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“Whole home” is a load decision, not a battery size. The phrase is used as if it were a product category, but two houses of the same floor area can need batteries several times apart depending on what is actually switched on during an outage and what happens when a motor starts. Sizing from the house gives the wrong answer; sizing from the circuit list gives the right one.
This page is for homeowners working with an installer, and for the installers specifying the system. It covers how to decide what gets backed up, why starting surge decides the inverter rather than the running load, and what has to be right on commissioning day.
Decide which circuits must stay on
The cheapest kilowatt-hour of backup is the one you do not have to supply. Most households, asked directly, want a short list: refrigeration, some lighting, internet and phones, the garage door, and often one heating or cooling circuit. Central air conditioning, electric ovens, tumble dryers and EV charging are the loads that multiply the size, and they are the ones most people are willing to do without for a few hours.
The output of this exercise is a backup panel schedule — a list of circuits and their loads. That list is what gets sized, and it is also what the electrician installs to. It is much easier to agree before the equipment is chosen than to argue about afterwards.
Two numbers, and one of them is surge
Every backup design needs the continuous load — what runs at once — and the surge — what happens for a few seconds when a motor starts. A house with 3 kW of continuous load and a 4 ton air conditioner can need 10 kW or more of surge capability, and the inverter is sized to the surge, not to the continuous load.
This is the single most common reason a backup system trips on its first real outage: the running load was added up, the starting current was not, and the system shuts down the moment the compressor kicks in. Listing every motor — air conditioning, well pump, refrigerator, garage door, pool pump — and whether it can be sequenced or soft-started prevents it.
How many hours is the real question
Capacity is hours times load, so the honest question is what an outage actually looks like where you live. Where outages are short and frequent, a few hours covers almost all of them and a large battery sits unused. Where outages are long and rare, or where the house is also off-grid, the calculation is about getting through a night or a day.
It is worth being specific. “I want to be comfortable for four hours and able to keep the refrigerator cold overnight” is a designable requirement. “I want the whole house backed up” is not, and it usually leads to buying capacity that is never used.
Where the battery goes is a code question
Location is constrained by clearance requirements, by whether the unit is indoor or outdoor rated, and by temperature. Garages are common and usually straightforward. Exterior walls work where the unit is outdoor rated and the clearance is respected. Interior living spaces are usually not acceptable, and unconditioned spaces that get very hot or very cold affect both performance and warranty.
Local requirements vary, and the installed position is what gets inspected. Confirming the location before the order removes the most common cause of a failed inspection.
Recharge: solar, grid, or generator
If outages are short, recharging from the grid afterwards is fine and the array is there for daily savings rather than for the outage. If outages are long, or if the house loses solar production during a storm, then either the array has to charge the battery during the outage or a generator input is needed.
Solar charging during an outage works, and it is worth understanding the limits: production on a stormy day is a small fraction of a clear-day figure, and many systems will not start the array at all until the battery is above a set state of charge. For multi-day outages, a generator input is usually the honest answer.
Two jobs, one battery
A battery that only does backup spends most of its life idle, which is a poor use of it. Most systems therefore also do daily self-consumption — charging from solar during the day and running the house in the evening — and switch to backup mode when the grid drops.
Choose the reserve setting with the selected battery’s operating and warranty limits in mind. Daily cycling, state of charge and temperature all affect ageing, so backup readiness and self-consumption should be balanced for the actual outage pattern. If outages are the main reason for the purchase, say so, because it changes how the system should be configured.
Specifications at a glance
Configuration reference
Swipe horizontally to view all columns →
| Item | Typical specification | What decides it |
|---|---|---|
| Usable capacity | Commonly 10 to 40 kWh per home | Which circuits are backed up and for how long |
| Continuous power | 5 to 12 kW typical | How much runs at once, not how much is connected |
| Surge capability | Two to three times continuous for a few seconds | Air conditioning, well pump and refrigerator starting current |
| Chemistry | Lithium iron phosphate (LFP) | Cycle life, safety and where it can be installed |
| Backup interface | Dedicated backup panel, or whole-service transfer | Which loads must keep running |
| Recharge source | Solar, grid, or generator | Whether an outage is expected to last more than a day |
| Cycle life | Confirmed against the selected model’s datasheet and stated test conditions | Depth of discharge, temperature, power and retained-capacity threshold |
| Enclosure | Indoor or outdoor rated, garage or exterior wall | Space, clearance and local installation rules |
| Compliance documentation | Applicable product listings, safety test reports and transport documentation confirmed for the selected configuration | Destination market, installation location and local authority requirements |
| Documentation | Load list, backup panel schedule, single-line diagram and commissioning requirements as applicable | Document scope and installer responsibilities confirmed in the proposal |
| Warranty | Battery 10 years; modules 12 years product, up to 30 years performance | Model and order terms |
Figures are selection references. Confirm the final ratings, dimensions, available certifications and warranty against the current datasheet and written proposal for the selected model or system.
Questions buyers ask about whole-home backup
How big a battery do we need for the whole house?
That depends on what is switched on and for how long, not on the size of the house. Working from a circuit list and a target number of hours gives a reliable answer; working from floor area does not.
Will it run the air conditioning?
Often yes, if the inverter surge rating covers the starting current, and if the rest of the house is not running at the same time. A soft starter on the compressor makes this materially easier and is a cheap addition.
Can it be added to solar we already have?
In most cases yes, either as AC-coupled storage alongside the existing inverter or as DC-coupled with a replacement hybrid inverter. Which one applies depends on the existing inverter, and it is worth checking before assuming.
How long does it last?
LFP batteries are commonly warranted for ten years and rated for several thousand cycles at 80 percent depth of discharge. Real life depends on how deeply and how often it is cycled and on the temperature where it is installed.
Does it need a permit?
Almost always, and the inspection will look at the installed position, the clearances, the labelling and the single-line diagram. Complete documentation supports inspection; acceptance depends on the installed system and the local authority’s review.
What to send us
Send the circuit list you want backed up with the motor loads marked, how many hours you want to cover, and where the battery could be installed. We come back with a capacity and inverter proposal, the backup panel schedule, and a written delivery date.
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List the circuits you want on backup
Send the circuits, how long they need to run, and the service panel rating. That is enough to size the battery properly and confirm what the installation will require.
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