Battery Bank Sizing


Sizing of batteries is critical to the performance of electrical items in any system. Insufficient capacity results in systems failure, poor battery performance and shortened battery life. Excessive capacity results in unnecessary weight, cost and space usage.

To ascertain the correct battery size a simple arithmetic calculation of power usage of each electrical accessory between charging periods (usually daily) is required. From this a calculation of current each accessory uses (amps) multiplied by the duration of use (hrs) gives the ampere hour consumption. Ampere hours is the unit of measurement of battery capacity.

It is a characteristic of lead acid batteries that regular discharges below 50% of capacity will result in a disproportionate reduction in life. When a battery is discharged, up to 85% of capacity can be restored relatively quickly. The remaining 15% required to bring the battery to full charge has to be “trickled” in at relatively low current rates resulting in a full charge time from, say, 50% depth of discharge (DoD), of around 6 to 8 hours. Therefore the best workable capacity results from a battery bank which is 2.5 to 3 times the daily consumption.  It is commonly recommended that capacities should be twice daily usage but this sizing results in discharges well below 50% and a significantly shorter recharge time because a larger battery can absorb greater ampere hours before the regulating voltage control causes a tapering down of the charging current.

Remembering that a battery simply stores power it is obvious that the charging capacity coupled with the number of charging hours is equally as critical to good battery performance. Insufficient charging system output or insufficient charging time will result in system failure. If a battery is operated at low levels of charge the battery efficiency is reduced. Failure to periodically bring the battery to full charge will result in reduced battery performance possibly to the point of failure.

Battery Sizing Calculation

List all of the electrical accessories in your system. Include either the current draw in amps or the power usage expressed in watts. This information can be obtained from the specifications contained in the appliance instruction manual or from the supplier. Take care to ensure that the true position is indicated. For example, you may have six lights in your system but realistically you only use three at any one time.

Because the battery capacity is expressed in Ampere/hours we need to convert any wattage figures into amps of load. This is simply done by dividing the watts by the system voltage. For example a 12V, 100W spotlight consumes 8.5A. 100/12=8.5

If in a boat or motorhome, when extending the figures into the “A/hrs/day” column, only extend the circuits which apply when the engine is not running. For example the engine clutch on an engine driven compressor drawing 8A would not be included as the current draw stops when the engine is turned off. However, these current demands need to be taken into account when calculating the available charging current and should be deducted from the alternator output.

Once all the accessories have been included and their individual consumption calculated, simply add the right hand column. This will provide you with the power usage. From this the battery capacity is established. The power usage calculated should represent between 33% and 40% of the total battery capacity. Please note, whilst this is generally a “daily” figure, individuals may decide that they only wish to run their charging system once every three days. This is possible provided the calculations reflect the number of hours of usage between charges.