I have two Delta Pros and extra batteries and the Smart Home Panel. FWIW, the transfer switch or lockout would be a great answer and would save a buck. If I were to start over I might choose that path, although the SHP provides some extra goodness.
What you get with the SHP is automatic backup (on ten circuits) in the event of an outage, and/or one can choose to run any or all of those circuits from grid or batteries from the app at any time, and the SHP will recharge the batteries at 3400W (each side) if that speed is selected by the user. The app also allows monitoring all of the circuits in real time, whether on grid or batteries.
If you are able to do any charging with solar, the SHP makes it very easy to add juice with solar and then choose to discharge that juice on circuits to save a bit on your electric bill. It would also make it easy to do the 'energy arbitrage' thing by charging the batteries from the grid overnight and run circuits with them during the day.
The SHP can also operate in EPS mode, in which it will switch to battery in under 20ms.
In my case, I started with the idea that I wanted to be able to run my furnace in an emergency in the winter, and boom, next thing I knew I bought Delta Pros and the SHP. I live in a house with awful solar potential and one (cheap) electrical utility rate, so the arbitrage and solar charging aspects will not do me much good. I think the automated aspects of the SHP, and ability to monitor all of it remotely were the things that keep me from buyer's remorse. I also like the idea that if I move I can have it uninstalled and take the entire thing with me to my next home.
In testing, I've been able to keep the house up and running for 52 hours straight. Furnace, fridge, chest freezer, reasonable use of lights and televisions, etc. for two full days and into a third before all of the batteries died and the SHP reverted to grid power. It all works together really nicely.
If you are simply looking to power the house when the power goes out, the transfer switch or panel lockout and receptacle would work, although in that case so would a properly-sized generator. I didn't want a generator because of the noise and proximity of neighbor houses to us, and I've been enamored of whole-house batteries for several years.