STC or NMOT Specs for sizing PV array?

Having a Sunsynk 8k system with approx 10kw solar array, that has been running for a year, I just watched a video explaining the “real world” performance we can expect from our PV panels.

Should we use the STC or NMOT specs when sizing our arrays ? There is a considerable difference.

I used STC when calculating max Voc and Isc, but surely NMOT should be more practical.

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I’m not up to speed with the abbreviations. (STC obviously isn’t Standard Telephone & Cable!)

I think for South African conditions, if your panels are aligned fairly well ( not facing south ) then you can expect to reach very close to STC at some point in the year. I have seen my panels produce between 90 and 95 % of its stc rating on clear days and this is not taking into account cloud edge effects. I easily get this during summer on my east / west array and during winter on my north array inclined 30 degrees.

So for designing a system and matching the panels to the mppt, it would be safer to use stc values. Btw, I have 2 5kw sunsynk inverters.

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No, not that one…
STC - Standard Test Conditions - 25 °C module temp, 1000 W/m2 Irradiance - Lab test conditions as I understand it.
NMOT - Normal Module Operating Temperature - 20°C Ambient temp, 800 W/m2 Irradiance - more in line with real word conditions.
Both are listed with panels Specs but NMOT has lower figures in Watts, Voc and Isc. My Canadian Solar CS6U -335 w has STC ratings of 335W, Voc 45.6 v Isc 9.54 A and NMOT specs are 246W, Voc 42.6 v and Isc 7.7 A.
Potentailly a considerable difference when calculating string sizes.

Thanks @Vassen.
How much did you oversize your panels ?
Seems that the general rule of thumb (from Ausie videos) is ± 30 % above Inverter AC power rating.

I use STC for the voltages and NMOT for the Isc. You con’t go over the V, so better not go over on that one. The MPPT will limit the current(I) if it needs to, but the NMOT is what I normally see from my pv 99% of the time.

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I’ve got about 6.5kw on 1 inverter and about 6.1 on the other but with combination of east west and north west.

I ran out of roof space though so not much chance for anything else…. Just more batteries. :frowning:

I presume you have mix of 2 types of PV panels?
I was advised by another PV user to buy all your panels at the same time to ensure they are identical…

i have nine 355 watt poly Canadian Solar on one mppt and six 540 watt mono JaSolar on the second mppt, as long as they are identical for each string, it should be ok

Yeah, I’ve been thinking the same thing, but leaning more towards panel replacement after say 5 years or so. Not because they’re not performing, but rather so that I can get more power out of the existing roof space.

(The eventual switch to electric cars will be a game changer for household electricity consumption: you’re going to feel a 7kw load when you topup, even if you do it incrementally. But let’s discuss elsewhere. :slight_smile: )

I think we’ll see a vibrant second-hand market emerge for good, but lower-wattage panels over the next few years. Which will of course be great for new users.

Here in Aus we do use the STC power rating to size the array. The Clean Energy Council (CEC) who regulate the solar industry here, require the array to inverter relationship to be within 33% or 75% inverter/array. The majority of systems are grid tied with a 5 kW max export rate so most systems end up with a 6.6 kW array.

9.3 Array peak power
The peak power of the array is calculated using the following formula:
Array Peak Power = Number of modules in the array x the rated maximum power (Pmp) of the selected module at STC.
9.4 Array peak power – inverter sizing
In order to facilitate the efficient design of PV systems the inverter nominal AC power output cannot be less than 75% of the array peak power and it shall not be outside the inverter manufacturer’s maximum allowable array size specifications.

I oversized my system using the required ratio but I had to use a 6 kW inverter to handle my 7.92 kW array (STC). This then meant I had to use a power meter to export limit my output to 5kW.

My inverter manufacturer was happy to work with an array 150% the size of inverter. However if I wanted to obtain the government incentives I had to stick to CEC rules therefore 133% array oversized only.

For info:

If I had to do it again, and if not limited by local regulations or utility I would use STC for Voc as it is potentailly damaging if exceeding the Inverter limit. Isc not so much since it will be “clipped” if necessary. Would not think twice to oversize by a considerable margin - 30 to 50 % - as long as Inverter Max Voltage not exceeded. Appreciate all the replies.

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