The days of storage determines how many days in a row the stand-alone system can handle a specific load without solar energy input. This expression has to do with system availability. Stand-alone PV power systems that use intermittent renewable energy as their main source of generation require battery energy storage, and the size of this battery storage has long been a crucial and occasionally contentious topic.

It is always required to size the battery in accordance with site-specific loads, customer requirements, and customer expectations because of the nature of stand-alone power systems.

Days of autonomy, which are defined in AS/NZS4509.2:2010 as the number of days the power system can operate without energy input from generators before exceeding the design maximum depth of discharge of the battery, is a term that the industry has adopted to describe the size of battery storage with respect to the site’s needs. Depending on a number of factors, the advised minimum autonomy period ranges from 2 to 5 days.

The cost of PV solar modules has, however, significantly decreased recently, and it is now far more cost-effective to add PV generation capacity than battery storage capacity. There is still some sun radiation available on cloudy days.

Even on cloudy days, a PV array with a rated capacity much greater than what is needed by the site loads might be able to produce enough energy to power the battery. Theoretically, fewer days of autonomy ought to be needed in this case.

What is Balance of Power?

Early in the 1990s, there were challenges in the business with vendors offering inadequately sized systems and customers disliking their standalone PV power systems. A design guideline was created and incorporated in SEIAA training courses after the Solar Energy Industry Association of Australia (SEIAA) launched its solar accreditation programme in 1993. If appropriate for the equipment, is suggested 5 days of autonomy with a maximum depth of discharge of 70%.

Also Read: What is the Average Solar panel Output Per day?

Five days was a common duration at the time since it gave a daily depth of discharge of less than 20 percent and allowed for a number of cloudy days before the system owner might need to activate a generator. Because system designs were frequently based on the average daily solar irradiation for the worst month of the year, the bigger storage capacity also took into account this fact.

Therefore, the battery would supply the deficit energy to the loads on days when the available irradiation was below average. Extra solar energy from sunny days would recharge the battery.


Elliot is a passionate environmentalist and blogger who has dedicated his life to spreading awareness about conservation, green energy, and renewable energy. With a background in environmental science, he has a deep understanding of the issues facing our planet and is committed to educating others on how they can make a difference.

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