Inverter sizing

[Lakester89]

When sizing an PV inverter do you apply the 125% to the dc input of the inverter? for example 26- 425w panels and a 11,800w inverter input.
425x26= 11,050 OK
425x26x1.25= 13825.5 Undersized.

Which is correct and if the 125% rule does apply can you point out a code section?

 

[ggunn]

When sizing an PV inverter do you apply the 125% to the dc input of the inverter? for example 26- 425w panels and a 11,800w inverter input.
425x26= 11,050 OK
425x26x1.25= 13825.5 Undersized.

Which is correct and if the 125% rule does apply can you point out a code section?

You multiply the module Isc (which is also the string Isc) by 1.25 to account for the possibility of excess insolation of the array per NEC 690.8(A)(1) when you are sizing DC conductors to an inverter; is that what you are thinking of? As far as sizing an inverter is concerned, it is typical to connect more PV DC rated power to an inverter than its AC power rating; to a point it doesn't hurt anything because the inverter will clip the power from the array that is cannot convert and it will perform better in the morning and afternoon. Most inverter data sheets will tell you what the maximum DC:AC ratio can be; typically it's about 1.5. Most of the systems I designed have a DC:AC ratio of around 1.2.

In answer to your question, though, the first number is correct; you would have a DC:AC ratio of slightly less than 1.0.

 

[jaggedben]

I'd consider that inverter oversized, i.e. likely a waste of money if there is a 10kW available instead.

As far as code, there is absolutely nothing in the code that proscribes what size inverter has to be used with what size array. It is entirely based on complying with inverter specs and manufacturer instructions. Other than that choices are made based on economics and solar production modeling.

The code does impose requirements on DC circuits based on the attached modules (or DC-to-DC converters) as ggunn described.

 

[ggunn]

I'd consider that inverter oversized, i.e. likely a waste of money if there is a 10kW available instead.

Conversely, if there is a way to add modules to the design, that might be a relatively cheap way to boost overall energy production from the system.

 

[ggunn]

I'll just add that the power rating of PV modules is determined by "flash testing" them under Standard Test Conditions, aka STC, which include a cell temperature of 25 degrees C and 1000W per square meter of sunlight equivalent radiation perpendicular to the module surface. Those conditions are rarely if ever seen by a real world solar array and always for only short periods of time; usually an array's output is significantly less than its STC power rating, especially averaged over time. Most arrays are oversized in comparison to the inverter's AC power rating to help counter this discrepancy in output.