How to Accurately Size a Battery for Solar Energy?


Off-grid solar energy supply has emerged as a popular choice for domestic, industrial, and municipal use in recent years. While there are different storage mechanisms for solar power, the method of finding out how large a battery is needed for a home/ commercial property is still the same. In general, the process of sizing a battery is carried out by figuring out the battery’s loading and run time autonomy. 

Notably, we need to make some allowances for the efficiency of the components in the system while changing the energy from the input source to the desired form. For determining the accurate battery capacity for the system, we further need to take into account various factors, such as the size of the individual load, the overall load, and the individual run times as well. 

The Autonomy (H) in Hours 

This refers to the time for which a battery operates without a recharge. Every load has autonomy of its own, and they can be labeled as h1, h2, h3 etc.

Factors to Consider While Choosing Battery Sizing

Sizing the battery requirement of a solar system requires a detailed approach, which needs to take into account the system’s voltage losses. Moreover, we need to understand that the solar battery’s capacity changes with regards to temperature – the lower the value of temperature, the lower the capacity. Also, the higher the operating temperature of the battery, the shorter will be its life.

Finding the Total and Average Load

This can be done by estimating its value from the equipment rating and also directly measuring the load.

Employing Average Load to Estimate Available Battery Capacity

Calculating the average load can be done by taking into account factors such as the inefficiencies, the run time, the time at which they occur at the discharge state, and the peak load. Once that is done, the battery’s available capacity can be found.

It is worth noting that the size of the solar battery is related to the input required for the battery to charge. Besides, the charger needs sufficient output current for recharging the battery to the point where it can complete the autonomy period. 

Also Read: A Clean, Green and Affordable Solar-Battery Combined Future Awaits India

Another factor worth considering is the efficiency of the charger and the battery. The efficiency of the charger will vary with the conversion losses from the power source, and more losses will occur as a result of the variation between the battery charging and discharge voltage.

A Useful Formula to Find Energy is as Follows:

Energy efficiency in Watt Hours = Amps x Volts x time

Coulombic efficiency in ampere-hours= Amps x time

Battery Sizing of Solar Battery and Charger

Solar battery sizing can be determined accurately once the output requirement and recharge characteristics are determined.

Total Watts including the inefficiencies removed from the battery = Total Watts including the inefficiencies added to the battery.

Sizing of the Battery = ((Time * Total Watt) / (Battery Voltage * Capacity Fraction)) * (Temperature compensation)

Other factors that influence the value of the battery size necessity are ambient temperature and the depth of the discharge and recharge to determine the life cycle and recharge time for battery operation.

Battery capacity can be expressed as a fraction. For example, if the SOC/state of charge’s minimum value is twenty percent and the maximum value is ninety percent, then the capacity fraction is 75% or 0.75.

Also Read: Steps to Top-Up Water in Inverter Battery – Dos and Don’ts

Note that for smooth operation, a contingency of +5% is added to the final answer from the battery sizing calculation formula.


 If you find this method for solar battery calculations tedious or time-consuming, then you can always get the accurate values from here, an easy-to-use solar battery size calculator.