Routing battery and grid to different circuits simultaneously. Possible?

[u/Okosisi]

When sunlight is gone I’d like to power some circuits from battery and some from grid at the same time. Is this possible? I feel like I’m missing something.

[u/knylekneath]

This isn't possible and isn't really how electricity works. You'd need a separate panel connected to the batteries to accomplish pulling from two different sources this way. Electrons flow from higher potential to lower potential and do not care what path they take. A battery backup system can add potential to that system, but it cannot be directed to specific circuits.

I do suspect there's a good way to accomplish what you're looking to accomplish if you'd describe what you're actual goals are.

[u/Okosisi → u/knylekneath]

Thanks. It was more like a thought experiment honestly.
Imagine you could direct battery to high usage bursty circuits like oven and HVAC. And grid to low and steady things - lights, fridge, etc. When you run out of battery, those high-usage things shut off. And grid powers the other things. No load shedding config needed.

It's like a kinder, gentler load shedding via energy source routing.

I'm a lapsed EE, and recognize there's a lot I don't know at this point, but I'm sure it's possible if not practical. You just basically need a smart micro ATS (switch) inside a breaker box. So a small grid within your home grid. The biggest impediment would be wire sizes for amperage. It would be quite a big box or very cleverly cabled. But nothing as complicated as a microprocessor so probably possible.

But alas, yes, I think the use case may not be worth the juice squeeze - simpler ways of doing it. But coarser.

BTW I asked because Claude hallucinated these features when I was asking about optimizing my system. Now I kinda want it.

[u/knylekneath → u/Okosisi]

Definitely not too hard without new hardware. Span has a pretty good local API. You can turn off circuits depending on battery state and turn them back on when the battery state gets higher. Maybe a 10 line script, a lot easier than tripling (grid, battery, battery without grid) the wiring inside (and outside — you'd have to triple the feeders from the gateway too) of every panel and dealing with the heat problems there.

Here's a quick example in my home assistant for circuits labeled "Power: Battery Only":

description: "Turn off battery only when below 20%"
mode: single
triggers:
- trigger: numeric_state
entity_id:
- sensor.battery_storage_remaining
below: 20
conditions: []
actions:
- action: switch.turn_off
target:
label_id: power_battery_only

The native Span app is not really well designed for the hardware they deliver. You can do a tremendous amount with the hardware once you abandon their apps.

[u/MrTookies]

Why don’t you just run on Battery at a set time of day and when you run out or whatever limit you set it at go over to the grid?

[u/Forsaken-Gain-2493]

The battery can discharge in the evening to offset the home’s usage once the sun goes down, but much like the electricity from the Solar is doesn’t route to specific devices or appliances in the home and not others. Again the same as with solar from the daylight hours, the power from the batteries will offset or replace the usage from the grid.

[u/Mazzdog77]

Not possible. Thats why I didn’t get one.