Monday, June 27, 2011

How to calculate the battery size for an inverter?

Nowadays , power cuts and the loadsheddings are on in whichever part of India you go to.  There is no other escape to get away from that, except to buy either an inverter or a generator for home use.


The size of the battery is calculated  and determined by the projected running time of the inverter and the  wattage load it is supposed to need. If the battery is not strong enough for the inverter  will cause the inverter  not to power up properly  and may also lead to discharging issues which may permanently  and irrepairably damage the bettery.

The formula for caculating the size of the battery needed is the  Inverter rating in watts divided by input voltage multiplied by the usage time.
That is, if the battery size is
Battery size = Inverter rating in watts ÷ input voltage x usage time (hrs)
e.g. 300W ÷ 12V x 5 hours = 125 Amp

Some other formulas that you should know are:
To Convert AMPS to WATTS:
WATTS = AMPS X 120 (AC voltage)
The result is a ballpark figure of wattage required for continuous load of device.

To calculate approximate Startup Load:
Starting Load = WATTS X 2
The result is a ballpark figure of wattage required for starting load of most appliances.
Devices like air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.

Formula to convert AC Watts to DC Amps:
DC Amps = AC Watts divided by 12 x 1.1
Continuous rating:
Maximum combined wattage + 20%.

Amp hour / Ampere Hour or the AH rating is a measurement of the electrical capacity the battery will store. Current multiplied by time in hours  equals ampere hours. Ampere hours is the most important point that should be looked in to  while choosing a battery for the inverter. Ampere our / AH ratings vary with the temperature  as we as the rate of discharge.  The ratings of the actual use  can change according to the efects of the temperature and the internal resistance.

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