How much energy will I lose from clipping with 270W micro inverters on 330W panels?

Let's do the calcs for a 16 panel, 5.28kW system in Melbourne (because that's where the original questioner lives).

I will do the calculations with an online solar calculator called PV Watts and compare the energy produced in 1 year from:

• a system with a inverter that is the same size as the panel array 
• a system with an panel array that is 22% larger than the total inverter rating (each 330W panel is 22% larger than the 270W microinverter rating)

And we can see which one wins.

System 1: 5.28kW of panels and a 5.28kW string inverter

I tell PVWatts I am in Melbourne, the panels are North Facing (0º Azimuth and 20º from horizontal).

To be sure there is no 'clipping' of the inverter I use a "DC to AC Size ratio" of "1". Which means the string inverter is the exact same size as the panel array.

The results: The string system will produce 7,246kWh of energy in a typical year.

System 2: 16 x 270W Microinverters and 16 x 330W panels

I tell PVWatts I am in Melbourne, the panels are North Facing (0º Azimuth and 20º from horizontal).

Critically I put the DC to AC Size ratio as 1.22 because The Panels are 1.22 bigger than the Microinverters (270W x 1.22 = 330W).

The results: This system will produce 7,238kWh of energy in a typical year.

Conclusion 

The system with the matched panel array and inverter size theoretically generates 8kWh more per year.

i.e. the smaller inverters make almost no difference to the energy yield of the system due to clipping.

In fact because micro inverters optimise each panel individually, I would expect at least 8% more energy from the Enphase system in practice - about 600kWh per year more from the micro inverters.

You can learn more about why oversizing your panel array size relative to your inverter size is a good idea here. It is a good idea whether you use micro inverters or string inverters.

Enphase have an old (but still relevant) white paper on inverter oversizing here. It says:

"Overall, annual losses to inverter saturation were insignificant. In 90% of the scenarios using the M215 Microinverter, including all scenarios with module nameplate ratings below 275W, annual losses were less than 0.5% (Table 3). Conversely, increasing module size resulted in significant gains in annual production, even while inverter size remained unchanged."