The preferred PV-system size is too large for the existing electrical panel. It isn't code compliant.
Conventional solutions are to: install a smaller PV system, downsize the main breaker, do a supply-side PV system connection, or upgrade the electrical service. These all have their drawbacks.
Why isn't the PV system code compliant?...
What's the electrical panel's main-breaker rating? What's the maximum continuous incoming current of the PV system and any energy storage system(s)? Multiply the latter by a 1.25 "continuous-load multiplier". This total can't be more than 120% of the panel's busbar capacity.
Example: If the main panel has a 100-amp main breaker and a 100-amp busbar capacity... a PV-system's maximum continuous current x 1.25 can't be over 20 amps.
100-amp busbar capacity x 120% = 120-amp power-source maximum
100-amp main breaker + 20-amp PV-system maximum = 120 amps
Why 120%? This is a judgement call, not a real-world threshold. Technically, the 100-amp busbar could be overloaded in this situation... if the main breaker and PV breaker were right next to each other. But if the percentage is over 100%, the PV breaker must be placed at the opposite end of the busbar from the main breaker. ...Realistically, the busbar won't be overloaded (because the total source power won't flow across the entire busbar).
An example, with the above contraints:
The inverter(s) maximum continuous output current x 1.25 must be less than 20 amps.
If installing Enphase IQ8+ microinverters (micros) and 400-watt PV modules... the maximum allowed system size is 5.2 kW.
13 micros x 1.21 maximum current per micro x 1.25 = 19.66 amps
13 modules x 400 watts per module = 5.2 kW
For some sites and some utilities, a service upgrade (for example, from 100 amps to 200 amps) is straightforward and affordable. But in many situations, it's time-consuming and cost prohibitive.
Here's an account of the latter:
I'm in the midst of trying to get an electric car charger installed in my garage.
I live in Las Vegas, in a 25-year-old subdivision. The home's 200-amp panel doesn't have enough spare capacity to add a charger (three old less-efficient HVAC units, pool pumps, hot tub, electric oven, clothes dryer, etc.)
So to add a charger, I have to get my home's service raised to 400 amps. That wouldn't be a big deal, except... the neighborhood's transformers are already over capacity. A few of the homes already added 400-amp services so they could add Tesla chargers, and there's no capacity left in the neighborhood.
Nevada Energy is willing to add a new transformer in the neighborhood -- but they want to charge me $20,000, plus the costs of trenching the new 400-amp line from the transformer to my house, plus the new panel, plus the charger. It's looking like a $40,000 installation.
I don't know how many homes & neighborhoods are in this situation, but I was surprised that a 25-year old neighborhood is having this problem.
In late 2023, in Oakland, CA (PG&E territory).
My prospect wanted a 7.2 kW PV system, but was limited to a 5.2 kW system because of a 100-amp electrical service. I submitted a service-upgrade application. A new 200-amp panel couldn't be installed in place of the existing 100-amp panel, because the existing panel was within 3' of the natural-gas regulator vent. It couldn't be installed on the other side of two windows, because the feeder's horizontal run can't be more than 10'.
The new panel could potentially be installed around the corner, on the street-facing wall. The reply from the PG&E rep on that:
I'll need to send it to the calculations department for approval which takes 4-6 weeks.
I've had the City of Oakland refuse front of home locations.
The new location would be closer to the utility pole. But the calculations department will need to confirm that a longer service wire isn't required.
It'd take at least a month to get utility approval. We'd then need to get City approval... we'd need to make sure it meets the City's aesthetic requirements. I don't know how long the City's decision process would take.
At this point, I don't yet know if the utility would require a transformer upgrade (at my prospect's expense).
This isn't good.
Instead of upgrading the electrical service, consider this approach (continuing with the scenario above):
Install a 200-amp subpanel (with a 200-amp+ busbar capacity). Move all load circuits to the subpanel. Put the PV and battery circuit(s) in the subpanel. The main panel is now just the main disconnect (with overcurrent protection).
The main panel's service-disconnect breaker is still rated for 100 amps. The subpanel breaker in the main panel is rated for 100 amps. The main breaker in the subpanel is also rated for 100 amps (despite the subpanel's 200-amp busbar capacity).
Add safety signage to the main panel, to indicate that no breakers besides the main breaker and subpanel breaker should ever be added to it.
Now the subpanel -- not the main panel -- is subject to the 120% rule. A PV system up to 36.8 kW can be added to it (if no batteries).
200-amp busbar capacity x 120% = 240-amp power-source maximum
92 micros x 1.21 maximum current per micro x 1.25 = 139.15 amps
100-amp main-breaker rating + 139.15 amps < 240-amp maximum ✓
92 modules x 400 watts per module = 36.8 kW
With the prospect's blessing... I decided to cancel the service-upgrade application, and install a high-capacity subpanel instead. The City approved the plan set. The installation and inspection went well (although the inspector didn't do much diligence). Everything has been working as expected in the four months since.
If electrical service needs to be upgraded later to support new loads... the high-capacity subpanel will simplify that process.
Or -- better yet -- the service upgrade can be avoided forever with flexible-load control (through an added energy-management system).
In any case, the subpanel installation is good future-proofing.
At the societal level...
By not upgrading the electrical service, the site isn't increasing the total load that the utility needs to support. This means we aren't increasing our dependence on the utility, and we aren't further justifying their costly infrastructure expansions.
To learn about alternatives to the approach above, check out this article by Bill Brooks.
Thanks to Jeff Spies and Planet Plan Sets for helping me navigate this, and for preparing a related plan set.