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Read the panel. Get the answer.
Most solar designs do not stall on hard engineering. They stall on a question nobody on site could answer. These are the 11 that come up most, each one ending in something you can go and look at.
Reading the service and the panel
What to look at, and what each label actually tells you.
Identify your service style at the meter
There are four arrangements, the difference is inside the enclosure, and picking the wrong one produces a drawing that looks right and is wrong.
Kills: “The grey box next to the meter is just "the panel". One box, one name, and you can tell what you need from the outside.”
MCB vs MLO: does this panel have its own main breaker?
Plenty of panels are main-lug-only. The conductors land straight on lugs and the protection lives upstream, which means there is no main breaker to write down.
Kills: “Every panel has a main breaker. It is the big one at the top.”
Service rating, busbar rating, main breaker: three numbers, not one
They are three different facts about three different things, they are often three different numbers on the same job, and the solar math runs on the gaps between them.
Kills: “It is a 200A service, so the answer to all three questions is 200.”
Spotting a feedthrough panel
ElectricianA feedthrough panel is fed through the service equipment lugs rather than through a breaker, and which interconnection methods are legal depends on knowing the difference.
Kills: “That second panel inside is just a subpanel.”
240V or 208V: which service is this?
A condo or a light-commercial unit fed from a shared three-phase service is 120/208, nothing about the outlets looks different, and a 208V job drawn at 240 sizes the strings and the inverter output wrong.
Kills: “Residential is always 120/240.”
The 4-photo site checklist
Four photographs, taken in the right places, answer service style, all three ratings, main breaker or lugs, service voltage, and whether there is a feedthrough.
Batteries and backup scope
What was sold, and what the hardware has to do to deliver it.
Whole-home or partial backup: which one is on this job?
Backup scope is a design decision with hardware consequences, and it is a load question rather than a battery question.
Kills: “The customer bought batteries, so the house stays on. Backup is backup.”
Powerwall chassis vs expansion packs
A chassis contains an inverter and a battery. An expansion is battery only. Three chassis and one chassis with two expansions are different systems that draw differently.
Kills: “"Three Powerwalls" means three of the same thing.”
What the gateway does to your service
Where a gateway is the FIRST disconnecting means after the meter, it becomes the service disconnect and takes over the neutral-to-ground bond and the grounding electrode connection. Where a meter-main or a separate disconnect already sits ahead of it, that equipment keeps the role and the gateway takes neither.
Kills: “The gateway is a smart meter. It is a monitoring box.”
Interconnection decisions
Where the solar ties in, and what the code allows there.
Interconnection methods in plain English
Where solar ties in is a code decision the busbar math forces, and the obvious spot is often the one the rules rule out.
Kills: “Interconnection is utility paperwork. On the drawing it is just where the wire lands, and any spot is fine as long as...”
When shrinking the main breaker is the right call
A smaller main breaker buys busbar headroom for solar under the 120 percent rule. It is a routine, code-recognised move, and its real cost is that it permanently lowers what the service can draw.
Kills: “It is asking me to shrink the main breaker on somebody's existing house. That has to be a bug, and I would stop and p...”