We are allowed to replace existing equipment. We can install an inverter something like 10% or 1kW larger without a new connection agreement.
The wattage you're allowed to install may be limited to either something based on consumption history, estimate for new construction, or a standard size typical for homes. If you have a load such as pool pump that you normally operate only some hours, you can crank up the consumption for one billing cycle.
When it comes to the wattage actually installed, maybe you can say you put in more than you actually did up front? Inspector may not care if final system size is smaller. Don't know if utility guy will count panels. Nameplate wattage of inverter is probably the main thing he considers.
Typically a system has all panels facing one orientation, so production ranges from 2 hours to 7 hours effective sun in various seasons. By paralleling two PV strings of different orientations, you can over-panel to 140% without clipping. That assumes 90 degree angle, 6 hours, between them. So 140% more watt hours per day from same inverter. Further over-paneling can be OK but will clip production during some seasons and hours.
You can put in grid-tie inverters which have Rule-21 frequency-watts. That should be many/most these days including SMA Sunny Boy, Enphase, and others. My system began with Sunny Boy SWR2500, but those didn't support the feature so I swapped for later (not current, not Rule-21) models which do have frequency-watts.
With such grid-tie inverters you will be able to later wire them through a battery inverter (Sunny Island, etc.). Not the cheapest system, but works well for me. Low priced hybrids would be cheaper. Another option is adding a Power Wall or Sunny Boy Storage (with external 400V battery). That gives you peak shifting but not battery backup. Adding a transfer switch and auto-transformer (e.g. SMA Automatic Backup Unit) works with Sunny Boy Storage and Sunny Boy for grid-backup.
The Sunny Boy Storage inverter is economical ($2500) and ABU is about $3500. The battery, available many sizes, is expensive but UL listed. These also have limited surge power for starting motors (9kW). For all these reasons I prefer a low-frequency 48V battery inverter.
If you only want backup during occasional power failures, not shifting peaks of utility consumption every day, a small UL Recognized AGM battery can work, costing $1250 to $5000. Battery provides surge to start big loads like A/C, and while the sun shines 100% of PV is available to run them. Or wait until UL Listed commercial lithium batteries are cheap and safe.
You can have some batteryless backup. If you put in a 7.7kW Sunny Boy with 10kW of PV panels, it offers manually enabled "Secure Power" of 120V 2000W on a dedicated outlet. That should run refrigerator and the like enough hours per day.
Separate UPS should take care of VOIP phone etc.
