Autonomy days and pv capacity

[sairam]

Why does increasing autonomy days, decrease the pv module kwp capacity ?

[André Mermoud]

The PV array is sized in order to provide sufficient energy during the worst conditions along the year, therefore based on the worst days series in your weather data.

Now if you diminish the autonomy period, the probability of "bad weather" series will significantly increase on short periods, therefor requiring a higher PV power.

In other words: if you have 2 day autonomy, your PV array should be sized for ensuring the energy needs during the worst 2-days periods of the year. If you have 10 days, the probability of better days duting this time is much higher.

In practice, PVsyst proposes an autonomy of 4 days. This may be slightly diminished in southern countries (but not less than 3 days), and significantly increased in northern countries with highly marked seasons.

When you undersize the batteries, they will work more. Then, the price of the battery pack is not only the initial investment, but also related to the ageing, i.e. the number of charge/discharges (replacement costs).

On the other hand, if you design a very big battery pack (more than 10 days), the PVsyst calculation will propose a reduced PV power, and the batteries may stay in discharged state during long periods, which is also not goos for their health. With the present cost of PV modules, it is wise to slightly oversize the PV array.