Battery Bank Size

Battery Bank Size

We can plan the overall size of our battery bank by using the daily energy requirement of the inverter to determine the size of the battery bank for 1 day's worth of energy.

From our load analysis we know our energy demands to be:
16.9 kWh per week = 2.41 kWh per day.

Required battery energy storage per day = 2.41/(battery discharge efficiency * inverter efficiency) = 3.05 kWh battery output.

Useful energy output from battery per day.	3.05	kWh
Simplified battery bank approximation
Battery array @12volt for 1 day at 50%DoD	508.32	ah
Battery array @24volt for 1 day at 50%DoD	254.16	ah
Battery array @48volt for 1 day at 50%DoD	127.08	ah

Note that the size of the batteries have been increased to take into account a Depth of Discharge (DoD) of 50%. If you do not understand amp-hours then see the notes section.

These battery sizes represent one day's worth of back up supply. If we require more Days of Autonomy (DoA) then we should multiply the battery size by 'n', the number of DoAs required. A typical off-grid system will usually have a four or five day autonomy. 

Different batteries have different current ratings, our battery bank design and indeed the type of battery should be selected carefully. Below are approximations of the current that might be drawn through various nominal voltages. Given the size of our max power load, we will probably need a 36 Volt or 48 Volt battery bank.

Maximum power load on battery	2.4	kW
Simplified battery bank approximation
Maximum current drawn from 12 Volt battery bank	200	A
Maximum current drawn from 24 Volt battery bank	100	A
Maximum current drawn from 48 Volt Barry bank	50	A

One model of battery we could use is the Rolls S12-240, a 12 Volt 215Ah battery, 8 of these could supply 17.7 kWh with 368 Ah at 48 voltage.