In electrician land, we get this question a lot: (Most have an existing 100 amp pg&e main service entrance panel.) A lot of East Bay homeowners ask some version of the same question: How do I know how big my new PG&E panel needs to be to accommodate my new EV, my solar, my Tesla battery, my ADU? The short answer is that you do not size a panel by guessing at the number of new things you want to add. You size it by doing an actual load calculation, checking the existing service, and looking at how all those systems interact under current code and PG&E requirements.
This is where people get tripped up. They hear 200 amps is the standard answer, so they assume that is always enough. Sometimes it is. Sometimes it is not. In older homes, the bigger issue is often not just the panel rating, but the entire service – meter, mast, weather head, service conductors, grounding, main disconnect, and whether the home has enough physical space and legal capacity for the loads being added.
What determines how big your new PG&E panel needs to be?
Panel size is not just about breaker spaces. It is about service capacity.
If you are adding an EV charger, solar, a Tesla Powerwall or other battery system, and an ADU, the electrician has to look at the connected loads and the calculated demand load of the property. That includes the main house, fixed appliances, HVAC, electric range, dryer, water heater, small appliance circuits, laundry circuit, and the new loads you want to add.
An EV charger can be a major load. A common Level 2 charger on a 50-amp circuit is not treated as a casual plug. If it is continuous duty, code sizing matters. An ADU can also be a major change because now you may be adding another kitchen, another water heater, another mini-split or HVAC system, and more lighting and receptacle load. Solar affects the panel differently than load equipment does because it is a power source, not just a power consumer. A battery system adds another layer because of backup loads, interconnection method, and equipment placement.
That is why one home with an EV and solar may still work on an upgraded 200-amp service, while another property with an all-electric ADU and multiple EVs may need a different design approach.
Why 200 amps is common, but not automatic
For many single-family homes, a 200-amp PG&E service upgrade is the practical target. It gives room for a modern load profile, more breaker space, and fewer compromises when adding EV charging, solar backfeed, battery equipment, and subpanels.
But 200 amps is not a magic number. If the house is small, the ADU is limited, and load management is used for EV charging, you may not need as much service as you think. On the other hand, if the property is going all-electric, with induction cooking, heat pump HVAC, heat pump water heating, EV charging, and an ADU with its own substantial loads, the design may need careful engineering even with 200 amps.
There is also a practical field issue. Some people say they need a bigger panel when what they really need is more spaces. A 100-amp or 125-amp panel can be physically full, but the real problem may be lack of breaker positions, not total service capacity. In that case, a subpanel or complete service upgrade may be discussed depending on the load calculation and condition of the existing gear.
How an electrician actually answers this question
The right way to answer How do I know how big my new PG&E panel needs to be is to start with a demand load calculation under the National Electrical Code, then compare that with the existing service equipment and PG&E service limitations.
That process usually includes square footage, kitchen appliance count, laundry circuits, nameplate ratings for fixed equipment, heating and cooling loads, electric cooking, dryer load, water heater load, EV charger size, ADU load, and any known future plans. If solar and battery are being added, the electrician also has to review the interconnection method and whether the existing panel is suitable for line-side or load-side connections.
This matters because solar and battery do not always mean you can get away with a smaller service. People often think solar offsets electrical use, so the service can stay undersized. That is not how service calculations work. Solar production is not treated as a free pass to overload old service equipment. The panel and service still have to be rated and configured properly.
EV, solar, Tesla battery, and ADU each change the job in different ways
The EV charger is usually the cleanest part of the calculation. It is a known circuit size with known continuous load rules. A 40-amp charger, 48-amp charger, or 60-amp circuit can move the service calculation significantly.
Solar is more about busbar rules, breaker placement, interconnection limits, and the condition of the existing panel. If you have an older panel, especially a problematic brand or a service that has already been modified too many times, solar may be the thing that finally makes panel replacement the safer and cheaper long-term move.
A Tesla battery or other battery backup system introduces equipment coordination issues. You may need a backup loads panel, gateway equipment, revised service layout, and proper working clearances. The battery may not increase demand the same way a new heating load does, but it definitely affects panel selection, service layout, and permit scope.
The ADU is where the job often gets bigger fast. A legal ADU usually means additional circuits, often a feeder or subpanel, and in many cases substantial new electric load. If the ADU has its own kitchen and laundry, that changes the calculation more than people expect.
Older East Bay homes need a different level of caution
In Oakland, Berkeley, Piedmont, and nearby areas, a lot of houses still have old service equipment, undersized mains, worn meter sockets, outdated grounding, and legacy panels that should have been replaced years ago. If the house has Federal Pacific, Zinsco, a fuse panel, or questionable additions done over time, panel sizing is only one part of the job.
In those homes, the service upgrade often has to include code corrections that are not optional once permits are pulled. Grounding and bonding have to be brought up to current standards. Service entrance equipment may need replacement. Clearances may matter. Meter-main combinations may be considered. Overhead versus underground PG&E service can affect the scope and schedule.
That is one reason homeowners get different prices from different contractors. One bid may only be thinking about putting in a bigger panel. Another is looking at the actual PG&E service, inspection requirements, and what will pass without future trouble.
When load management can help
Not every property needs a massive service upgrade. Sometimes smart load management makes the numbers work.
For example, an EV charger can be set up with energy management so it reduces charging output when the house load is high. In some cases, that can avoid an otherwise expensive upgrade. Likewise, a battery system can be designed around backup priorities instead of trying to support every load in the house at once.
This is where experience matters. There is a difference between jamming new equipment onto an old service and designing a system that works in real life, passes inspection, and does not create nuisance trips or future upgrade problems.
Questions to ask before you commit to a panel size
Before anyone tells you that you need 125 amps, 200 amps, or more, ask a few direct questions. Was a formal load calculation done? Is the existing service entrance equipment being evaluated, or just the breaker panel? Will the EV charger be full-power or managed? Will the ADU be all-electric? How is the solar being interconnected? Does the battery require a backup loads panel or service reconfiguration? Are there existing panel safety issues that make replacement necessary regardless of load?
If those questions are not being answered, you are probably not getting a real panel sizing recommendation. You are getting a rough guess.
The safest practical answer for most homeowners
If you are planning a new EV, solar, Tesla battery, and ADU, there is a strong chance a 200-amp service and new PG&E panel will be the most practical starting point for review. That is especially true if the home has an older 100-amp service, limited breaker space, or outdated equipment.
Still, no honest electrician should promise that number before checking the actual loads, the existing service configuration, and the utility requirements. Some jobs need a full service upgrade. Some need a new main panel plus subpanel. Some can use load management and avoid overspending. Some older panels need replacement for safety before any new equipment is even discussed.
The right answer is not the biggest panel someone can sell you. It is the panel and service size that safely supports your house, your ADU, your EV charging, and your solar and battery equipment without creating the next problem. If you are doing this once, do it with a real calculation, a permit-ready design, and a contractor who understands both code and PG&E service work.
