Porter Ranch, CA · January – August 2023 · LADWP / LADBS

Porter Ranch: two models, two different answers.

The same roof, the same panel count, a week apart — and the projected output moved by more than a thousand kilowatt-hours.

Written by the Cali Energy project team · California contractor license 1032379 · Updated September 2026

Production estimates are model output, not measurements, and two models given the same roof can disagree by a wide margin. On this Porter Ranch project an early figure for twenty-seven modules came back near 18,250 kWh a year, a public modeling tool gave a similar number, and a re-run a week later produced about 17,000 for the same array — so the azimuth, the tilt and the shading assumptions were checked before anything was ordered. Cali Energy settled the design at twenty-four REC 410 W modules with a SolarEdge inverter and a derate of the main breaker, and the system was signed off in June.

At a glance

System

24 × REC 410 W, about 9.8 kW

Inverter

SolarEdge 10 kW

Main breaker

Derated to 175 A

Design revisions

Four, before anything was ordered

Extra approval

Homeowner association

Utility / AHJ

LADWP / LADBS

The story

This project spent four months in design and eight days in installation, and the four months are the useful part.

The first pass was twenty-seven modules at four hundred watts with a microinverter behind each, modeled at roughly eighteen thousand two hundred and fifty kilowatt-hours a year. The household compared it against a public modeling tool and got a number within a hundred kilowatt-hours. So far, so ordinary.

A week later the same array was re-run and came back at about seventeen thousand.

That is a gap of more than a thousand kilowatt-hours a year on an array nobody had touched, and it is worth explaining what it means, because owners see two numbers and reasonably conclude that one of them is dishonest.

Neither is a measurement. A production estimate is a model fed four things: where the roof faces, how steeply it slopes, what shades it and at what hours, and what the weather does at that location across a year. Change the azimuth by ten degrees in the input and the answer moves. Change the assumed shading from ten percent to fifteen and it moves again. The models are not lying to each other; they are answering slightly different questions.

So the question we asked was not which number is right. It was which input changed — and the answer was in the azimuth and tilt used for the run.

What an owner should take from that: when a quote shows an annual figure, ask what shading percentage and which roof planes it assumes. Two quotes that differ by fifteen percent usually differ in assumptions, not in equipment, and the assumptions are the part you can check.

The design moved three more times after that. Twenty-seven modules became twenty-two when the household looked at its actual consumption, which was closer to twelve thousand seven hundred kilowatt-hours than to eighteen thousand. Then the module changed: REC 410 W panels with a single string inverter, which fit the roof planes better than twenty-seven of anything.

The final set was twenty-four modules, about nine point eight kilowatts, with the main breaker derated to 175 A so the backfeed fit the panel it was joining.

Two other things belong on the record. The association reviewed and approved the layout before the permit went in, which on this tract is a separate track with its own calendar. And a section of roof tile was damaged enough to need replacing, so that went into the contract as a written change rather than a verbal promise — the difference matters when somebody asks, two years later, who agreed to what.

The utility work request opened in late May. The installation was done inside a week, the final inspection passed on the seventh of June, and permission to operate came through on the twentieth.

In August the household reported a leak and we were on the roof the same day. That is the other half of the job that quotes rarely mention: an array lives on a roof for twenty-five years, the weather keeps working on everything around it, and whoever put the system up should be the one who comes back and looks when water shows up anywhere on that roof.

The array has run on permission to operate since the twentieth of June, and the household still has the company that built it on the other end of the phone — which is the half of a twenty-five-year purchase that only shows itself years after the signing.

Cali Energy designed, permitted, installed and serviced this array in Porter Ranch.

How it ran

Four months of design, eight days of installation. The four months are where the system was actually decided.

What we did

• Chased down why two production runs disagreed instead of picking the better number.
• Sized the array against the household's actual consumption, not the first layout.
• Put the tile replacement in the contract as a written change.
• Took the association approval before the permit rather than alongside the install.
• Came back for the leak report in August.

System specifications

Is your project like this one?

• You have two quotes with production numbers that do not agree.
• Your roof faces more than one direction and nobody has explained the trade-off.
• Your property is inside a homeowner association.
• Something was promised verbally and is not written in your contract.

Cali Energy in Porter Ranch

Frequently asked questions

Why do two solar quotes show different annual production for the same roof?

Because both are model outputs, not measurements. The model is fed the azimuth, the tilt, an assumed shading percentage and a weather file. Change any of those inputs and the answer moves. A fifteen percent difference between quotes usually comes from different assumptions rather than different hardware.

What should I ask about a production estimate?

Which roof planes it uses, what azimuth and tilt it assumes for each, and what shading percentage is applied. Those three answers explain almost every gap between two estimates, and they are things you can check against your own roof.

Should the system be sized to the roof or to the bill?

To the bill, with the roof as the constraint. On this project the first layout would have produced far more than the household used; sizing came down once the actual consumption was read, which is the order it should happen in.

What is a main breaker derate and why would my project need one?

It is a reduction of the main breaker rating so the solar backfeed and the main together stay inside what the panel's busbar is rated to carry. It is a normal part of connecting an array to an existing panel, and it belongs on the drawings rather than being discovered at inspection.

Does Cali Energy install solar in Porter Ranch?

Yes. Porter Ranch is inside the City of Los Angeles, so permits run through LADBS and interconnection through LADWP. Most tracts up here also have an association review, which we treat as its own track from the first week.

What this would look like on your roof

Related

• They wanted the same system as the neighbor Porter Ranch · 17 modules · the bill said something different Solar
• The array did not fit the panel, so the main breaker got smaller Granada Hills, CA · 31 modules · de-rated to 175 A instead of a new panel Solar
• The array went on the west roof, not the east Santa Clarita · 8.14 kW · the side changes the output, not the count Solar
• The flashing around the chimney came first Porter Ranch · roof repair before the array, and a lender waiting on a letter Solar

This timeline reflects this property and this permitting path. Updated September 2026.