USERS | System Integration
Doc #OI0003 Part #004122 Rev 04 2010
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17
Battery Sizing
Off Grid Stand Alone
Sufficient battery capacity is imperative for optimum operation of your system. Working out the correct battery
pack size that will suit your needs involves a number of factors including the daily discharge required of your
system. The peak surge of the appliances etc and should be determined by your Selectronic Authorised
Installer. A rough “rule of thumb” is to start with your expected daily requirement (in kilowatt hours or kWh)
divide it by the nominal system voltage, double this figure to allow for 50% depth of discharge and multiply the
result by 5 to allow for five days autonomy.
As stated before there are many other factors to consider including battery efficiency and temperature effects
the above is simply a rough guide.
Batteries for use in Off Grid applications will generally be “Deep Cycle” type.
Grid Connected
Effective sizing on grid connected systems, depends on a number of factors, including frequency of grid
failure and the export potential of your solar array.
If your system is predominantly designed to provide power during blackout periods, then your battery capacity
should be designed around your average power consumption and the longest power outage you are likely to
experience.
As an example, if your usual combination of appliances draw 2 kW with a 48V battery, and the longest power
interruption in your area in recent years is 3 hours then
To cover for various inefficiencies you would most likely choose a 300 amp hour battery pack.
Once again, this is a rough guide and you should consult a Selectronic Accredited Installer for a total system
design.
Batteries for use in On Grid applications would generally be “UPS” type.
Generator Sizing
Your SP PRO can be programmed to accommodate almost any size of generator; however too small a
generator can result in wastage of fuel. A well sized generator will be able to supply all your normal loads and
have enough reserve to use the maximum capacity of your SP PRO to charge batteries. Remember that the
quality of power produced by the generator will be feeding through to your AC Loads - sensitive appliances
may suffer if power from generator isn’t “mains” quality.
ExAMPLE
If your usual house load is around 2 kW and you are using a SPMC 482 inverter, a generator capable of
providing 8 kW would be an appropriate choice ,this will normally mean a 10 kVA generator. Your Selectronic
Authorised Installer will be able to advise you in this matter.
= 41.66 x 2
(to allow for 50% depth of discharge)
x 3 hrs = 249.96 amp hours of battery capacity
2000 (2kW)
48 (battery volts)
=
93.75 x 2
=
187.5 x 5
=
937.5 amp hours battery capacity is required.
4500
48
