Parameters:

• Load type (ampere or watt): Select the unit in which the load will be specified, ampere or watt.
• Load (watt): If the load type is watt, specify the load in watt, for example 100 W. Use and average value if its a cyclical load.
• Load (ampere): If the load type is ampere, specify the current in ampere, for example 10 A.
• Voltage (V): Specify the battery voltage, if the load type is watt.
• Required duration (hours): Specify the duration that the load must be supplied for.
• Battery type:  Select the battery type,  lead-acid or lithium-ion.
• Remaining charge (%): Specify the required remaining charged. To prolong the life of a battery, a lead-acid battery should not frequently be discharged below 50 %, and Lithium-ion battery below 20%. Note that 0% is a flat battery and 100% if a full battery.

Result:

• Required battery size (Ah): The recommend battery size in Ah.

Current calculation:

• If the load is specified in watt, the current I is calculated as:

I=PV

$I={\displaystyle \frac{P}{V}}$

where, P is the power in watts, and V is the voltage in volts.

Lithium-ion batteries:

• The required battery size Bli−ion

  $B_{li-ion}$

   

   for litium-ion batteries is calculated by the battery size calculator as:

Bli−ion=100⋅I⋅t100–Q

$B_{li-ion}={\displaystyle \frac{100\cdot I\cdot t}{100–Q}}$

where, I is current in ampere, t is the duration in hours and Q is the required remaining charge in percentage.

Lead-acid batteries:

• The battery rating of a lead-acid batteries is proportional to the discharge rate. They are typically rated for a discharge rate over 20 hours. For example, a 20 Ah battery can deliver 1 A for 20 hours, which amounts to 20 Ah.
• The rating of the battery reduces by approximately 62% if it is discharged in 1 hour. For example, if the same 20 Ah battery is discharged in one hour, it can only deliver 12.4 A for one hour, which amounts to 12.4 Ah. The battery size calculator derates the battery linearly based in this characteristic.
• The required battery size Blead−acid

  $B_{lead-acid}$

   

   for lead-acid batteries is calculated by the battery size calculator as:

Blead−acid=100⋅I⋅t(100−Q)⋅(0.02⋅t+0.6)

$B_{lead-acid}={\displaystyle \frac{100\cdot I\cdot t}{(100-Q)\cdot (0.02\cdot t+0.6)}}$

where, I is current in ampere, t is the duration in hours, and Q is the required remaining charge in percentage.