You have solar panels on your roof and suddenly the power goes out. Will your fridge, lights and router keep working?
The answer depends on how the system is designed. The panels, even when combined with a battery, do not by themselves provide backup power. You need compatible equipment, a suitable electrical setup and a clear choice of the appliances you want to keep running.
In this guide, we explain what to ask for during the system design process, so you know in advance what will happen during a power outage.
Why does a standard solar system stop working when the grid goes down?
A conventional grid-connected solar system operates in coordination with the electricity grid. When the power supply is lost, anti-islanding protection prevents your system from feeding electricity into the grid in an uncontrolled way.
This protects the grid and the people who may be working on it. HEDNO includes anti-islanding protection in its technical requirements for inverters used in energy storage systems.
To keep some of your home’s circuits running, the system needs a designed backup mode: these circuits are safely isolated from the public grid and supplied by the appropriate system. This is not done by bypassing the protection systems.
To better understand the role of the equipment, read what an inverter is in a solar system.
A battery for savings and a battery for backup: What’s the difference?
A battery can store energy for you to use later, reducing the amount of electricity you buy from the grid. Backup power, however, requires additional capabilities in the overall system.
Before choosing a solution, ask for confirmation of:
- The backup function of the specific inverter model.
- The battery’s compatibility with this function.
- The required isolation and switching equipment.
- The circuits that will be powered during an outage.
- Power limitations and start-up conditions.
The word “hybrid” in a quote does not describe all these capabilities. For example, Fronius documentation on Full Backup sets out specific requirements for the inverter, battery and installation.
So ask for a written description of the function you are buying, along with the equipment required to enable it.
Does the whole house need to be powered?
Start with your actual priorities. During a short outage, you may mainly want to keep:
- The fridge or freezer.
- A few essential lights.
- The router and work-from-home equipment.
- Selected outlets for small appliances.
The installer will consider setting up suitable backup circuits. SMA describes the distinction between supported and other loads when designing a backup system.
An electric cooker, water heater, car charger or heat pump needs a separate assessment. Their simultaneous operation also matters, as does the power some appliances require when starting up.
“Whole-home backup” does not mean unlimited simultaneous power consumption. For a three-phase home, also clarify which phases are supported and what limitations apply.
How many hours will you have power?
You need to distinguish between two measurements:
- Power in kW: how many appliances the system can support at the same time.
- Available energy in kWh: how long it can power them.
Suppose that, when the outage occurs, 5 kWh are available to power the appliances, after taking discharge limits and losses into account. If the average total consumption is 0,5 kW and there is no new solar generation, a simple calculation gives approximately 10 hours.
This is a hypothetical educational example, not a promise of backup duration or a recommendation for battery size. In practice, consumption varies and is affected by the appliances, temperature, battery condition and system settings.
Do not calculate backup duration simply by dividing the battery’s nominal capacity by a rough estimate of consumption. Ask for a scenario based on the actual battery charge level at the time of the outage.
What happens if the battery is almost empty?
A battery may already have used much of its energy for everyday consumption when the outage occurs.
Discuss whether the equipment allows you to reserve energy for backup and how this affects your day-to-day use of the stored energy. There is no one suitable reserve level for every home.
Ask what happens when it reaches the minimum permitted level and how the system restarts. The SMA backup operation manual, for example, describes a power interruption at low charge and specific restart conditions. Details vary by model.
Can the panels supply you with power during an outage?
In a suitably designed system, solar generation can power supported devices and charge the battery. However, you should not assume that every existing inverter will continue generating power in backup mode.
There are also specific limited backup power solutions without a battery, such as Fronius PV Point, when sufficient solar power is available. These are not equivalent to full home coverage and do not provide power at night without storage.
Ask separately what happens in cloudy weather, at night, and after the system has shut down completely.
Is backup power the same as a UPS?
Not necessarily. Switching to backup power may involve an interruption in supply. Automatic switching does not mean your devices will not notice the change at all.
For computers or other equipment that must not restart, ask for an assessment of the switching time and whether a suitable UPS is needed. UPS units also differ in this respect, as Schneider Electric explains.
If you support equipment that is critical to health or safety, a specific assessment of its requirements and the manufacturer's instructions is required.
What to ask before you agree
Add a clear description to the quote:
- Which devices are supported and for what indicative duration scenario.
- What the continuous power is and what the startup limitations are.
- Whether switching to and returning to the grid are automatic or manual.
- Whether the panels operate during an outage.
- What battery reserve is provided and how the system restarts.
- What equipment and work are included, and what the handover test involves.
The inspection and demonstration must be carried out by the responsible professional. Do not attempt to make changes to panels, wiring, or protective devices yourself.
For a broader assessment of a proposal, see our guide to comparing two solar PV quotes.
Frequently asked questions
You have panels and a battery. Do you definitely have backup?
No. The corresponding function must have been planned and installed, with compatible equipment and suitable isolation from the grid.
Can you use all your devices as normal?
Only within the limits of the design and equipment. Ask for a specific list of supported loads and limitations.
Can you add backup to an existing system?
It may be possible, but the inverter, battery, electrical installation, and compatibility must be checked. Adding a battery is not enough.
Does backup mean complete energy independence?
No. Its purpose is to support agreed needs during an outage. Designing an off-grid solar PV system involves a different mode of operation.
Plan backup around your needs
The useful question is specific: which devices do you want to keep running, and for how long?
At Solar Home, we can assess the property, energy consumption, and existing or proposed equipment so we can discuss a suitable solution. Contact us to have your system assessed.
Sources and technical references
- HEDNO — energy storage systems and technical characteristics of inverters.
- SMA — backup system design and backup operation conditions for a specific model.
- Fronius — backup power options and Full Backup requirements.
- Schneider Electric — switching times for different types of UPS.
References to manufacturers explain technical differences. The capabilities of each installation must be confirmed based on the exact model, configuration, and technical design.
