Summary: Rooftop solar power plants can be connected to the grid. This enables the combined system to use solar power during sunshine hours and grid power when solar power is not available.

Did you know that solar power systems can be used along with power from the traditional grid? How do these “grid-tied” systems work?

Most times, when we talk of rooftop solar power plants, we visualize a simple set of solar panels that generate electricity for a home that has no electrical grid connection. Thus, there is a perception that residential and rooftop solar power are mainly for those consumers who are not served by the electricity grid.

This is not entirely correct.

Indeed, rooftop solar power can be used in situations where there is no grid available – this is in fact known as offgrid solar. But solar from the rooftop can also work together with the electricity grid.

When a rooftop solar power plant is tied to the grid such that the two systems – solar power plant and the grid – work together, it is called a grid-tied rooftop solar power plant.

Grid-tied rooftop solar power plants are a great way to combine the advantages of solar power and grid power.

One can easily see the benefits of having a grid-tied rooftop solar power system.

On its own, rooftop solar may not be able to satisfy all your load requirements. But tied to the grid, it can provide a powerful combination. Let’s consider the following scenarios:

  • Let’s consider a hotel that has solar panels installed, but no batteries. It is night time. Without batteries, solar power cannot supply its electricity requirements during the night. But the grid power can. The grid-tied solar power system hence takes all the electricity required from the grid during the night for this hotel.
  • Let’s next consider a college that has a large capacity solar power plant installed on its rooftop. It is a sunny day and the solar power system on rooftop is having a merry time, generating enough electricity for all the power requirements of the college. Great. The grid-tied system simply uses all the power from solar and takes nothing from the grid.
  • Finally, let’s look at a house like yours or mine that has installed solar on its rooftop. It is evening time, the sunlight has started to fade, and the solar power system generates only 30% of the total amount of electricity your house requires. No sweat. The grid tied solar system takes 70% from the solar panels and the rest from the grid.

The three scenarios explained above shows how a grid tied system provides the twin benefits of eco-friendly solar power and reliable grid power, all the time satisfying your electricity requirements.

Questions from the Curious Cat

  • Can I also use a battery in a grid-tied solar power plant?

Yes, you can. In this case, the system has to intelligently decide when to discharge the battery and when to charge the battery.

  • In a grid-tied solar power plant, what happens to the solar power generated that is in excess of consumption?

If the grid-tied power plant has batteries included, the excess electricity generated is fed to the batteries until the capacity of the batteries is reached. If there are no batteries in the system, the excess power is exported to the grid!

  • Can a grid-tied solar power plant generate power when the grid does not work?

Typically, grid-tied solar power plants do not work when the power supply from grid fails.

The reason is that, for the grid-tied solar power plant to work, it needs a reference voltage, which is under normal circumstances provided by the grid power. Consequently, when the power from grid is unavailable,  the system does not work.

However, when a reference voltage source is available (from a battery or from a diesel generator), the solar power system operates smoothly even during times of grid non-availability. Thus, grid-tied solar power plants that are connected with diesel generator sets or batteries can operate during all times, whether or not grid power is available.

  • Which is a better system – a grid-tied solar system without batteries or an offgrid solar system with batteries?

Both the options provide a back-up source of power for the solar power system – the grid is the backup for the grid-tied system, and batteries form the backup for offgrid systems.

It is hence difficult to answer which is better, and the answer depends on the particular circumstances.

Usually, off-grid solar power systems are recommended only for applications with very small loads, or loads that are used only infrequently (a solar water pump, for instance). If these systems are especially located in regions where the grid is highly unreliable (say a remote village), an offgrid solar system with batteries could be the better option.

However, where the power consumption is consistently high for long periods (say, 12 hours a day), and the grid power is fairly reliable, grid-tied solar power without batteries will be most times a better option.

In cases where the power consumption is high but the grid power is not available for only a couple of hours everyday, a grid-tied system with batteries could be considered.

So, in short:

–> Small loads (less than 5 kW) + required only infrequently + highly unreliable grid power = Offgrid solar with batteries

–> Medium and Large loads (50 kW and above) + required for long periods in a day + reliable grid power = Grid-tied solar without batteries

–> Medium and Large loads + required for long periods in a day + somewhat unreliable grid power = Grid-tied solar with batteries

  • What are the limitations for a grid-tied solar power plant?

One of the important limitations is that a grid tied solar power plant will not work when the grid does not work. This can be a serious limitation for developing and underdeveloped countries where the grid is unreliable and does not supply power for considerably long periods during the day.

While this limitation can be overcome by having a battery in the system, such an addition increases the overall cost as batteries are expensive, especially when used in medium and large rooftop solar systems (25 kW and above).

Please also note that grid-tied rooftop solar that has a diesel generator tied to it can also work when the grid is not available, provided the diesel generator is running during that time.


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