The most commonly used devices for emergency power are currently the SMA Sunny Boy Smart Energy (or SBSE, on one phase) and the Sunny TriPower Smart Energy (or STPSE, a three-phase device). We explain the current possibilities and also provide an insight into what will be possible in the future.

Three-phase: STPSE with backup

Most homes in the Netherlands are equipped with a three-phase electrical connexion from the grid, which is why a three-phase inverter such as the STPSE is a logical choice. The STPSE does not have just one AC connector, like a regular inverter, but two. One is the 'regular' AC connector that connects the inverter to the meter cupboard; the other is a special backup output to which a select number of consumers can be connected; these consumers are always supplied with power, even when the public grid is down.

Schematic overview of the connections of a STPSE inverter. The blue line is the 'normal' AC connection, the red is the backup connection

The picture above shows what the two different AC connections look like. The blue line indicates the ‘normal’ connection. This is the connection of the STPSE to the distribution board and via the distribution board to the public grid. All ‘ordinary’ loads are connected to the meter board, i.e. the loads that are not so important that they need to remain powered during a power failure. In the picture, this is for instance a sauna.

The red line is the backup output. In the picture, this line runs back to the distribution board, and the essential loads, such as a refrigerator, are connected to this line.

What is clear from the picture is that the backup loads are always powered from the inverter. Even if there is no sun on the panels and the batteries are empty, the power supply to the backup loads still runs from the public grid via the inverter.

Seamless transition

If the grid fails, this will result in everything on the blue line being without power. But the inverter switches to backup mode very quickly, which keeps the red line energized. The image below shows what happens: as soon as the inverter detects that there is no more voltage on the blue line, it opens relays 1 and 2, and immediately closes relay 3. From that moment on, the red line is fed by the inverter with energy from the solar panels and the batteries. The switchover happens very quickly: within 20 milliseconds. As a result, the consumers on the backup output do not even notice that there is a power failure.

Schematic representation of the backup function with the relays inside the inverter. In the event of a power failure, relays 1 and 2 open, and relay 3 closes immediately afterwards. From that moment on, the backup consumers no longer have a connection to the grid and are only supplied by the inverter

What should we pay attention to with this backup?

The automatic backup works perfectly as long as sufficient power can be supplied to the backup loads. The amount of available energy depends on the presence of the sun, and on the energy stored in the batteries: if the batteries are empty and it's dark outside, the backup loads will of course also be without power. It's possible to set a minimum reserve in the batteries for these types of situations, for example 10% of the total capacity. This does mean that this capacity cannot be used for anything else, like e.g. storing excess solar power.

Furthermore, the power requested by the consumers must never be greater than what the inverter can supply. Note that the power consumption for a 1-phase load may never be more than ⅓ of the total power. This means, for example, for an STPSE5.0 with a capacity of 5 kW, that the maximum power requested on the backup output per phase may never exceed 5000/3 = 1700 Watt! That's more than sufficient for a refrigerator or freezer, for example, but there are also some devices that can require a lot of power for a short period of time, such as a kettle. If more power is drawn on a phase than the inverter can supply in backup, it will go into failure mode. It will immediately try to start up again, but as long as the requested power remains too high, it will immediately go back into failure mode. Therefore, you should switch off the devices that require too much power.

The maximum power on the backup output is divided over the three phases. Therefore, a maximum of one third of the maximum power can be drawn from each phase

Single phase: the Sunny Boy Smart Energy with backup socket

The Sunny Boy Smart Energy is the single-phase hybrid device from SMA. This device also has an option for emergency power, which SMA has called the ‘Secure Power Supply’. This solution works with one or two separate sockets that are connected directly to the inverter, with their own switch. The socket is normally de-energized. If there is a power failure on the grid, you have to press the switch to energize the socket. The device can supply a maximum of 3600 Watts (on one phase). Where the backup with the three-phase inverter runs automatically, with the single-phase inverter it is a process in which you have to perform some actions yourself (such as connecting the plugs of the essential consumers to the emergency power socket).

SMA Sunny Boy Smart Energy hybrid inverter with two battery modules. The red arrow points to the emergency power socket with a switch next to it. This socket is normally not live: only when the switch is pressed, the socket can supply power from the solar panels and the batteries.

The backup socket of the single-phase SBSE, unlike that of the three-phase STPSE, also works if only solar panels and no batteries are connected to the inverter.

Coming soon: the backup box for single phase systems

As described above, the current emergency power option for the single-phase device does not run automatically: you have to do something yourself to activate it if the public grid fails. However, a new backup box will soon be on the market that can make the switchover happen automatically, called the ‘SMA Backup 1P’. This device will usually be mounted underneath the SBSE as in the picture below. It can make the switchover from mains power to emergency power happen automatically, but only for one phase. In contrast to the manual system with 1 or 2 sockets, this system costs a lot more in materials and installation costs.

SBSE inverter with the new SMA Backup 1P backup box underneath

Important: the warning stickers!

When we install solar panels in a home, we always make sure that this is immediately clear on the meter cupboard, with the help of a sticker. This is important for the fire brigade, so that they know immediately that there are still devices in the home that can supply voltage, even when the firefighters switch off the main switch.

When a house has a home battery, it is even more important that the fire brigade can see this immediately, and that they can switch off the voltage immediately. If there is an emergency power supply, the fire brigade must be able to recognise this at a glance and switch it off. That is why Zonnefabriek often places extra switches on the back-up output with a sticker to warn that there may still be voltage on sockets. It can sometimes seem like too much of a good thing, but we do this deliberately, with a view to safety in all possible situations.

Emergency power: know your options

As you can see: there are quite a few different options, each with their own advantages and disadvantages. Although the power in the Netherlands rarely goes out, it can still be a good idea to have an emergency power option in your home, just in case. More and more customers now have an emergency power option in their home, with or without automatic switching. Does this sound like something for you? Then contact us, we will be happy to tell you more about it.