# Shouldn't System Capacity Be Driven by Smallest of Inverter Size and PV?

#1

Hi,

Got a question…

If I’ve got 6,600 watts worth of PV (22 * 300W) on 5,000 watts of inverter then, as far as I’m concerned, my system capacity is 5,000W – as that’s the max amount of power I can get out of it.

If, on the other hand, I have 4,400 watts worth of PV on 5,000 watts of inverter, then the system capacity is 4,400 watts.

In other words, I believe capacity should be calculated on the min of inverter and panel capacities – otherwise my ‘efficiency’ figure is skewed.

What do you think?

Cheers!

#2

The definition of the term “System Capacity” depends on context. In the case of PVOutput, it is the rated DC Power of your panels at standard test conditions (STC) which is what you would find on the data sheet for your panels. That said, it’s rare that you have a day where the cell temperature is exactly 25°C with 1000 W/m^2 of solar irradiance and an air mass of 1.5 which is the industry standard STC for PV Panels.

Inverters also have a nameplate rating which is usually stated at 30 or 40°C ambient temperature. The inverter will usually derate itself above it’s rated temperature and may be able to output more power at lower temperatures.

Basically what I’m trying to say is that there is no single right answer. In my opinion, since the PV Panels are the source of power, their power rating at STC is appropriate to use as the system size. Everything else that affects the conversion of this DC Power to AC Power (temperature, irradiance, inverter clipping, inverter efficiency, wire resistance) are simply efficiency factors. As long as we understand the way in which the system capacity is defined, everything else falls into place.

#3

well i have a 5.050kw inverter and 6625kw of panels which at max NOCT put out 202 watts each so 202X 25 =5050 whatts the same as the inverter so i call mine a 5.050 KW system.
so i entered my panel size as 202 whatts each and that gives me a better idea of how my system is performing.

so in short, if you want your system to be driven by inverter size then Just derate your solar panels till the total watts adds up to your max inverter output, example, my inverter is 5.050 kw max output.
panels were 25 X265 watts each for 6625 watts total ,derate panels to 202 watts each = 25 X 202 =5050 kw same as my inverter.

jrbenito so if you have 5kw of panels and a 5 kw inverter you proberly only get about 4.2 kw of output from the system anyway.

#4

NOCT is Nominal Operating Cell Temperature which is defined as 800 W/m^2 irradiance, 45°C cell temperature, 1 m/s wind speed and ambient of 20°C. There’s nothing wrong with using this number either, just as long as you know what you’ve used to set your system capacity. Your just trying to set a baseline value that you can compare your actual output against.

#5

I agree with Mr Fusion - if the efficiency and target stats are based on the total theoretical output of the panels (6.6kW in my case) I’ll never get close as my inverter maxes out at 5kW.

This isn’t an unusual scenario, at least in NSW, where 5kW is the maximum we can attach to the grid (for single phase at least) and 33% overloading of the panels to maximise when you’ll get 5kW is becoming the norm.

@bankstownbloke, ideally PVoutput would provide the option to select how your system’s maximum should be calculated.

Just my 2c

#6

Same here. My system is 3.4kw, but the inverter is rated to only 3kw. At best the system generates a few watts above 3kw and gets no where near 3.4.

#7

There are no plans to change the way this is recorded or allow an option for some systems to use inverter size instead.

#8

Supposing that 33% overloading is used to get most out of 5KW inverter when there is no ideal condition, hence it do make sense to use total PV installed as system capacity. From the system standpoint you put 33% more panels trying to reach 5KW most of the time (as opposed to install only 5KW PVs on a 5KW inverter) hence total installation target is to maximize average output. From the investment standpoint one spent over 6KW knowing it will produce at most 5KW. IMHO PV is good measure as if you compare two system at same conditions, the 6KW pv on 5KW inverter will perform better in average than a 5KW PV on 5KW inverter (thats is why one would spend more in PV capacity than her inverter is capable of).

#9

This ^ yes. These days, particularly with batteries involved, the instantaneous kW rating of the panels is less important than the number of kWh generated throughout a day. Even if you are flatlining the inverter for a bit, as @jrbenito said, 6kW of panels will generate more kWh during the day than 5kW of panels, so it is still valid to measure the system capacity by the nominal panel capacity, even if it is higher than the inverter can take at peak.
This also flows through the efficiency measure, and the total generation as ranked in the PV Ladder - the efficiency of the 6 kW PV system will be higher than the efficiency of a 5 kW PV system, even if both are limited by a 5kW inverter, due to the higher kWh generated through the day.

#10

Hi, again. Just want to address one common misunderstanding that I see a lot - 5kW is NOT the maximum which can be attached to the grid on single phase.

It is in fact quite trivial to obtain approval for 10kW on a single phase (30kW on 3-phase).

I know this as I just helped a mate set up exactly that. 13.2kW of Jinko Black PV, & 2 x Zeversolar Zeverlution 5,000s, all feeding a single phase 100A service. No limiting device required.

And all this for less than \$8k from Captain Green who did a great job!

If you want to verify, check out page https://www.ausgrid.com.au/Connections/Apply-for-a-connection/Application-forms-and-guides

The guide for NECF05 specifically states:

We are currently improving our forms for connecting micro embedded generation. While this is progressing, please submit the NECF05 form for systems up to 10 kilowatts per phase or compliant systems under 30 kilowatts. Please refer to NS194A for more information on eligible systems.

And in the reference document NS194A. This states specifically:

Ausgrid will approve the low voltage connection of an IES to a premises connection point provided that the total rated inverter nameplate rating is no greater than 10kVA (@230v). This requirement is also applicable if there is an existing multiphase supply to the premises.

Cheers!