BATTERY BANK SIZING

Let's assume that the load of the system you are planning to implement is as follows:

THE ELECTRICAL LOAD: 2 x 12V CCTV Cameras - rated at 5W Peak. During winter time the cameras will make use of a 15W heater to keep camera protected from the cold weather. Also being used is a 12V CCTV recording device and this is rated @ 60W peak. It is assumed that the CCTV cameras and recording unit will operate for 12 hours per day, 7 days per week.

Let's also assume that the equipment is drawing the peak power for the entire 12 hours per day then we can work the calculations as follows:

CCTV = 2 x 5W x 12h per day = 120Wh per day (for the cameras)

PLUS

Recording Unit = 1 x 60W x 12Wh per day = 720Wh per day (for the recording unit)

PLUS

CCTV Heaters = 2 x 15W x 12h per day = 360Wh per day (for the camera heaters in winter)

Total Load (Wh) = 120 + 720 + 360 = 1200Wh per day or 1.2Kwh per day.

Allowing for a 20% safety factor and system losses means that we need to provide an average of 1440Wh per day on average.

AVERAGE TOTAL load = 1440Wh per day (Average)

Battery Storage required.

To calculate the battery size required to provide for 5 days of back-up in the event there was no sun or wind, the battery bank would need to be rated as follows:

(Total Load x Days of Storage) / (System Voltage x Efficiency after system losses)
= (1440 x 5) / (12 x 0.8) or 750Ah at 12V.

Put another way.

The battery bank size is determined by the DAILY WATT-HOUR requirements and the desired number of DAYS of storage capacity required.

Let's assume the following:

The load is 1440 watt hours per day or 1.44kW.

Note: losses are taken in account.
 

Hence 1440 * 5 (required storage in days) = 7200 watt hours
 

DIVIDE by System Voltage = 7200 / (12 * 0.8)
 

Amp Hours of storage required?= 750Ah at 12V

There are many battery sizes available as we have used a 200Ah battery for this example:

Battery storage available: 200Ah @ 12V

NUMBER OF BATTERIES REQUIRED = 750 / 200 = 3.75 220AH batteries.

Naturally, 3.75 batteries would increase to 4 batteries.
 

Therefore four (4 x 200Ah) batteries would be required to provide 5 days backup at a discharge rate of 1440 watts per day in a 12V system.

Put another way you can do the following:

BATTERY SIZING WORKSHEET - Option 2

Another way of working out the calculation is to use the following calculation

1. Determine total watt-hours per day required from your load calculation.
2. Determine the number of days of storage (backup) required. NOTE: 5 days storage is considered normal and this equates to the number of cloudy days in a row.?This figure could be as low as 3 or as many as seven or even 14 depending on the location. Remote sites may be more or less that this.
3. Multiply line 2 by line 1. (storage x watt-hours).
4. Determine depth of discharge (DOD) of the battery. 80% should be considered the maximum for deep cycle batteries.
5. Ensure that your batteries incorporate low temperatures by multiplying the answer in line 4 by the factors shown in the table below using the lowest expected weekly average temperature.

Battery Temp.(F?)	Multiplier	Battery Temp.(F?)	Multiplier
80	1.00	40	1.30
70	1.04	30	1.40
60	1.11	20	1.59
50	1.19

6. Find the watt hour capacity of your selected battery. Watt hours = voltage x ampere hour capacity.  Example: GEL deep cycle,12 volts x 200 amp-hours = 2400 watt hours
7. Divide line 5 by line 6.The result is the number of batteries required.
8. Round number of batteries to fit system voltage.

Example: A 12V system requires 1 x 12 volt battery; a 24V system requires 2 x 12 volt batteries or 4 x 6 volt etc. Work out your load using our evaluation form before determining battery size.
 

View the load evaluation form

View the Marine Load Calculator

 

STEP	DESCRIPTION	EXAMPLE	YOUR ACTUAL FIGURES
1.	YOUR DETERMINED LOAD	2000 watt hours
2.	YOUR REQUIRED STORAGE IN DAYS	5 days storage
3.	WATT HOUR STORAGE REQUIRED	10,000
4.	DEPTH OF DISCHARGE (DOD)	10,000 / 0.8 =12,500
5.	INCLUDE OPERATING TEMPERATURE	12,500 x 1.19 = 14,875
6.	YOUR BATTERY CAPACITY	2400 watt hours
7.	NUMBER OF BATTERIES REQUIRED	6.19
8.	NUMBER OF BATTERIES (ROUNDED UP)	8