Critical Loads Subpanel Design Guide

Here is the uncomfortable truth about battery backup: the battery is rarely the reason an outage goes badly — the circuit list is. Two homes can buy the identical 13.5 kWh battery, and one rides out 20 hours of outage while the other dies in six, purely because of what their electricians put on the backed-up subpanel. The critical loads subpanel for battery backup is not paperwork. It is the sizing decision wearing a different hat.

This guide walks circuit by circuit through what belongs on the backed-up panel and what stays on grid-only power, with realistic wattage budgets, the surge and heating loads that ambush good plans, and the math that turns a circuit list into a runtime number. By the end you will be able to hand an electrician a prioritized list and know exactly what your battery buys you. Costs are 2026 US market ranges; get itemized local quotes.

Critical loads subpanel for battery backup: why it decides everything

Most residential battery systems are installed as partial-home backup: the battery feeds a dedicated subpanel — the critical-loads panel — while the rest of the house stays on grid-only circuits. When the grid fails, the transfer equipment islands the subpanel and everything on it keeps running. When the grid returns, everything reconnects. The elegant part is selectivity: by choosing circuits instead of backing up the whole service, a mid-size battery delivers a day of meaningful coverage instead of four hours of whole-house normalcy.

The alternative, whole-home backup, routes the entire service through the battery system. It is simpler to explain and far more expensive to execute: it demands enough continuous power output to start every motor in the house at once and enough energy to feed every habit. More than nine in ten battery-with-solar homeowners are better served by partial backup — the industry's open secret is that whole-home battery backup usually means a few hours of power followed by a long wait for sunrise, unless the battery bank is enormous.

So the subpanel is where the project is won or lost. Choose too many circuits and the battery drains by dinner. Choose too few and the family camps in one room during a three-day outage, resenting the five figures in the garage. The goal is a panel that covers genuine needs — food safety, heat or cooling within reason, light, communication, medical equipment — at a consumption rate the battery can sustain.

The three-tier rule for battery backup circuits

Before touching individual breakers, sort every circuit into three tiers. Tier 1 is survival: refrigerator and freezer, well pump or sump pump if the house depends on them, the heating system's blower and controls, medical equipment such as CPAP machines or powered medical devices, and a lighting circuit or two. These ride the entire outage, no compromises. Tier 2 is comfort: Wi-Fi and networking gear, phone chargers, a few outlets, the kitchen's small appliances, the garage door opener, and in hot climates a bedroom mini-split or window unit for sleep. These stay on, but you would shed them first if the battery ran low. Tier 3 is convenience — and it stays off the backed-up panel entirely: electric dryers, ovens and ranges, EV chargers, pool equipment, central air conditioning (with exceptions), electric water heaters, and baseboard or resistance heat.

The tier boundaries are personal, and that is the point. A home on a well has no choice about the well pump — no power, no water — while a municipal-water home barely thinks about it. A household with a sump pump in a high-water-table basement treats that circuit as life-safety; a slab-on-grade home in Arizona does not have one at all. Medical equipment always lands in Tier 1 and gets its own dedicated circuit where possible, sized so nothing else can trip it. Walk your panel with this framework and the decisions get much easier: every circuit has to earn its tier, and Tier 3 is not a failure — it is the discipline that makes the battery work.

Circuit-by-circuit wattage budgets

Now the numbers. The table below gives realistic running wattages for common residential circuits — what the circuit draws in steady operation. Starting surges are separate and often two to five times the running figure for motors; the next section covers those. Use this table as a planning budget, then have your licensed electrician verify against your actual equipment nameplates, because a 20-year-old well pump and a new one can differ dramatically.

Circuit	Typical running watts	Typical tier
Refrigerator	100–200 (cycling)	1 — back up
Chest freezer	100–150 (cycling)	1 — back up
Gas furnace blower + controls	300–800	1 — back up
Well pump (1 HP)	1,000–1,500	1 — back up if on a well
Sump pump	500–1,000	1 — back up if flood-prone
Medical equipment (CPAP, concentrator)	30–500	1 — dedicated circuit
LED lighting, whole house	100–300	1–2 — back up
Wi-Fi router + networking	20–60	2 — back up
Microwave	1,000–1,500	2 — back up; brief, heavy
Garage door opener	500–700 (cycling)	2 — back up
Mini-split, one head	800–1,500	2 — back up for sleep comfort
Electric range / oven	2,000–5,000	3 — leave off
Electric dryer	3,000–5,000	3 — leave off
EV charger (Level 2)	7,000–11,000	3 — leave off
Central AC (3–4 ton)	3,000–5,000	3 — usually leave off
Pool pump (single-speed)	1,500–2,500	3 — leave off
Electric water heater	4,000–5,500	3 — leave off

Read the table as a story: the entire Tier 1 survival set — fridge, freezer, furnace blower, lights, medical equipment — typically draws 600 to 1,500 watts combined at any moment. That is the beautiful fact at the heart of battery backup: keeping a family safe and comfortable through an outage costs about the same as running a hair dryer. The Tier 3 loads are what bankrupt the plan. A single electric water heater draws more than the entire survival set combined, every minute it runs.

The ambush loads: surge, resistance heat, and simultaneity

Running watts lie by omission. Motors draw a starting surge — locked-rotor current — that can be three to seven times their running draw for a few seconds. A well pump running at 1,200 watts may demand 4,000+ watts to start; a 3-ton AC compressor can surge past 10,000. The battery's inverter must absorb the largest surge it will ever see on top of everything else running at that moment, or it shuts down to protect itself — even with energy still in the tank. This is why installers care as much about the battery's continuous and peak kW ratings as its kWh capacity, and why soft-start devices on big motors are one of the best few-hundred-dollar upgrades in backup design.

Resistance heat is the other ambush. Electric furnaces, baseboard heaters, heat strips in air handlers, and the backup elements in heat-pump water heaters all convert electricity to heat at a ruinous exchange rate: a 10 kW furnace strips a 13.5 kWh battery bare in about 80 minutes. If your home heats electrically, the honest options are keeping heat off the backed-up panel (with a plan for staying warm — a gas fireplace, a wood stove, or simply a cold-tolerance strategy), or switching the heating source itself before buying the battery. Do not discover this on outage night.

Simultaneity is the quiet third. The microwave (1,200 W), the well pump starting (4,000 W surge), and the mini-split compressor kicking on (2,500 W surge) can all coincide at 6 p.m. — and that coincidence, not the average, is what the inverter must survive. Good installers add a 20–25 percent headroom margin above the calculated simultaneous peak. Ask your installer what peak they designed for and how they got there; the answer should involve your actual loads, not a rule of thumb.

Smart panel or simple subpanel?

There are two hardware philosophies for implementing the tier system. The traditional approach is a straightforward critical-loads subpanel: the electrician physically moves the chosen circuits' breakers into it, and the battery backs up everything in that panel. It is simple, legible, and cheap — a few hundred dollars of panel hardware plus labor. Its limitation is rigidity: what is in the panel is backed up, what is not is not, and changing your mind means another electrician visit.

The newer approach is a smart electrical panel — products in this class replace or sit alongside the main panel and offer per-circuit monitoring and software-controlled shedding. With one of these, the tier system becomes dynamic: the panel can drop Tier 2 circuits automatically when the battery falls below a setpoint, keep Tier 1 alive to the last, and even let you change tiers from an app. For homes with big discretionary loads or evolving needs, that flexibility is genuinely valuable. The trade-off is cost — smart panels run several thousand dollars installed — and complexity. A traditional subpanel chosen well, by contrast, never needs a firmware update.

Our guidance: if your tiers are stable and your discretionary loads are modest, the simple subpanel is the better value. If you have heavy loads you want available-but-sheddable (a workshop, a hot tub, a second HVAC zone), or you expect the household's needs to change, the smart panel earns its premium. Either way, the circuit-selection discipline in the previous sections is what makes it work — software cannot fix a panel full of Tier 3 loads.

Turning the circuit list into a runtime number

Here is the payoff calculation. Add up the average hourly consumption of your Tier 1 + Tier 2 circuits, multiply by the hours you need, and compare against the battery's usable capacity (derated a bit for inverter losses and the reserve you keep for a second outage day). A worked example: Tier 1 survival set averaging 800 watts, plus evening Tier 2 comfort adding 400 watts for six hours — roughly 12–14 kWh over a 16-hour overnight stretch. A 13.5 kWh battery covers that with margin. The same battery feeding a whole-house panel with the dryer, oven, and AC available dies before midnight.

Run this math twice: once for the overnight stretch and once for a full 24 hours with modest daytime discipline. If the battery covers the overnight with margin, you have a sound design — daytime solar (if installed) extends it further. If it does not, the fix is almost never a bigger battery first; it is a stricter circuit list. Every 1,000 watts you remove from the backed-up panel buys roughly an extra hour on a 13.5 kWh-class system. That is the exchange rate to memorize.

A note for households backing up medical equipment: treat those circuits as untouchable Tier 1, on dedicated breakers, and size the battery so the medical loads are covered with wide margin even if everything else must be shed. Some owners report exploring whether backup-power equipment for medical needs has any tax treatment — the IRS's medical-expense rules are the relevant reference — but treat that strictly as a question for a tax professional; never let a salesperson promise a deduction. This is not tax advice.

The mistakes electricians see every month

Ask any installer what goes wrong and the same stories repeat. The most common: the homeowner insists the electric range stays on the backed-up panel “just in case,” and the first time someone preheats the oven during an outage, the inverter trips and takes the refrigerator down with it. Next: the garage subpanel got backed up wholesale, including the EV charger, which then helpfully charges the car off the battery at 7 kW all night. Then the classic — the well pump was included for water, but nobody checked its starting surge against the inverter's peak rating, so every pump cycle is a coin flip.

The subtler mistake is panel bloat over time. The critical-loads subpanel is installed beautifully in 2026; by 2029 someone has added the new hot tub circuit, the workshop heater, and the holiday-lighting circuit to it because it had spare breaker slots. Label the subpanel clearly — “BACKED UP — do not add heavy loads without review” — and tell every future electrician what it is. A laminated circuit directory inside the panel door costs nothing and prevents the most common form of backup decay.

“The battery doesn't care how important a circuit feels. It only cares how many watts it draws. Size the panel to the watts, and the feelings take care of themselves.”

Your next steps: from circuit list to quote

Do the homework before the electrician arrives: walk your main panel, list every circuit, and assign each a tier using the framework above. Note the nameplate wattage of the big motors (well pump, sump, HVAC) and whether your heat is gas, heat-pump, or resistance — that single fact shapes the whole design. Then get two to three itemized quotes from licensed electricians or certified battery installers. Each quote should show the subpanel hardware and labor, any panel-upgrade work, the battery equipment sized to your circuit list with the simultaneous-peak calculation shown, permits and inspection fees, and utility interconnection handling.

Ask each bidder three questions: what simultaneous peak did you design the inverter for, what happens when a Tier 2 load surges during an outage, and how do I shed loads if the outage outlasts the battery. The installer with crisp answers to all three is the one who will still be right when the grid goes down. Costs are 2026 US market ranges; get itemized local quotes — and remember that the cheapest quote with a bloated circuit list is the most expensive system you can buy.

Frequently asked questions