Essential Loads Panel: What It Is and What to Put on It
Essential Loads Panel: What It Is and What to Put on It
An essential loads panel (also called a critical loads panel or backup loads panel) is a second, smaller electrical panel that sends battery power only to the circuits you pick during an outage. Your refrigerator, internet, lights and a few outlets stay on. The big power hogs stay off, so the battery lasts longer. For most San Diego homes with solar and a battery, it is the sensible way to design backup. Getting the circuit list right is the part that decides whether your outage is a minor inconvenience or a dead battery by midnight.
We design these panels as part of battery projects across San Diego County, and the questions are always the same: what goes on it, how is it wired, and what does the city require. This guide answers all three.
TL;DR
- An essential loads panel is a subpanel fed by your battery system. During an outage, it powers only the circuits moved onto it.
- Choose circuits by priority: refrigeration, internet and communications, medical equipment, lighting and a few outlets first. Heavy loads such as air conditioning and electric cooking change your battery math the most.
- Two numbers set your limits: power (kW) for what runs at once, and energy (kWh) for how long it runs.
- In San Diego the work must meet the California Electrical Code, pass inspection before wiring is concealed, and meet SDG&E interconnection rules. We handle permitting and SDG&E interconnection for you.
- Partial-home backup on an essential loads panel is the usual design. Whole-home backup is possible when battery capacity matches your loads.
Table of Contents
- What an essential loads panel does
- Which circuits belong on it
- Power and energy: the two limits that matter
- How the panel is wired
- San Diego permits, inspections and SDG&E rules
- Essential loads or whole-home backup?
- What battery backup costs
- Our approach to essential loads panels
- Planning a backup panel with us
- Our view: design the list first, then the battery
- Keeping the system ready: service and maintenance
- FAQ
- Sources
What an essential loads panel does
Your main electrical panel feeds every circuit in the house. A battery connected to that panel alone would try to feed everything, including the air conditioner, the oven and the dryer, and would run out quickly or trip on overload.
An essential loads panel solves this by splitting the house in two. The circuits you care about most are physically moved out of the main panel and into a second panel sitting next to it. That second panel is the only one the battery backs up.
In normal conditions, the backup panel gets its power from the main panel, so you notice nothing. When the grid goes down, a transfer switch disconnects your home from the grid and routes battery power, plus solar if the sun is up, only to the backup panel. The circuits left in the main panel go quiet until the grid returns.
This is also the idea behind load shedding. The U.S. Department of Energy’s Solar Energy Technologies Office factsheets note that identifying priority loads helps stretch battery duration in longer outages, and that shedding can be done manually or automatically through a critical-load panel. We cover the day-to-day side of it in our post on battery load shedding.
Which circuits belong on it
Start by listing what you could not live without for 24 to 72 hours, then rank it. A simple ranking works well: life-safety and food first, comfort second, convenience last.
Commonly chosen essential loads include the refrigerator, internet equipment, medical equipment, and selected lights and outlets. Water well pumps and heating controls belong on the list if your home depends on them.
| Priority | Typical circuits | Why it matters |
|---|---|---|
| First | Refrigerator and freezer, medical devices, router and modem, phone charging outlets | Food safety, health, staying connected |
| Second | Lighting in main living areas, garage door opener, a kitchen outlet, furnace blower or well pump if you have one | Daily function and safety |
| Third | A single mini-split or one zone of air conditioning, EV charger at reduced output | Comfort, with a clear effect on battery run time |
| Usually left off | Electric oven or range, electric dryer, pool pump, electric water heater, whole-house air conditioning | Large power draws that can drain a battery fast |
Walk through your breaker panel with a marker and label each breaker as essential, nice to have, or off. Count the number of breakers you want. A typical essential loads panel handles a limited number of circuits, so the list has to be realistic.
Three practical tips:
- Think in rooms, not appliances. If the kitchen outlet that feeds the refrigerator also feeds the microwave and toaster, the whole circuit moves over. Your electrician sorts this out when the circuits are mapped.
- Include the router and a few outlets in a living area. In an outage, people want light, a charged phone and information.
- Plan for electric vehicles separately. An EV charger can pull more power than everything else combined. If you own an EV, tell us up front so the design accounts for it.
Power and energy: the two limits that matter
Two specs decide what your backup panel can do.
- Power, in kilowatts (kW): how much your battery and inverter can deliver at one moment. This limits what can run at the same time.
- Energy, in kilowatt-hours (kWh): how much the battery stores. This limits how long things run.
Appliances with motors, such as refrigerators, well pumps and air conditioners, briefly draw several times their running power when they start. Your inverter has to cover that surge as well as everything else running. Electric cooking, water heating and air conditioning can substantially change both your power and energy needs, so check simultaneous demand and starting requirements against the inverter rating.
Here is a simple illustration with round, made-up numbers, not a quote. Suppose a battery system can deliver 5 kW at once and stores 10 kWh. Your essential loads average 1 kW over the day (refrigerator cycling, lights, internet, a few devices). That is roughly 10 hours from the battery alone, and longer if solar recharges it during the day. Add a 3 kW air conditioner running for a few hours and the same battery empties much faster. That is exactly why the air conditioner is a decision, not a default.
We go deeper on the relationship between the two numbers in matching kWh to kW for reliable whole-home backup.
How the panel is wired
The details belong to a licensed electrician working from a permitted design, but understanding the layout helps you read a proposal.
- The backup panel is installed next to the main panel. It has its own breakers for each essential circuit.
- Selected circuits are re-routed. Each essential circuit’s wire is disconnected from the main panel and landed in the backup panel. This is why circuits must be chosen before installation: moving them later means reopening the work.
- The battery system feeds the backup panel. In normal operation the backup panel is powered through the system from the main panel. In an outage the transfer switch isolates the home from the grid first, which keeps your backup power from flowing back out to utility lines where it could endanger line workers.
- Labeling and documentation. Every breaker is labeled, and the equipment is marked so that a technician, inspector or firefighter can see how the system isolates.
Some systems skip the separate physical panel and use a smart panel or a gateway with built-in circuit control that can shed loads automatically. The goal is the same: protect the battery from loads you do not need during an outage.
If your existing main panel is full or too old, an upgrade may come first. Our post on main panel upgrades for solar explains how that affects your timeline and utility reconnection.
San Diego permits, inspections and SDG&E rules
Backup wiring is not a weekend project, and in San Diego it is regulated at three levels.
Electrical code and inspection. The City of San Diego’s residential solar inspection guidelines state that the installation must comply with the California Electrical Code and that required inspections must be obtained before work is concealed. That matters for a backup panel, because the re-routed wiring has to be visible when the inspector arrives.
Service upgrades. Per the same guidelines, when a project includes an electrical service upgrade, an SDG&E service order and a completed City of San Diego Circuit Card must be on site. If a separate utility disconnect is required, an SDG&E letter of approval must also be available.
Utility interconnection. Battery systems must meet SDG&E’s interconnection requirements under Rule 21. Non-exporting systems can qualify for an expedited process if they meet specific criteria, including being inverter-based and using UL 1741 listed equipment.
Faster city permitting. In 2023 the City of San Diego launched an online instant permit program for residential solar and battery storage projects up to 38.4 kW on single-family homes and duplexes. Eligibility depends on the project, so the paperwork route is confirmed during design.
These steps are the main reason homeowners hire a contractor for backup panels. We handle all permitting, SDG&E interconnection and HOA approvals, so you are not chasing three offices. If your home is outside the City of San Diego, the jurisdiction and its requirements may differ, and we work across the county from Oceanside to Chula Vista.
Essential loads or whole-home backup?
Partial-home backup uses a backup load panel for selected circuits. Whole-home backup powers everything, typically through a smart energy management panel that manages loads, and it needs enough capacity to start and run all the appliances you want, for as long as you want.
The tradeoff is straightforward:
| Essential loads panel | Whole-home backup | |
|---|---|---|
| What runs in an outage | Circuits you choose | Whole house, often with load management |
| Battery size needed | Smaller | Larger, matched to your biggest loads |
| Best for | Refrigeration, communications, lights, medical devices | Homes with electric cooking, well pumps, or air conditioning you want to keep running |
| Decision to make | Which circuits to include | How much capacity and how many batteries |
Clean Energy Group’s energy storage guide points out that it is typically not cost-effective to size storage for every load a building has, which is why a critical-load subpanel is the common approach. For many homes we start with an essential loads panel and design the battery around it. If your loads call for whole-home backup, we size the system for that instead.
What battery backup costs
Our published price for battery storage is $12,000 to $16,000. The final number depends on the battery brand and quantity (we install Tesla Powerwall, Enphase IQ and Franklin WH), your panel’s condition and how many circuits go on the backup panel. For current Powerwall pricing, see our post on Tesla Powerwall costs in San Diego.
Our approach to essential loads panels
We do the design, engineering, installation and service with our own crews, and every system is custom designed by our in-house engineering team around your roof, your energy usage and your goals. For a backup panel, that means we walk your breaker panel with you, agree on the circuit list, size the battery to it, and put the timeline in writing before you commit.
If you are comparing quotes you already have, put your essential circuit list next to each one. It shows quickly whether a proposal was designed around your home or around a standard package. You can see our storage options on our battery storage page, and the consultation is free.
Planning a backup panel with us
We design backup systems for homeowners who want outage protection and want their battery to work with solar under NEM 3.0, where self-consumption matters. Our Battery Storage service is priced at $12,000 to $16,000, and we take the permitting and SDG&E interconnection off your hands. If you are planning a new roof, we can handle solar and roofing with one crew under one warranty.
Bring your latest SDG&E bill and a photo of your main panel to a free consultation, and we will map out which circuits belong on the backup panel and what battery size fits them. Start at sandiegosolar.com.
Our view: design the list first, then the battery
Most backup disappointments come from the same mistake: someone buys a battery first and decides what it should run second. We do it the other way around.
The circuit list is the real design document. Once you know that the refrigerator, the internet, a medical device and a handful of lights are non-negotiable, the battery size follows from arithmetic instead of guesswork. Everything you can leave off the panel is battery capacity you do not have to buy.
We also think the people who do the design should do the work. In 30 years we have never used a subcontractor, so the person who mapped your circuits is on the team that installs and services them. If something does not hold power the way you expected during an outage, there is one company to call, and we have been in San Diego since 1996.
Pair that with NEM 3.0, where using your own solar and stored energy is what saves you money, and the essential loads panel does double duty: it protects you in an outage and keeps your battery working in a sensible daily pattern.
Curtis Williamson
Keeping the system ready: service and maintenance
A backup panel only helps if the system behind it works on the day the grid fails. Preventive maintenance for solar and storage systems, according to the National Laboratory of the Rockies’ operation and maintenance best practices, includes keeping wires secure, monitoring system health, confirming charge control settings suit the battery type, and checking cooling so enclosures stay within temperature limits.
The same publication lists common reasons for service on solar-plus-storage systems: declining capacity, battery replacement planning, and unexpected outages where the system fails to keep power on designated essential loads. If your backup circuits dropped out in the last outage, that is a reason to call.
We offer a diagnostic visit at $150 to $250, and the solar and inverter repair services on our repair page cover simple repairs at $200 to $500 and inverter replacements at $1,500 to $3,500. Panel replacements run $300 to $800 per panel.
FAQ
Is an essential loads panel the same as a critical loads panel?
Yes. Essential loads panel, critical loads panel and backup loads panel are different names for the same thing: a subpanel that holds the circuits your battery powers during an outage. The names vary by manufacturer and installer, but the function does not.
Can I add circuits to the panel later?
Often you can, if the panel has open slots and your battery has the power and energy to support them. Each added circuit raises the load, so the system should be re-checked first. It is easier to plan the full list during the original design.
Do I need a permit to wire a backup loads panel in San Diego?
Yes. The work must comply with the California Electrical Code and pass inspection before wiring is concealed, and battery systems must meet SDG&E interconnection rules. We handle the permitting and SDG&E interconnection for our projects.
Should I put my air conditioner on the essential loads panel?
It depends on the size of the unit and your battery. Air conditioning can substantially change the power and energy your system needs, and starting surge adds to the demand. A single mini-split is much easier to support than whole-house cooling. We size it against your battery during design.
How long will my battery power the essential loads?
Divide your battery’s usable energy (kWh) by the average power your essential loads draw (kW). Solar recharging during the day extends that. Your actual figure depends on which circuits you choose, which is why we start with the circuit list.
Sources
- DOE Solar Energy Technologies Office factsheets on energy best practices
- City of San Diego residential solar photovoltaic systems inspection guidelines
- SDG&E Rule 21 interconnection requirements
- City of San Diego instant permit program for residential solar and battery storage, 2023
- Clean Energy Group, Energy Storage 101
- Best practices for operation and maintenance of photovoltaic and energy storage systems