# Stand Alone Systems Pre Sizing

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Hello

I have one question about the logic behind the presizing help. Which parameters are considered in the suggested capacity calculation? When I make this calculation on my own, regarding the efficiencies and the SOC used by PVsyst, I obtain higher value.

Thanks!

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• 3 weeks later...

I agree with you at a first glance but it is possible to match your value with the one obtained by PVsyst. In example, if you have a consumption of 10 thousand Ah per day, and it´s needed an autonomy of 2 days and on the other hand you need to incorporate in your design a DOD of 70% maximum. The calculation is as follows:

Battery Capacity = (10000Ah/day X 2 days)/ (0,7) = 28571 Ah;

Then, taking into account the self-discharge and efficiency of the battery, we have: 28571 Ah X 0,95 = 27142 Ah

Comparing this value to the one obtained by PVsyst Simulation:

Go to the main menu - Preferences - Hidden parameters - Category: Standalone System Presizing : change the value of "SOC minimum Threshold" to 30% (if you want to fix the maximum DOD at 70%) and also in the same Category change the value of "Battery Capacity: C100/C10 ratio" to 120%. With these values it is obtained a suggested value of 26510 Ah; we have only 2,38% oversized capacity comparing our calculation to the PVsyst simulation.

Note: The lower value we have in C100/C10 ratio, the higher value we obtain in capacity , but the usually ratio is between 120 and 130%. We must to fix some values in the Hidden parameters menu if we want to obtain the expected values or at least similar.

Kind regards

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• 1 year later...

Hello

I was wondering about the same thing as when i manually calculate the battery size i get higher values than those estimated by PVsyst

I dont think it is correct to include the losses to the battery capacity as these losses will only affect the PV array system size.

And even if we apply the self discharge, this should increase the manaully calculated value rather than decreasing it

Which also makes me think why and how does PVsyst accounts for the battery energy efficiency in the pre-sizing of the battery capacity although it should only affect the PV array

Thanks

Amir

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The estimation of the Battery capacity is a rough calculation of course.

It is computed with very uncertain parameters, like the admitted DOD, or the ratio of the C100/C10 capacity definition, or even the capacity specification itsef !

Now for taking the efficiency into account, you are partially right.

The capacity is indeed related to the State of Charge (or vice-versa).

Now when charging, you have a loss due to the internal resistance, and also the gassing at high SOC.

When discharging, you have a loss due to the internal resistance.

Both are strongly related to the instantaneous charging or discharging current.

For the required capacity evaluation, we should indeed take the discharging losses into account.

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I dont quite get though how and why the C100/C10 is conisdered into the battery capacity pre-sizing

Thanks

Regards,

Amir