How Much Battery Do I Need for My Rooftop Solar Power Plant?

Need for batteries

Batteries are required for rooftop solar power plants to increase the reliability of power generation from the solar power system. As solar is an infirm power source, batteries are required to ensure uninterrupted power supply from the rooftop solar power plant.

How much battery is required?

The capacity of the battery (also called battery sizing) can be quite tricky. The reasons are as follows:

  • Depends on load –The size of the battery is dependent on the size of the load, firstly. Larger the amount of electricity your house or factory consumes, larger is the battery size for the same extent of backup required.
  • Depends on type of rooftop solar system – It depends on whether your rooftop solar system is an offgrid system (not tied to the grid) or a grid tied system. It is easy to see that an offgrid solar power system will require a larger battery bank than a grid tied system (where the grid power is also supplied along with solar power), because in the case of the offgrid system, the battery is the only backup for cloudy days and night times.
  • Depends on extent of backup required – The size of the battery also depends on how much back up capacity (or “days of autonomy” as it is called in industry parlance) you require. If you require a battery backup for just 1 hour, the size of the battery required will far less than a system where the backup is required for 3 hours.

Recommendations on Battery Sizing and Use

For rooftop solar, for regions that have a stable and reliable grid, batteries might not be required at all, as they significantly add to the cost and do not provide any significant advantages. If the grid is available to you at all times that the solar power is not, why would you need a battery?

Where the grid is not reliable and the region faces frequent blackouts, use of batteries is recommended if you wish to have uninterrupted power supply, but size your battery optimally so that you get uninterrupted power while your cost is minimized.

For medium and large scale rooftop solar installations (100 kW and above), batteries are not usually recommended except in small capacities, unless these are special cases. The reason is that using batteries on a large scale could end up doubling your cost of solar power for the extent that the batteries are used. Just imagine a 1 MW rooftop power plant using batteries for 30% of the power supplied by solar. That could mean doubling the cost of power for almost 500,000 units per year.


Battery sizing depends on a number of factors. As batteries add significant cost to your system and to the solar power generated, size your batteries with care.

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India Renewable Energy Expert