Critical Loads Panel or Whole Home Backup: The Question to Ask Before You Sign a Battery Quote

A critical loads panel backs up only the circuits moved into a small second panel, while whole home backup can supply every breaker in the house. Before signing a battery quote, get two answers by email: which circuits keep power when the grid fails, and whether the battery reduces your entire household's draw during normal operation.

Get your free solar and battery estimate

Solar Calculator Canada is an independent Canadian platform that provides free solar and battery estimates and matches homeowners with vetted installers. We do not install systems. If you request quotes through our estimator, your details may be shared with vetted installers in our network, and we may earn from installer matching.

Put two battery quotes side by side and the equipment lists can look identical while the systems behave nothing alike, because the behaviour is set by the wiring rather than the battery brand. This guide explains both configurations in plain language, shows how each one shapes outage coverage and bill savings, and gives you five questions that expose what each quote actually delivers.

An analogy sets the frame. A critical loads panel is a packed emergency bag: it holds exactly what was chosen in advance, nothing more. Whole home backup is the full pantry during a storm: everything is available, and rationing is your job. Neither is wrong. They are different products for different homes, and quotes rarely say which one you are buying.

A critical loads panel is a small second electrical panel holding only the circuits chosen for battery backup. An electrician mounts it near your main panel and relocates your chosen circuits into it, typically refrigeration, the furnace fan, the internet router, a sump pump and some lighting. When the grid drops, the battery serves that panel and nothing else. You may also see this called a backup subpanel, a backup loads panel, an essential loads panel or partial home backup.

The defining trait is that the backup list is set when the system is installed. In a blackout, the subpanel circuits run and everything else waits. If you decide next year that the heat pump or the EV charger belongs on backup, that is a paid electrician visit, not a settings change.

Whole home backup is a battery configuration that can supply a home's entire electrical panel during an outage. The connection point is typically at the service entrance, between the utility meter and your main panel, upstream of every breaker in the house, so any load can be carried by the battery within the limits of the inverter's output. In an outage the choice of what runs moves to you: turn the big draws off at the panel and the stored energy lasts far longer than a fixed list would allow.

The tradeoff is straightforward. Full coverage needs more inverter output and usually more stored energy to be worth having, and work upstream of the main panel involves more coordination with your utility. Electric ranges, dryers and resistance heating will empty any battery quickly, so whole home backup is about choice during an outage, not unlimited power.

The single most useful question hides a second one, because a home battery does two separate jobs and the wiring for each is decided separately.

Job one is outage coverage: which circuits receive battery power when the grid is down. Job two is bill savings: whether, on an ordinary day with the grid connected, the battery discharges against your full household load or only against the subpanel. Rate arbitrage means filling a battery during low-priced hours and drawing it down during high-priced hours, and it only pays to the extent the battery can displace what your home is actually consuming when prices are high.

The detail that separates two otherwise identical quotes: in some designs, the system's metering point sits at the service entrance even though only a subpanel is protected in a blackout. Those systems track the entire home's draw in normal operation and can discharge against all of it. In other designs, the battery only ever serves the subpanel, and the rest of the house never uses stored energy at all. Same battery, and a very different bill at the end of the year.

Email these five questions to every installer quoting you, and keep the replies. Each answer narrows the decision.

1. In an outage, which circuits are protected? I want the list as it appears in the design documents.
2. On a normal day with the grid connected, is the battery discharging against my full household load or only against the subpanel circuits? Point to the metering location in the drawings that makes it true.
3. What is the inverter's continuous and surge output, and which of my large loads exceed it?
4. If the inverter itself breaks down on a day when the grid is fine, does grid power keep flowing to my panel on its own, or does someone need to come out and bypass it? I want the documentation page that answers this.
5. What would it cost later to change which circuits are backed up, or to move from one configuration to the other?

A vague reply to the second question is a reason to discount the savings forecast attached to that quote. Any savings number attached to a battery quote is an estimate, not a promise. Ask for the assumptions behind it. Written replies also give you something to hold the installer to after the work is done, which is part of how we vet installers for our network.

One honest caveat before you spend anything. If outages in your area are rare and brief, your province has no time-varying rate worth arbitraging, and you have no solar to store, a battery is a hard purchase to justify on economics alone. Backup value is real, but it should be priced as insurance, not savings.

Ontario is the clearest illustration in Canada because its Ultra-Low Overnight (ULO) price plan carries the country's widest regulated spread. These are the prices set by the Ontario Energy Board, effective November 1, 2025:

Source: Ontario Energy Board, November 1, 2025 Regulated Price Plan prices.

The table below shows what that spread is worth under stated assumptions. Assume a battery with 13.5 kWh of usable capacity, 90 percent round-trip efficiency, one full cycle per weekday, a design that discharges against the full household load, and a home that actually consumes at least the delivered energy between 4 PM and 9 PM, which averages about 2.4 kW across those five hours.

Source: Solar Calculator Canada estimate, August 2026, using the assumptions above. Delivery charges are unaffected, battery degradation is not counted, weekend cycling against the much thinner 5.9 cent spread is excluded, and results scale with battery size and how consistently the home consumes during the expensive hours. These are estimates, not guarantees, and the installer's design governs what is achievable.

Now the wiring point. That estimate assumes the battery is discharging against everything the house consumes between 4 PM and 9 PM. If the system's discharge is confined to a critical loads subpanel, the only usage the battery can displace is whatever the subpanel happens to be running in those hours, and adding more storage does nothing to widen that ceiling. Everything in this math is self-consumption, which is sometimes called load displacement: the battery reduces your own draw and nothing is exported. None of it depends on net metering, which is a separate billing arrangement for exported surplus with rules that differ by province.

You can compare Ontario's plans in our guide to TOU and ULO rates, and model your own usage with our peak shaving calculator. Outside Ontario, the same logic applies with smaller numbers: provinces with flat or tiered residential pricing offer little rate-shifting value, so battery value there rests mainly on outage protection and solar self-consumption. Our overview of electricity rates across Canada shows where time-varying plans exist.

Get your free solar and battery estimate

On older services, yes, and this is worth raising before quotes are drawn. Canadian Electrical Code Rule 64-112 caps the combined rating of the breakers feeding a home panel's busbar at 125 percent of what the busbar is built to carry. On a 100 amp service the math works like this: 125 percent of 100 amps is 125 amps, the main breaker already accounts for 100 of them, and the remaining 25 amps is all the room an inverter breaker gets. At 240 volts, with the standard continuous load derating, that translates to roughly 5 kW of inverter output through the panel.

A supply-side connection lands between the meter and the main panel, upstream of the busbar entirely, so the 125 percent arithmetic never enters the picture. In Ontario the governing limit becomes the micro-embedded generation threshold: amendments to the Distribution System Code, the connection rulebook administered by the Ontario Energy Board, set that threshold at 12 kW as of May 1, 2026, and projects at or under it follow the streamlined connection stream.

These limits describe what is possible on paper, not what any utility has approved. Connection rules vary by province and utility, the design and the work belong to a Licensed Electrical Contractor under ESA in Ontario, inspection applies, and utilities approve connections on their own terms with no outcome assured. Ask each installer which connection method their quote assumes and what your specific service supports.

Pricing varies with every quote, so the useful comparison is what drives cost in each configuration rather than a single number.

In most quotes the battery itself is the largest line item, and the wiring delta between the two configurations is often small relative to total system price, which is exactly why it deserves an itemized line. Have each installer price the identical battery both ways so the wiring difference shows up on its own line, and compare hardware options in our guide to the best solar batteries in Canada. Incentive programs change frequently and are never guaranteed, so verify current program status before counting any rebate in your math.

A critical loads panel buys certainty about essentials at the lowest wiring cost. Whole home backup buys flexibility and capacity margin. Bill savings follow the metering and discharge design rather than the backup label, and two quotes with the same battery can carry very different savings for exactly that reason. Collect written answers to the five questions, and the right choice for your home usually becomes obvious.

Back to the analogy: the emergency bag and the full pantry both get a household through a storm. What no quote should leave unclear is which one you are paying for, and whether the battery is working for you on the thousands of ordinary days in between.

For a first estimate of what a battery could save on your own address, usage and rate plan, run the free estimate on this site. It models both rate plans and shows the assumptions behind every number.

Get your free solar and battery estimate