What Happens When a Home Solar System Is Disconnected from the Grid

Most electrical installations in homes are designed to provide power to the entire home or a part of it. They are also designed to disconnect from the power grid in case of a power outage. This is crucial because transformers used in electrical installations work both ways. If the power grid goes down, power from the 120 or 240 volts of your home solar system would be boosted to 1300 volts or whatever is delivered to the transformer from the power company. This can be dangerous for utility workers on nearby poles.

Isolation Examples

There are examples of isolated structures like the BP station at Drury just south of Auckland where the motorway service center was built too far from the National Grid to connect it. The entire station is powered by solar panels on the roofs and is operational 24/7. Surprisingly, it has been working seamlessly for years. There are also a number of isolated houses in similar situations, using solar power without requiring a grid connection.

Grid Tied Systems and Safety Regulations

The vast majority of residential photovoltaic (PV) solar power systems in populated areas are "Grid Tied" systems. These systems feed varying DC current into an inverter, which tries to extract as much power as possible from the solar panels and then converts that to AC power that is pushed into the main breaker panel. However, they rely on the grid to regulate the voltage and frequency. If the grid is not present, the system will shut down.

This requirement aligns with safety rules, as the electronics in most inverters cannot form a stable grid on their own. Additionally, the law mandates that they must shut down to protect power line workers. If your solar system were still pushing power, it could energize a fallen line that is being repaired. This is why it#39;s crucial for grid-tied systems to disconnect during a power outage.

Off-Grid Solar Systems

Some systems are designed to operate off-grid. These require a battery-based inverter capable of AC coupling to form a local grid. For such systems to function, the battery inverter must be more powerful than the maximum power that the PV solar inverters can produce. The battery bank must also handle the full power of the entire solar array.

For a 4800-watt solar array, I use a 48-volt battery system. The battery must safely accept 100 amps of charge current. My grid-tied system has an approved transfer switch, ensuring it meets safety requirements. During a power outage, the system switches to a battery-based local grid, providing power until the grid stabilizes. This process isn't perfect, but it does work. The local grid is less powerful than the national grid, causing minor flickering in LED lights and occasional blinks with sudden load changes.

Charging and Battery Management

When the battery becomes full, the system needs to reduce power from the solar inverters to prevent overcharging. This is typically achieved through frequency shifting. As the battery approaches full charge, the inverter gradually raises the local grid frequency. New solar inverters often respond to this increase, slowly reducing their power output.

While my system attempts to do this, the solar energy begins to reduce output but not quickly enough. A few inverters shut down due to unstable grid conditions. In older inverters, this would trigger a shutdown, but with the current loads running on the batteries, the system successfully operates. When the batteries run down slightly and the frequency returns to normal, the solar system automatically recharges the batteries. This cycle continues seamlessly, ensuring power during outages.