Stop any more electrical work before you talk to a company that specializes in powering a house with a generator. Have them consult and suggest how things should be partitioned at the panels ... otherwise you will just pay to have the work redone later when it doesn't work like it should.

There is no generator that can power a whole house for long without running out of fuel. Will you have a propane tank? We have a 500 gallon buried propane tank and it would barely last 3 days if we tried to power the whole house like normal ... and that is assuming we had topped off the propane tank before the storms hit (hurricanes).

We just made the decision to go with solar and batteries instead of getting the generator. We get benefits from the solar every day, unlike a generator that can go unused for a year or more, but still needs to be maintained. We were close to the eye of hurricane Milton and solar panels in the area did very well, with hardly any damage, which made the decision easier for me.

It probably depends on your heating and air conditioning system and fuel source. Your electrician could be right, and you won't ever draw over 100A. You must have reasons for a 400A service?

I don't get it. Just use one panel and turn off/don't use the equipment you don't need during the power outage.

"You must have reasons for a 400A service?"

Well, large houses with multiple heatpump HVAC systems draw a heck of a lot of power (current) especially on startup. But I agree, unless the generator is on natural gas, I wouldn't try to power everything in the house off it.

@clt3, many of us aren't lucky to be hooked up to a natural gas line, so most fuel sources for the generator will run out at some point. Most of our neighbors who put in generators upgraded to a 1000 gallon propane tank (buried).

We had friends without power for 10 days after hurricane Charlie. They kept the fridge running and an outlet or two for lights and a small fan at night, but they only had small gasoline powered generators.

The other advantage of solar & batteries is that it is quiet.

If we have an outage for several days we will probably have to sleep in a guest bedroom on the other side of the house, because our neighbor has their very large generator hooked up on the same side as our master bedroom ... it is LOUD. Luckily they usually don't use the house during the summer/hurricane season and it doesn't seem to be set up to turn on automatically when power goes out.

If you have variable electrical rate charges for time-of-day batteries without solar can pay for themselves and supply you in an outage. EVs are coming and some are out with capabilities to power house loads or connect to the grid.

"What’s the best practice here?"

Speak to a few local electricians.

This is a quick and easy battery inverter solar option for $1700.

You can add solar if you want to.

Make sure you place your batteries at least 4 ft off the ground ... knowledge gained from recent storms Helene and Milton! Batteries and salt/brackish water are not a good combination.

I think this largely depends on your loads and intent for the system. Is this intended for short durations, or is it intended to get you through a major weather event where the power is off for days? Power off for days and you will run into fuel problems unless you have natural gas or unless you trim your emergency load to the basics.

I understand that in many cases, one panel can be reasonable.

Hard partitioning what things will get power, and what won't, during an outage may not stand up over time and different owners. My home is primarily natural gas, so a single panel would make sense. Even with the central AC on, the whole thing would top out at 50 amps max.

You have some extraordinary loads if you have a 400 amp residential service. Even in a large house, that's something other than lights, the fridge, AC, perhaps even a well. If it's a large load that could automatically come on, or someone might turn on, it would need to be partitioned. Someone is going to get in the hot tub, or max out the electric range and the wall oven, or something if it's not locked out during an outage.

Such loads might be:

• All electric major appliances (stove, water heater, dryer)
• A whole house electric tankless water heater,
• vehicle charging
• serious shop (welding equipment)
• lots of backup electric resistance heat for a large heat pump

Electricians load balance your 2 panels. Putting all your key systems on one panel isn't recommended per my electrian. Panels are loaded over capacity with the expectation that not all the things wil be on at once. Putting "systems" on one would lead to imbalanced panels if your HVAC, fridge, oven, water heater, well pump, etc are all on one and your (LED) lighting circuits on another.

Get the 2 switches and then selectively turn off breakers if you have an extended outage.

OP hasn't been back for two months. Mustn't have been much interested in the responses.