have to be controlled and connected.

Michael Ebel

In addition, you have to bear in mind that a three-phase inverter capable of handling unbalanced loads is required for grid backup with single-phase loads, something that in many three-phase photovoltaic or hybrid inverters has only extremely limited functionality, or once again results in significantly higher costs. For grid backup, with three-phase inverters you can use only a third of the nominal power per line conductor, which is usually insufficient for many larger or medium loads with a high inrush current. An inverter with 5 kilowatts of power then only has 5 kilowatts divided by 3, equaling 1.66 kilowatts per line conductor.

Battery temperature is crucial to the service life of a battery. Is it better to install an outdoor system in the carport to help keep its temperature low? 

In Germany, most domestic storage systems are currently installed indoors. An installation in the basement will enjoy stable temperature conditions over the course of a year, which has a positive effect on the service life of the battery.

Installing a domestic storage system outdoors is only possible if the manufacturer has also approved this, as Tesla has done, for example. To keep the average temperature of an outdoor installation low, the battery should not be exposed to any direct solar irradiation. However, in winter the temperature should also not fall below 0°C too often, as otherwise the battery will need to be heated. Simply put, for the service life the decisive factor is the average annual temperature at the installation site. If the temperature in a shady, protected outdoor area is lower than that in the attic for example, an outdoor installation could be beneficial.

Why was the charge/discharge power on the Sunny Boy Storage set to 2.5 kilowatts and not to 3.3 kilowatts as on the Tesla battery?

Sunny Boy Storage and Tesla Powerwall

The battery inverter power should only be 30% to 50% of the photovoltaic inverter power. This is enough to temporarily store 99% of the excess PV current in the battery, even with a feed-in limitation of 50%. Because typical photovoltaic system values in Europe are usually between 5 and 10 kilowatt-peak, and most typically around 6 t0 8 kilowatt-peak, 2.5 kilowatt-peak battery inverters are ideal for more than 90% of domestic storage systems. On the other hand, the inverter must, above all, also be able to adapt to the typical consumption behavior of the household. Here, approximately 80% of the household consumption is between 3,000 and 7,000 kilowatt hours. This means that the average power consumption is between 350 and 800 watts. During the evening and overnight, when most of the battery discharging takes place, this is significantly lower, meaning that the inverter must be highly efficient, especially in the power output range from 250 to 750 watts. A battery inverter that is too large will, thanks to the higher no-load consumption always associated with this, also always be less efficient at very low power. Here, the Sunny Boy Storage 2.5 plays to its strengths with efficiency of approximately 95%, even at 250 watts.

My wholesaler has said that currently multiple Sunny Boy Smart Energy inverters can not be installed in parallel. This is apparently due to the Sunny Home Manager. Is that right?

No more than one Sunny Boy Smart Energy inverter can be connected in parallel.

Will the Sunny Boy Storage also come with an emergency power function?

[Updated, August, 25, 2021] This function was originally planned. However, it was not implemented because the inverter power of 2.5 kW without any overload turned out to be insufficient for supplying many loads. So it would have been very expensive in terms of price compared to the integrated emergency power function of the battery inverter Sunny Boy Storage 3.7.

How does the use of a heat pump in a single-family home affect optimum battery performance?

A heat pump increases electricity demand over the course of a year, and therefore the ideal inverter power and battery capacity also increases. For the Sunny Boy Storage, in the future this can be very easily factored in by installing two or up to three devices with two or three batteries on two or three different line conductors.

If I want to use photovoltaics and battery-storage systems mainly for directly charging electric vehicles, what is the best solution and design for this? Does it then always make sense to install AC systems?

Yes, because the charging devices in electric vehicles all charge from the AC utility grid and the wall boxes also always have only an AC connection. In theory of course, it would also be possible to charge the car battery directly using DC/DC power controllers, which would also be somewhat more efficient. However, the problem is the fact that neither the PV nor the battery voltages, or indeed the communication between the charging device and the car battery is standardized. The AC charger in the car can’t be dispensed with either, because then the possibility of charging the car via a normal outlet at some point in the future is no longer an option This means that an AC coupling is currently the only sensible option here too.

How high are the recycling costs, if batteries are discarded at a later date?

The take-back of batteries is already regulated by the legislation on batteries. The manufacturer must take back the batteries free of charge. Here, it should be ensured that the battery manufacturer has joined a common take-back system and has already paid the appropriate fees for this when the products were placed on the market. This is the only way to ensure that even if the battery manufacturer becomes insolvent, returning the batteries is still possible, free of charge.

Do you have any further questions?

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