The issue, overall, is transfer architecture
The idea of a "universal transfer switch" per se doesn't exist, at least not in the turn-key form you're after. Why? The issue is that multimode (grid-tied/off-grid switchable) solar systems use a different transfer architecture from standby generators. In particular, if you have a standby generator (save for some exotic systems), you will never be running it in parallel with the utility's generation, so the transfer switches used with standby generators, whether it's a 6kW light-duty portable genset or a 600kW emergency generator at a hospital, are double throw switches that provide total isolation between the utility mains and the generator connection.
However, solar power systems in this day and age are expected to be able to deliver power to the grid when a grid connection is available. In order to be able to do this when the grid is around, while still providing safety from backfeeding when utility power goes away, multimode solar systems use a single throw transfer means. This can either be in the form of a dedicated box, such as the Backup Gateway used with the Tesla Powerwall system, or integral with some other part of the system, typically the inverter-charger used to manage the batteries in "open architecture" multimode systems using 24 or 48VDC battery banks. While many of these inverter-chargers also support selecting a second AC input instead of the first, it does not change the overall thrust of the setup; namely, that when the transfer means is closed, the solar system is permitted to generate power in synchrony with whatever source is feeding the "mains" side of the inverter.
Trouble comes where neutral meets ground
A further issue comes from the fact that most portable generators are required to meet the OSHA specifications for construction use, which require neutral and ground to be bonded at the generator so that everything works as expected when you're using the generator as a sole source of power, say on a jobsite. However, your house also supplies a neutral-ground bond at its main breaker panel (service disconnect enclosure), so in order to avoid trouble with parallel bonding means when you add the generator to the picture, you either need to
- Remove the neutral-ground bond from the genset (something that's not always possible/provided for, and even when it is, creates a bit of a trap for someone who wishes to then use that generator for portable service), or
- Use a transfer switch that switches neutral, thus isolating the generator's neutral/ground bond from the bond in your main panel and eliminating the problem.
However, while switched neutral manual transfer is quite feasible to achieve, switched neutral automatic transfer is quite hard. In particular, while 3-pole double throw enclosed (UL98) switches are available at a reasonable price and a few switched neutral interlocked-breaker manual transfer panels (Eaton CHGEN...RSN, Reliance Controls PanelLink X, and some setups using Square D QO panels and parts) also exist, single phase automatic transfer switches that are not made for RV use universally have solid neutrals, and so do most multimode inverters. In fact, there's only one UL 1741 compliant multimode inverter available on the market at the time of this writing, and that's the 120V version of the Victron MultiPlus/MultiGrid; however, that inverter has several downsides that make it difficult to use in North American applications, especially those involving multiple power sources:
- It's 120V-only, requiring two inverters to be stacked for split phase service
- It lacks a secondary AC input, unlike it's bigger brother (the Victron Quattro, which hasn't yet been UL 1741 tested/listed), or the competing solid-neutral UL1741 inverters for that matter
- It only supports 24V DC, again unlike the Quattro, as well as the competing inverters from Outback and Schneider, which support 48V DC
- Its listing itself is...questionable when it comes to US usage, as the test lab that listed it isn't OSHA-approved as a Nationally Recognized Testing Laboratory (they're CSA-accredited, but that may not be good enough for your Authority Having Jurisdiction).
Permanent standby generators, of course, can be wired either with a bonded or a floating neutral without these issues, and are shipped in a floating-neutral configuration to allow use with stock two-pole residential transfer switches.
There is a glimmer of hope here though
The good news, though, is there is something that comes close to a "universal inlet" for most generators upwards of 8-9kW, whether they be light-duty portables or heavy-duty mobile generators. In particular, while NEMA never wrote a standard for twist-lock receptacles and plugs over 30A, the State of California did, and it has become a de facto standard throughout the electrical industry for this application. The resulting inlet for 120/240V split-phase applications is known as a CS6375, and is available either wrapped neatly in an inlet box from multiple manufacturers (Reliance Controls PB50, Eaton EGSPIB50, Midwest U050N, GE/ABB T050N), or as a kit that fits a 2.5" KO (Reliance Controls PI50).
While this limits you to pulling 50A at 240VAC from your generator, no matter what size it is, it's an adequate solution for most residential standby power needs. (Larger commercial applications use single-pole cam-lock type connectors, but these require significantly more attention in handling than residential power inlets and outlets do, and thus are generally only used in environments where appropriate personnel training can be guaranteed.)