https://www.solahart.com.au/products/solar-power/solahart-silhouette-premium-plus-pv-systems/solahart-silhouette-m/ gives specs for a Silhouette panel (SOLAHART415S5) which is rated to produce 415Wp (STC) and 312Wp (NMOT). The spec sheet is dated Jan 2023. I think it possible that the second quote is for an installation with 21 of these panels – which are being oversold by an indicative (unwarranted) power they’d produce when insolation is higher than the STC of 1000W/m2.
I think it also possible that the second quote is for an installation with 21 SOLAHART400V2 (being oversold as a 440W panel because that’s what it is likely to produce at 1100W/m2). See Solahart Premium PV systems
And I think it possible that Solahart is contracting to install some panel that isn’t yet revealed on their website, and which produces 440Wp at STC.
The first quote might be for an installation with 22 x DH144NB-440. As specified at Das Solar Modules Datasheet PDF, Price, Review, Dimensions, 420W, 425W, 430W, 435W, 440W Solar Panel, these panels are warranted to produce 440Wp at 1000W/m2 (STC), and 323Wp at 800W/m2 (NMOT).
All to say that, if a contract doesn’t clearly specify the panel that’ll be installed, it’s anyone’s guess what panels will actually be installed. However it would seem that the first quote may be for an array that’ll produce (22)(323Wp) = 7.1kWp when insolated at 800W/m2, whereas the second may be for an array that’ll produce (21)(312Wp) = 6.6kWp at 800W/m2 of insolation.
There’ll be power losses in the wiring and in the inverter. In my case (in Auckland NZ), the inverter losses on partly-cloudy days were the main reason why my array produced far less than the headline figure I had been quoted on my contract… but I had no contractual recourse because the panels were still performing to spec after 6 years, and because the inverter (Goodwe 3000NS) isn’t warranted for operating efficiency under variable-cloudy conditions. I rather doubt any more-modern inverter’s warranty covers that… but I’d guess that more-modern inverters respond much more quickly to changes in insolation than the Goodwe 3000NS (which first went into production more than 10 years ago). Gory details at MPPT is problematic for my domestic rooftop PV in Auckland – cthombor
All to say that I think it’s something of a pig-in-the-poke when purchasing a PV system, because these are complicated systems with complicated performance attributes which will play out rather differently in every installation.
A closing note: I’d suggest searching Australia’s equivalent of NZ’s Companies Register, to distinguish a solar installer which has been running continuously for a decade or more, from one which has a pattern of declaring bankruptcy every few years and then rebirthing (as a phoenix company) which adopts the name of its predecessor as a dba. Don't Get Swooped by Phoenix Companies | Solargain Blog