Powering an ADU and an EV Charger on One Service

Austin has an unusual density of backyard units — casitas, garage apartments, granny flats, whatever the household calls them. It also has a lot of electric cars. Increasingly the same property wants both, and that combination produces a specific problem that neither on its own does.

The mistake is treating them as two projects

Here is the sequence that costs money, and it is common.

The ADU gets built. The electrician working on it sizes the feed for the unit, checks the service, and confirms it works. Everything is correct.

Eighteen months later the household buys an EV. A different electrician runs a load calculation and finds the service is now short — because the first calculation was correct for a house plus an ADU, and this is a house plus an ADU plus a car.

Now you are paying for a service upgrade that could have been anticipated, and possibly for panel work that partly duplicates what was done for the ADU.

The fix is not complicated: calculate for the end state at the start, even if the purchases are years apart.

What a combined calculation looks at

An accessory dwelling unit is a serious load. It has its own cooling system, its own water heater, quite possibly its own range and dryer. Depending on how it is built and metered, some of that may be diversified against the main house and some may not.

A charging circuit is also a serious load, and unusually it is a continuous one — near-maximum draw for hours, which is a different profile from anything else on the property.

Together they can comfortably exceed what a 100 or 125-amp service was ever intended to carry. On the central Austin lots where ADUs are most common — Hyde Park, Bouldin, Travis Heights, east Austin — that is precisely the service size you are most likely to find.

That is why this comes up here more than in the newer suburbs, where 200-amp services are standard and the headroom exists.

Metering, which is a decision not a default

Whether the ADU gets its own meter or a subpanel from the main service is worth deciding deliberately.

Separate metering makes sense when the unit is genuinely let and the tenant should be billed for their own consumption. It involves your utility, and the requirements differ across the six providers serving this area.

A subpanel from the main service is simpler and cheaper, and it suits family accommodation, a home office, a studio, or a unit that is let with utilities included.

The choice affects the charger question, because a separately metered ADU is a separate service with its own capacity, while a subpanel shares the main one.

The charger often belongs on the ADU

This is the practical insight that saves the most money, and it gets missed.

Backyard units usually sit nearer the alley or the rear of the lot — which is very often where the car actually parks, particularly on the alley-access lots where this configuration is most common.

If the ADU is getting a subpanel anyway, feeding the charging circuit from that subpanel is frequently much shorter and simpler than running a separate long circuit from the main panel at the house. One trench, one distribution point, two purposes.

Deciding that during the ADU design costs nothing. Discovering it afterwards means a second trench across the same garden.

One trench, and the tree in it

Which brings up the constraint these lots share.

The trench feeding an ADU has to reach the back of the lot. On a century-old central Austin lot, the back of the lot is where the mature trees are.

A protected heritage tree is one with a trunk of 19 inches or more, and its critical root zone extends outward a foot for every inch of that diameter — a two-foot trunk protects a circle roughly 24 feet across. Trenching that significantly disturbs that ground needs review even where the tree is untouched.

So the route needs walking, and it needs walking once, for everything that is going in the ground. Opening that trench twice — once for the ADU and again for the charger — means dealing with the same protected zone twice for no reason.

The order to do this in

If you are building an ADU and expect an EV within a few years, tell whoever is doing the electrical design. It costs nothing to include in the calculation and it can save an entire service upgrade.

If the ADU already exists and the car is arriving now, the load calculation still comes first, and the answer may be a load-management device rather than a new service. Where the gap is modest, managing when the charger draws is far cheaper than growing the supply.

And if Austin Energy serves you, the rebate applies to the charging installation regardless of which panel it comes off — a licensed electrician and a permitted, inspected job.