 # DC AC ratio

• Is the DC AC ratio calculated as per

1. KWp / Active Power (kW )

or

2. KWp / Apparent Power ( kVA )

• It depends on the definition and usually varies between different companies and institutions.

Please note that the first ratio varies over inclination, orientation and of course location. Thus the value can easily get higher for an E-W-orientation and/or a higher inclination angle.

Second version additionally heavily varies on local utility requirements and has to be considered separately.

Anybody using a fixed value for both values has no clue what he's doing.

Have you also tried asking this question in the international version of this forum under http://www.photovoltaicboard.com/, which would be english speaking?

Ciao

Retrerni

• thanks for your feedback ,
I understand what you mentioned here ; I am wondering if there is a standard for projects requiring Cos phi .,
As Cos phi is not always required ( as depending on utility requirement ) should I calculate the DC to AC ratio based on kVA or kW ?
I have also asked the forums which you have recommended .

• Well, I would recommend you to use none of it.
I do recommend to us a kWp to kW(dc_in) as a function of inclination, orientation and location. E.g. for southern Germany the factor could be 95% for a roof with an optimal angle of 35° facing south. In some regions in our world this could be >1 (e.g. south of USA)!
Then there is the inverter efficiency, which you can approximate by using the European Efficiency (at least if the MPP-Voltage at higher temperature is close to the optimum input voltage of the inverter), giving the kW(dc_in) to kW(ac_out).
Last there would then be the cos(phi), giving the kW(ac_out) to kVA(ac_out).
So for a site specific underdimensioning rating of 95%, an inverter EU-Efficiency of 97% and a cos(phi) of 0,9 the overall ratio (kWp to kVA(ac_out)) would be 0,95 * 0,97 * 0,9 = 83%.
As you can see all of those three values could significantly vary thus a general rule of thumb makes no sense.

Ciao

Retrerni

• Sorry, made a typo in the above post. The correct formula is

overall ratio (kWp to kVA(ac_out)) : 0,95 * 0,97 / 0,9 = 102%.

Ciao

Retrerni