Standard Test Conditions vs Real life
Photovoltaic panels are rated at "Standard Test Conditions" (STC):
- Irradiance = 1000 W/m²
- Cell temperature = 25°C
- Air Mass = 1.5
Those conditions are achieved in testing laboratories, and basically never happen in real life. Modules need to be cooled down in order for them to stay at 25°C.
1000W/m² is a lot of irradiance, and if it ever happens, it will be at a high ambient temperature. Photovoltaic modules get hotter under high irradiance, so a module under 1000W/m² at 25°C ambient temperature might have a cell temperature of around 60°C. Cell performance decrease with increasing temperature, so your 300Wp modules will almost certainly never achieve 300W once installed on your roof.
Here is the datasheet for this Growatt inverter:
Growatt 5000MTL-S has a maximum recommended DC Power of 6150W and a max AC output of 4600W.
Depending on your location, this inverter might indeed be too weak for the solar panels. At the very least, the company in charge of the design should check if there's a mismatch.
Is it dangerous?
An undersized inverter shouldn't be dangerous for your inverter or your house, though. According to this SolarPowerWorld article ("Solar inverters and clipping: What DC/AC inverter load ratio is ideal?"):
During times when the DC input power is too high, the inverter will
raise the operating voltage of the modules to pull the array off of
its max power point and reduce the DC power.
With a Euro-efficiency (a.k.a. average efficiency) of 97.4%, the Growatt 5000MTL-S will radiate at most ~150W as heat.
Orientation, strings and MPP-tracker
You wrote in a comment:
The house is slightly L-shaped. I assume the majority of the panels
will face north or south, with some on the east portions of the
building. Slope I'd say is ~30° but that's a guesstimate
It means that not every panel will produce the same power at the same time and it might lead to further losses:
- panels in series (a string) have to deliver the same current. They will only produce as much current as the worst panel. The proposed panels have bypass diodes in order to mitigate this effect, but it would be better to only connect together modules which are unshaded and which have the same orientation.
- The proposed inverter has 2 MPP-Trackers, which can optimize the power output of two strings separately. It shouldn't be used with 3 different orientations. South orientation wouldn't make much sense in Australia anyway.
Ask for a simulation
There are are handful of reliable photovoltaic system simulation softwares (e.g. PV*SOL, PVSyst, NREL SAM, INSEL, ...).
Any serious PV company should have at least one license and be able to quickly simulate your planned installation, in order to check if there's any mismatch: not just for peak power, but also for maximum current, maximum voltage or MPP voltage range.
As a result, you'd get a:
- potential energy yield [kWh/a]
- a performance ratio [%]
- a specific energy yield [kWh/(kWp.a)]
The energy yields depend on your location so it's not possible to give any estimate without more information. There are some easy to use online tools if you want to check yourself (e.g. PVGIS, PV*SOL online, NREL PVWatts Calculator).
If the performance ratio is lower than 85%, it probably means there's a problem with the design.