Samlex Solar SCC-30AB Mode D'emploi page 26

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GENERAL INFORMATION: BATTERIEs
Sizing the Inverter Battery Bank
One of the most frequently asked question is, "how long will the batteries last?" This
question cannot be answered without knowing the size of the battery system and the
load on the inverter. Usually this question is turned around to ask "How long do you
want your load to run?" and then specific calculations can be done to determine the
proper battery bank size.
There are a few basic formulae and estimation rules that are used:
Formula 1
Formula 2
Formula 3
The first step is to estimate the total AC watts (W) of load(s) and for how long the
load(s) will operate in hours (H). The AC watts are normally indicated in the electrical
nameplate for each appliance or equipment. In case AC watts (W) are not indicated,
Formula 1 given above may be used to calculate the AC watts by multiplying 120 VAC
/ 230 VAC by the AC current in Amperes. The next step is to derive the DC current in
Amperes (A) from the AC watts as per Formula 2 above. An example of this calculation
for a 12V inverter is given below:
Let us say that the total AC Watts delivered by the12 V inverter = 1000 W.
Then, using Formula 2 above, the DC current to be delivered by the 12 V batteries =
1000 W ÷10 = 100 Amperes.
Next, the energy required by the load in Ampere Hours (AH) is determined. For
example, if the load is to operate for 3 hours then as per Formula 3 above, the energy
to be delivered by the 12 V batteries = 100 Amperes × 3 Hours = 300 Ampere Hours (AH).
Now, the capacity of the batteries is determined based on the run time and the
usable capacity. From Table 3.2. "Battery Capacity versus Rate of Discharge" shown
above, the usable capacity at 3 Hour discharge rate is 60%. Hence, the actual capacity
of the 12 V batteries to deliver 300 AH will be equal to: 300 AH ÷ 0.6 = 500 AH.
And finally, the actual desired rated capacity of the batteries is determined based on
the fact that normally only 80% of the capacity will be available with respect to the
rated capacity due to non availability of ideal and optimum operating and charging
conditions. So the final requirements will be equal to: 500 AH ÷ 0.8 = 625 AH (note
that the actual energy required by the load was 300 AH).
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Power in Watts (W) = Voltage in Volts (V) x Current in Amperes (A).
For an inverter running from a 12 V battery system, the DC current
required from the 12 V batteries is the AC power delivered by the
inverter to the load in Watts (W) divided by 10 & for an inverter
running from a 24 V battery system, the DC current required from the
24 V batteries is the AC power delivered by the inverter to the load in
Watts (W) divided by 20.
Energy required from the battery = DC current to be delivered (A) x
time in Hours (H).