Top 3 Factors for Sizing a Solar PV Battery Bank

Batteries (and battery banks) are critical components of solar PV systems – especially for stand-alone/off-grid setups. Sizing them correctly is important but not easy. Getting it wrong leads to excess costs or insufficient power availability. Also, batteries may get over- or undercharged and thereby damaged.

Factor 1: Total Energy Consumption

Calculating the total energy consumption for all connected loads is a time consuming process. It involves anticipating energy consumption patterns based on the time of day and season – in advance, and anticipating unexpected demands that will exceed the average energy requirement.

However, it’s the most important criterion. An undersized system will never be able to supply adequate energy backup, leading to energy shortage and an unsatisfactory experience with the solar PV system.

On the other hand, an oversized system – while being able to supply all of the energy demand – will be needlessly expensive and might not be fully charged often enough, which could in turn lead to battery damage.

Factor 2: Amount of Backup Required

The longer the time the battery bank is expected to supply power (or the energy autonomy the system is expected to provide), the greater its size.

However, for grid-tied or hybrid systems, the battery time will be relatively short (from a few hours to typically a couple of days at the most) as grid outages do not generally last for more than a few hours.

Conversely, for off-grid systems, larger battery banks will be required to cover (potentially) as many as 3-4 days of total loss of sunlight (such as during extended rains). However, to keep the bank size within reasonable limits, such systems are usually paired with an alternative source of charging, such as a diesel generator set or a kW scale wind turbine. This is because not including an alternative charging source will generally lead to an unnecessarily large increase in battery bank size, the size of the solar array required and the overall system cost.

Factor 3: Number of Strings

As the number of strings (battery connections in parallel to each other) increases, it becomes progressively more difficult to maintain the connections and equal resistance between all the strings. In fact issues show up as early as when 3 or more strings are combined.

Therefore, it the number of strings of batteries should be kept to a minimum (ideally, to two strings). Usually, batteries with low voltages (and consequently higher charge capacities) are used to achieve the objective.

To read more about solar PV battery bank sizing, click here.

Aniruddha Bhattacharjee
Aniruddha Bhattacharjee
Aniruddha has been a renewable energy researcher and report writer for over 3 years. He has also been a content developer for multiple websites, including portals on travel and tourism, restaurants and personal finance. He holds a bachelor's degree in mechanical engineering and a master's in endangered species conservation. Apart from renewable energy, Aniruddha is a keen motor sports and aviation enthusiast, and a beginner in photography.
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  • Gregory Smith

    Solar array size and battery system size have to worry about multiple things, including one factor you omitted that is seriously the most vital of dual factors and those are voltage drop and temperature rise. While your current rises with radiance the temperature also rises not just for modules, but also for batteries, so doing the calculations for voltage drop and battery temperature effects factor in largely when right sizing the module string sizes and also the type of battery heat levels and even the type of batteries you use. While AGM batteries reduce the maintenance issue, they are very subject to overcharging, so the calculations get very cutting edge when very large loads have to be covered by such systems. Also surge power is again an issue you do not cover, because such power demands result in dramatic power needs requiring the battery controller to do double duty to ensure loads are covered. And if your system is grid b tied, demand charges are seriously expensive when the Utility has to cover them, thus a very robust system of PV solar and battery backup that can work in tandem is a must. So paying attention to STC conditions and elevated temperature are demanding more from both battery controllers and inverter technology.

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