PV backup power: emergency socket, whole-house backup, off-grid system
In short: A standard PV system switches off in a power cut, even in sunshine. It can only keep supplying power if the inverter is built for it. There are then three levels: an emergency power socket for individual appliances, backup power for the whole house via a transfer switch, or an off-grid system that runs entirely without the grid. A storage unit is usually part of it.
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What levels are there?
If the grid fails, a grid-tied inverter immediately disconnects from the grid and stops the grid feed-in. In this way, it protects people working on the power line. A PV system therefore does not supply electricity on its own during a power cut, even when the sun is shining. This requires an inverter with an emergency power function and, in most cases, a storage system.
| Emergency power socket | Back-up power for the whole house | Off-grid system | |
|---|---|---|---|
| supplied | individual devices connected to their own output | the household via the domestic distribution system | anything that is connected, permanently without mains power |
| requires | Inverters or storage units with an emergency power output | Hybrid inverters with storage and switching in the sub-distribution system | Off-grid inverter with battery |
| Network | normal when connected to the mains; the output switches on in the event of a failure | Normally connected to the mains; the house is disconnected in the event of a power cut | without mains power |
| Installation | Electrical contractor | Electrical contractor | Electrical contractor |
What does an emergency power socket do?
Many hybrid inverters have their own emergency power output, often referred to as EPS. If the mains supply fails, it continues to power the connected devices – such as a fridge, router or heating control unit – from the storage system and modules. The power output of this connection is limited; this is specified in the data sheet.
- The Growatt Aura 5000, a storage unit installed alongside an existing inverter, has its own output for grid failure with 2,500 W, or 3.5 kVA for short periods. According to the manufacturer’s English data sheet, the switchover takes less than 20 milliseconds.
- In the case of the Fronius Symo GEN24, this path is called PV Point: a basic emergency power function with 3,000 VA; according to the data sheet, the switchover takes around 15 seconds.
- For the balcony power plant, the Growatt NEXA 2000 and the Anker SOLIX Solarbank 3 E2700 Pro have a dedicated output for power cuts (Growatt up to 800 W, Anker up to 1,200 W); the devices are connected to this output.
How is the whole house supplied with emergency power?
In the event of a power cut, the house must be disconnected from the grid and supplied from the storage system to power the entire household. This is achieved by switching over in the sub-distribution board:
- Automatic in the inverter: According to the data sheet, the hybrid inverters in the complete PV kits with storage – Growatt SPH, Solplanet ASW H and Trina TRH – switch to the storage system in less than 10 milliseconds. The Trina TRH supplies all three phases. The qualified electrical contractor determines which circuits are connected to the emergency power output.
- With a backup power box: The Growatt SYN 100-XH-30 is available for the Growatt MID-XH series hybrid inverters (11 to 30 kW). According to the manufacturer, it switches over in less than 0.5 seconds and feeds up to 40 kW from the storage system.
- With a transfer switch: With the Fronius Symo GEN24 Plus, power supply to the entire household is referred to as ‘Full Backup’. The Fronius backup power transfer switch (Backup Switch 1PN/3PN, 63 A, for Fronius GEN24 Plus) is provided for this purpose; according to the manufacturer, it is switched manually.
How long the electricity lasts depends on the charge level of the storage unit and on consumption. A household that keeps its washing machine, cooker and heat pump running in emergency mode will drain a storage unit significantly faster than one that only powers lights, a fridge and the heating control system. Information on what size of storage unit is appropriate can be found under ‘Electricity storage: How big should it be, and how much does it cost?’.
What is an off-grid system?
An off-grid system has no grid connection. Modules, the battery and the off-grid inverter supply the loads directly, for example in a garden shed, a cabin or an alpine hut. The range includes the Growatt SPF 3500 ES and SPF 6000 ES Plus; according to the data sheet, the SPF 6000 ES Plus has an output of 6 kVA, accommodates up to 8 kWp of module power and is designed for indoor use (IP20). They can be found under ‘Off-grid inverters’.
What needs to be clarified before making a purchase?
- Which devices need to remain running? This determines the required power and whether an emergency power socket is sufficient.
- Can the inverter do this? Whether and how a device continues to supply power in the event of a mains failure is specified in the data sheet; some functions require accessories such as changeover switches or backup power boxes.
- Who installs it? The switchover involves work on the domestic electrical installation. A hybrid inverter and a permanently connected storage unit are integrated into the domestic electrical installation; this is carried out by a qualified electrician. The grid operator and the Federal Network Agency provide information on registration with the grid operator and in the market master data register.
To find out which inverter is suitable for your system, see ‘Hybrid, string or micro inverters: which one is right for you?’. Complete systems with storage and emergency power function can be found under ‘Complete PV kits with storage’.
