Solar Fraction (SF) > 100% in stand-alone PV + battery systems – clarification on E_User vs E_Load definition

[Rafael Vargas]

Hello PVsyst team and community,

I would like to clarify a conceptual point regarding the Solar Fraction (SF) in stand-alone (off-grid) or PV + battery simulations.

From the documentation and forum discussions, I understand that:

• E_Load represents the energy demand of the user (defined load).

• E_User represents the energy supplied to that user.

• Unused energy accounts for PV energy that cannot be used when the battery is full and the load is already covered.

• SF = E_User / E_Load.

However, in several simulations of PV + battery systems with no loss of load (LOL ≈ 0), I observe Solar Fraction values slightly above 100% (e.g. 101–103%), even though E_Load is fully covered and all excess PV production is already accounted for as “Unused energy”.

Conceptually, this raises a question:

If E_User represents the energy supplied to the defined user load, should it not be limited to E_Load, such that SF ≤ 100%, with all additional PV energy going strictly to “Unused energy”?

I understand that this may be related to the internal calculation sequence (i.e. E_User being evaluated before curtailment due to battery full), but I would appreciate clarification from the PVsyst team on:

1. Why E_User can exceed E_Load in the annual balance, leading to SF > 100%.

2. Whether SF > 100% is intended as a design diagnostic indicator of PV overcapacity, rather than a physically bounded performance metric.

3. If there is any recommended way to interpret or report SF in such cases, especially for non-technical audiences.

Thank you in advance for any clarification.

Best regards,

Rafael V

[André Mermoud]

First of all, the definition of the "Solar Fraction"  SF = EUser / ELoad  is valid for Grid-connected systems. 

In Stand alone systems this is more complex: it is the  Output of the PV system (EOut converter), minus the battery energy balance: 

SF =  ( EOutConverter - EBattCharge + EBattDischarge )  / ELoad.  

The battery balance (EBattCharge - EBattDisch) includes the difference in SOC between the beginning and the end of the interval, and the battery losses. Therefore:

-  Over la short period, the Solar fraction is not relevant because of the battery SOC difference, 

-  Over a long period, the SOC difference contribution becomes lower, but may still alterate the real value of the Solar Fraction. 

Now the battery losses are extremely  difficult to evaluate, especially due to the variability of the effective capacity of the battery according to the Charge/Discharge rate: in PVsyst this is an approximation, which may sometimes lead to "positive" battery losses in some cases (depending on your load profile definition). This seems to be the case in your project, where the differences are very low (some few permille). 

See the Help   https://www.pvsyst.com/help/physical-models-used/batteries/battery-model/battery-efficiency.html?h=battery