# Gain in Battery Energy (instead of loss) while Discharging

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Hi,

I have added storage to my PV system, the screen shot of storage windows are at the bottom of this message.

Looking at 8760, there is a negative loss (or gain) while discharging, you can see it highlighted in yellow in the image below. This loss is added to the battery balance to get the discharged energy:  EBatDis=EbatLss+SSOCBal

What is the source of this negative loss or gain?

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You should be aware that the calculation of the State of Charge (SOC) of the battery is extremely difficult and uncertain.

This depends on the effective capacity of the battery pack, which itself depends on the instantaneous current (charging/discharging rate), temperature, possibly effective SOC at this time, etc. Keeping all these parameters into account in an instantaneous way leads to very strange and erratic charging/discharging balances along the day (or the year), especially when the discharging conditions are highly varying, which is the case in Peak shaving

Therefore we had to find a compromise, by evaluating the effective average capacity during the discharging phase, and keeping this average for the calculation during the next charging. This works rather well for long-term balances.

Moreover the Charging/Discharging current relation to the energy is dependent on the battery voltage, which is itself a function of the current (through the internal resistance), the battery efficiency, the SOC, the temperature). It is probably this contribution which involves little negative balances (-1.7%) in your hourly results.

NB: You have defined a PNom of the inverters equal to the Max. power as calculated from the clear sky model (374 MW). Now you have several hours exceeding this power. To my opinion, this means  that your climatic data are far above the clear sky model. Please check them.

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Thank you André

However I am a bit confused and your answer raised two more questions for me:

1. Is there a place where we can define how the nominal power (PNom) of inverters is calculated? Is it the user that defines the clear sky model should be used?
7 hours ago, André Mermoud said:

You have defined a PNom of the inverters equal to the Max. power as calculated from the clear sky model (374 MW)

2. In the quality check of my weather data there is no warning regarding clear sky model. Although the graph shows that there are months that my GHI is higher than clear sky model. Is the "Check data quality" enough for data quality check? (related screen shots below)

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The Inverter nominal power is a property of the Inverter that you have used.

The Clear sky model is based on the geographic situation of your system (latitude, longitude, altitude). It doesn't depend on your weather data.

Your weather data seem indeed to be quite correct.

I don't know why you have such overload losses above the calculated maximum Clear sky power value. Please send us  your full project, using "Files => Export project" in the main menu  (e-mail  support@pvsyst.com).

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I sent the project!

Thank you Andre!

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I had asked the project because I had a doubt about the E_Grid and EOutInv values, on the 05/06/90 10:00 .. 14:00 records.
Normally, PVsyst calculates  the production during the best hour of the best clear day of the year (according to the clear sky model), for an advice about the sizing of the PNom you should define for your inverters.
You have indeed specified your inverters pack using this value (374 MW), and this is “a priori” correct.

With this definition, we expect that the real production exceeds the PNom value only very exceptionally.
In your case, it exceeds it during 5 consecutive hours, which I found very suspect. This is the reason why I asked for checking the input meteo data – which seem indeed excellent.
With your project I found the explanation:  you have defined a bifacial system,  so that the real power may exceed this PNom value.

I had not anticipated this possibility when defining the “Max. output power (clear sky)” value.  This should be done in a future version.