DC to AC ratio for Projects

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DC to AC ratio for Projects

Postby solarfarmer4 » 26.12.2016, 10:31

Is the DC to AC ratio calculated as per

1. KWp / Active Power (kW )

or

2. KWp / Apparent Power ( kVA ) ?
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Re: DC to AC ratio for Projects

Postby Retrerni » 28.12.2016, 00:00

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

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)! Thus the value can easily get much lower for an E-W-orientation (e.g. 70%) and/or a higher inclination angle.
To make things worse, also the local temperature range has to be considered here as well.
Please note that e.g. for india there is a heavy difference between Ahmedabad and Itanagar from irradiation point of view.
Regarding inclination there is a big difference between Srinagar and Trivandrum for the optimum roof inclination angle (or a different correction factor).

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). As there are devices available actually only having a bit more than 90% up to 98% this is also not a constant (and for not optimal designed strings should be considered here as well). Depending on the irradiation distribution over the year instead of the European Efficiency the CEC efficiency can/should be used, whichever fits best.

Last there would then be the cos(phi), giving the kW(ac_out) to kVA(ac_out). This is usually between 1 and 0,9, in some strange requirements even 0,7. It heavily varies on local utility requirements and has to be considered separately.

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. Anybody using a fixed value for any of it usually has no clue what he's doing.

Ciao

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Re: DC to AC ratio for Projects

Postby Retrerni » 28.12.2016, 03:43

:oops: Sorry, made a typo in the above post. :roll: The correct formula is

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

Ciao

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