CALCULATING BATTERY SIZE AS A BACKUP STORAGE DEVICE

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

THE 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 weather. Also in use 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 = 120 Wh per day (for the cameras) plus

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

Heaters = 2 x 15W x 12h per day = 360 Wh 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)

Calculating 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.

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