Were both installed at the same time? By Tesla or someone else?
First off, the panel rating is under optimal conditions, so it's very unlikely that you'd ever see that actual number. Depends on lots of factors such as panel orientation, tilt, temperature, shading, lattitude, etc. Also, it's not uncommon for the total panel power to be more than the inverter(s) can actually handle. For example my 13.32kWp array feeds 2 5200W inverters, so realistically the best I'd ever see is 10.4kW (and I've only seen that due to cloud edge effect, on a normal clear day the peak is more like 9kW). So a question is what is the output capacity of the microinverters, and how many do you have, it should be simple math after that.
But if Tesla did the whole shebang, there is a chance that they might have dialed-down the solar output to ensure it couldn't be greater than what the PWs can handle when charging, which is 5kW per PW. For example in my system I think part of the reason why they split it across two 5200W inverters is because I have 1 PW (and said I planned to add another in the future), so one inverter is paired with the PW and the other is on the opposite side of the Gateway, so in the event of a power outage that second inverter simply goes away. There is a risk if you have more solar capacity than the PWs can absorb during charging (and your household consumption doesn't make up the difference) that things could become overloaded during a grid outage (really it shouldn't cause anything more than the inverters to shut down, but it might lead to voltage spikes until it hits whatever voltage the inverters would need to see to shut down).
Looking at your production graphs on both Enphase and Tesla's app would probably help, if Enphase continues to rise above 15kW while Tesla doesn't it might just be a measurement/configuration issue. And I'd also look for clipping, does the system rise and suddenly stop at 15kW, or does it just happen that your daily peak ends up around 15kW and it's a nice curve otherwise?
