Inside a Concord Solar and Battery Project: Design Choices, Backup Power, and Monitoring

Gelfer Electrical Services, CSLB #1159194 — Project writeup, Concord, CA

Homeowner Goals

This homeowner came in with a clear set of priorities. They wanted to reduce dependence on PG&E, have real backup power during outages — not just a critical-load subpanel keeping the refrigerator on, but whole-home backup — and make better use of solar production throughout the day rather than exporting cheap midday power and buying it back at peak rates. They also wanted to be able to export surplus solar once PG&E interconnection was approved, and to have visibility into how the system was actually performing at the individual panel level, not just a total production number.

That combination — meaningful storage, whole-home backup capability, and module-level monitoring — shapes every decision in this project.

System Size and Equipment

Here are the specs. I'll be direct about what went in and why each piece was chosen.

Solar array: 6.72 kW DC

Sixteen REC 420 W panels. REC makes reliable modules with good efficiency in a standard residential footprint. Sixteen panels at 420 W lands just under 6.75 kW, which is a practical size for a home with meaningful daily consumption and a 14.3 kWh battery to keep charged.

Inverter: EG4 18KPV hybrid, 12 kW AC output

The 18KPV is a serious piece of equipment for a residential job. It handles grid-tied operation, battery charging, and backup simultaneously — that's what makes it a hybrid inverter rather than a standard grid-tied unit. The 12 kW AC output capacity means it can run a real house during an outage. Not just the lights and the refrigerator, but the loads a household actually depends on, up to the inverter's output limit and subject to available battery charge.

The hybrid architecture also means the inverter manages the relationship between solar production, battery state of charge, grid import, and export natively. There's no add-on equipment needed to coordinate those functions.

Battery: EG4 Outdoor PowerPro, 14.3 kWh lithium iron phosphate

LFP chemistry is the right call for a storage installation that cycles daily. Better cycle life than NMC, more thermal stability, and the outdoor-rated enclosure means it can be sited where it needs to go without requiring a dedicated indoor space or a temperature-controlled environment.

14.3 kWh is a meaningful amount of storage for a home in this size range. It covers evening consumption on a typical day and provides a real reserve for outage use.

Module electronics: Tigo TS4-A-O optimizers with CCA/TAP monitoring

Each panel has a Tigo optimizer. This gives the system three things: rapid shutdown compliance (required by California electrical code), panel-level power optimization, and module-level monitoring. The monitoring piece is worth explaining separately — see below.

Why Module-Level Monitoring Matters

With only inverter-level data, you know how much power the system is producing in total. You don't know which panel is producing less than it should, or why. A partially shaded panel, a soiling issue, a connection problem — all of those can drag down overall production without triggering any alarm at the inverter level.

Tigo's CCA/TAP system reports individual panel production. If one panel is underperforming relative to the others, the homeowner can see it on the monitoring dashboard. That makes it possible to catch problems early rather than noticing a year or two later that production has been lower than expected.

Combined with EG4's system-level monitoring — which covers battery state of charge, grid import and export, and inverter operating status — the homeowner has a complete picture of how the system is running on any given day.

PG&E Interconnection and Export

The interconnection process with PG&E takes time. During installation and commissioning, the inverter is temporarily configured for zero-export: solar serves the home and charges the battery, but nothing goes to the grid. That's the required commissioning configuration while interconnection approval is pending.

Once PG&E approved the interconnection and sell-back, we reconfigured the inverter to allow export. Normal operation now works as expected: solar serves the home first, charges the battery according to the programmed settings, and exports remaining surplus energy to PG&E. The homeowner gets the bill credit for what they export.

This is worth spelling out because "zero-export" gets used loosely in the industry. Some systems are configured for permanent zero-export — usually because of HOA requirements, utility rules that don't allow export, or an owner's preference to keep all production on-site. This system is not zero-export. It was zero-export during commissioning, which is a temporary state. It now operates with full export capability.

Roof and Installation Details

Mounting: RT-APEX rail-less system

Standard solar installations use aluminum rail running across the roof, with clamps holding the panels to the rail. Rail-less systems like the RT-APEX attach directly to the roof structure with individual standoffs for each panel. The result is a lower profile with less visible hardware. On a home where aesthetics matter, it's a cleaner look.

Critter guard

A perimeter guard was installed around the array. This isn't cosmetic — it keeps birds, squirrels, and debris from getting under the panels. Without it, the gap between the panel and the roof becomes nesting space. Nests can damage wiring and restrict airflow that helps keep panel temperatures down. It's a detail that adds maybe an hour to the installation and saves a service call later.

Attic conduit routing in EMT

Where practical, conduit was routed through the attic in EMT rather than run on the exterior of the house. This reduces the amount of visible conduit on the outside of the building — a detail that matters on finished residential construction where exterior conduit runs along a painted wall or trim area.

EZ Solar rooftop junction boxes

These provide clean, protected penetrations at the roof-to-building transition. Where rooftop wiring enters the building, the junction boxes handle the transition properly — weatherproof, accessible for service, and cleaner than field-fabricated solutions.

What the System Delivers

Pull it together and the installed system gives this homeowner:

• 6.72 kW of solar production capacity

• 14.3 kWh of battery storage, outdoor-rated, cycling daily

• Whole-home backup through the EG4 18KPV's 12 kW AC output — subject to available battery charge and the inverter's rated capacity

• Module-level monitoring through Tigo, so individual panel performance is visible

• PG&E interconnection with export approved — surplus solar goes to the grid and generates bill credits

• Rail-less mounting, attic conduit routing, rooftop junction boxes, and critter guard — durable and unobtrusive from the street

Questions Homeowners Should Ask Before Choosing a Solar and Storage System

If you're evaluating a solar and battery system with backup power, here are the questions worth getting clear answers to before you sign anything.

What loads does backup actually cover? A lot of storage systems are sold on headline kWh numbers without a clear answer to this. Know exactly which loads the inverter will run during an outage and for how long, based on your actual consumption.

Is the inverter a true hybrid or an AC-coupled add-on? True hybrid inverters manage solar, battery, and grid in one unit. AC-coupled systems stack a battery inverter onto an existing grid-tied inverter. Both work, but they have different backup behavior, efficiency characteristics, and commissioning requirements.

How does the system handle export, and what were the commissioning steps? Interconnection with your utility takes time. During that period, most installers configure the system for zero-export as a temporary state. Once approval comes through, the inverter is reconfigured to allow export. Make sure you understand what state your system is in and what the final operating configuration is.

What does monitoring actually show you? Day-to-day monitoring is how you catch underperformance before it becomes a long-term production loss. Ask what the monitoring platform shows — battery state of charge, solar production per panel, grid import and export — and how often the data updates.

What are the commissioning steps, and has your contractor done this before? A hybrid system with battery backup takes more time to commission than a grid-tied solar-only install. The inverter needs to be verified for correct backup behavior, battery communication, export settings, and charge programming. Ask your contractor to walk you through what they check.

What are the warranty terms, and who handles a claim? Equipment warranty and labor warranty are separate. Know the manufacturer's terms for the inverter and battery, and confirm whether the contractor coordinates warranty service or you're on your own with the manufacturer.

Request a Solar or Battery Assessment

If you're in the Bay Area and evaluating a solar and storage system, I'm available for a paid Solar & Battery System Inspection ($300, 90 minutes) or can review a project proposal before you sign. Submit project details — address, what you're looking to do, and any equipment or utility information you have — at gelferelectrical.com.

Licensed California C-10 electrical contractor, CSLB #1159194. Serving the Bay Area, with selected Mendocino County solar and off-grid projects available by arrangement.