Solar battery backup on ADU while connected to main house

[Justin F.]

Hello!

Hoping to get some advice on powering a detached 860 sq ft 2bd/2ba ADU (accessory dwelling unit) in California.

My primary house has a 100A electrical panel supplied by underground powerline. When I installed solar on the primary house (with no battery backup), I had to replace my existing electrical panels (main and sub-panel) because they were outdated. I wanted to upgrade the electrical panel to 200A but PG&E said I had to trench to upgrade the power lines to handle the additional power. They also said I had to move the existing electrical panel 36 inches away from the gas meter if I wanted to upgrade to 200A or higher. Some contractors have mentioned costs of ~$20K to trench. I was also told it'd be pretty expensive to relocate the main electrical panel because they would have to run new lines.

That being said, I called Alameda county and asked if it was a requirement for me to upgrade my 100A electrical panel if I were to build a detached ADU. They said there wasn't a requirement but the only requirement was the ADU had to be connected to the main house. I believe the other option if for the ADU to have it's own power but that would actually involve more trenching since the ADU is in the backyard so that's also cost prohibitive.

I doubt a 100A panel will be sufficient to power the main house and an ADU. ADU design/build firms and ADU contractors all mention I need to upgrade the main electrical panel to 200A to power the ADU.

My question is...rather than spending $20K on upgrading the main electrical panel to 200A, can I spend that money to install solar panels on the ADU (which I'm required to for detached/non-manufactured home) with battery backup? Essentially making the ADU self-sufficient but still connected to the main house. The ADU would run off the solar batteries and if necessary pull electricity from the main house.

Is this a viable solution?  If so, I'm surprised none of the contractors have mentioned this.

Thanks in advance!

[Tanya F.]

What's the estimated electrical usage for the ADU?

Lighting, probably not much, but heating/cooling and hot water? Insulate it really well!

What's the roof area? South facing? Not shaded by trees? i.e. how many watts of panels will fit on it?

[Justin F.]

@Tanya, not sure how much electricity the ADU will be using. The main house is about 1,720 sq ft. Last 3 months I averaged about 405 kwh/mth. For the most part, the solar on the main house covers that usage (12 LG350 panels). The difference is main house has gas range, furnace, and water heater and ADU will be all electrical.

ADU will most likely have 3 mini-split systems for cooling/heating and on-demand hot water heater. Not sure how much power those appliances use. The ADU is half the size of the main house but since it will be all electrical, I'm guessing usage would hover around 300 kwh/mth? Current panels on the main house face SW. ADU solar panels would also face SW. I should be able to fit 12 panels on the ADU's roof. Was looking at the Tesla 4.08 kW solar panels. The solar panels will not be shaded by any trees.

If the main house only has a 100A electrical panel and I add an ADU, is the concern with tripping the circuit breakers if too many appliances are used at once? If I have solar battery backup, am I able to tell the house to use the batteries first before drawing from the grid? I know battery backup typically kicks in if there's no power from the grid but I wonder if it can work the other way around. I'd be willing to add battery backup to the main house as well if it will help with the situation.

I'm trying to avoid having to spend a bunch of money digging up dirt if there are better ways to use that money (i.e. solar).  Another advantage is the 26% tax credit for installing solar and solar battery backups which helps reduce my costs whereas $20k trenching to replace the powerlines is a sunk cost.

[Eric Spond]

Check out Enphase solar battery as an option. Great products.

[Tanya F.]

Seems like a viable plan, but take all this info to a solar company and HVAC professionals, and see if their estimated demand and solar production calculations are in agreement.

Really do insulate well, though. You don't need to go to passivehouse level, but go for more than what code requires. It will help make your plan viable from an HVAC perspective.