% Losses at STC - percent of what?

[JamesLenton]

Hello,

I am looking to include estimated losses from a power systems study to make my PVsyst report more accurate.

In the report the total losses are given in kW, to get this to a % losses at STC, what power should I divide this by?

1. Total installed capacity (DC) - this is what seems right
2. AC inverter capacity 
3. Export capacity (less than inverter capacity in this case)

I can't find the answer on the PVsyst help pages, hope you can help me.

Kind regards,

James.

[Michele Oliosi]

Hi,

For ohmic resistance losses, it is difficult to convert the report total losses to a % at STC.

Indeed, at STC the current is higher, and therefore the losses should be higher as well.

This dependence on the external conditions is also marginally present for other losses as well.

When you say that the losses are given in kW... do you mean the PVsyst report or the power systems study ?

[Michele Oliosi]

One quantity that is easier to export for the resistance losses specifically is the resistance. If you have this value you can transfer it to PVsyst easily.

[JamesLenton]

Hello Michele,

The losses I am referring to are the losses in the power systems study, which are given to us in kW.

I have this for the DC ohmic losses, AC cable losses from inverter to transformer, AC cable losses from transformer to PoC and for the transformers.

To enter the cable losses into PVsyst, I divided each by the total installed capacity to get a percentage and then entered this as the % loss at STC with the correct wire sizes. Is this the best way to go about it?

Kind regards,

James.

[Michele Oliosi]

But your power systems study is for an STC operation ? Or for realistic conditions.. If it's the latter, these kW losses will be underestimated if you compare them to the capacities.

[JamesLenton]

I have just double checked and the studies were carried out at maximum power. So that would represent STC, meaning taking it as a % of TIC is then the correct input.

Thank you.

[Michele Oliosi]

Okay ! Then yes I would divide them by the related maximum power.
DC cabling loss -> DC capacity
AC cabling loss up to transformer -> AC capacity
AC cabling loss transformer to POI -> transformer capacity

[JamesLenton]

Thats very helpful, thanks again!

James.