You could hook it up with an extension cord if you wanted. I wouldn't recommend it, but you could.
It all depends on how much load you want to draw from it. Sub panels typically don't have as much amperage available to them as main panels. The 10k, in theory, should have as much power available to it from the grid as it's capable of providing. Which is 10kw. So that's probably a 50a dp breaker.
Lets say you install the 10k, hook it up with a 20a breaker, a bunch of batteries, and some solar panels. You are operating in SBU mode, it's night time, and your batteries go flat. Suddenly it's time to charge the batteries AND supply your current loads. If you set your battery charge rate at 20a, and you still want to keep the power on to power your fridge, freezer, lights, heater, whatever, it will overwhelm that circuit.
Remember, charging batteries is a set current when running for grid, and that amperage is added to whatever you are consuming right now. Cloudy days exist, sometimes for weeks on end. Winter exists, and that lasts for a good while, where the days are fairly short, but the nights are very long.
This is a diagram I did for someone recently. The grid must come in and be attached to something. If you don't want to rip apart your main panel, buy another main panel and supply it from the inverter. You don't put the OUT from the inverter to your main panel.
Depending on consumption, you may also want to stick things like ac or electric dryer on the main and not on the inverter.
