The two numbers that matter
Capacity, in kilowatt-hours, is the size of the tank. Power rating, in kilowatts, is the size of the pipe.
A large battery with a modest power rating will run your fridge, lights and networking for a long time and will refuse to start a central air conditioner. Neither number rescues the other.
Ask what the continuous power rating is, and what the surge rating is, because motors draw far more starting than running.
What "backup" actually means
A battery does not silently take over your whole house. During an outage, it powers a defined set of circuits that were deliberately moved onto a backup subpanel — often called a critical load panel — during installation.
Deciding what goes on that panel is a real design decision made before installation, and changing it later is electrical work.
Whole-home backup exists, costs considerably more, and needs a power rating high enough for everything you might run simultaneously.
When storage makes financial sense
Storage stops being an emotional purchase and starts being an arithmetic one under specific conditions.
- Poor export compensation. Where the utility pays little for exported electricity, storing it and using it yourself is worth more than selling it.
- Sharp time-of-use spreads. If evening electricity costs much more than midday electricity, a battery arbitrages that gap every day.
- Frequent or long outages. Where reliability is genuinely poor, the value is partly non-financial and that is a legitimate reason.
- Demand charges or export limits. Some tariffs and some interconnection rules make storage the thing that lets the solar work at all.
Chemistry, in one paragraph
Most current home batteries use lithium iron phosphate (LFP) or a nickel-manganese-cobalt (NMC) chemistry. LFP generally trades some energy density for longer cycle life and better thermal behaviour, which is why it has become common in stationary home storage where physical size matters less than it does in a car.
You do not need to choose a chemistry. You need to read the warranty, which expresses the manufacturer’s own view of how long it will last.
How the work runs
- Load assessmentWhich circuits you actually want running in an outage, and what they draw. This drives both numbers.
- SizingCapacity from how long you want to ride through; power rating from the largest simultaneous load.
- Design and placementBatteries have temperature, clearance and sometimes location restrictions set by code. Garage interior is common; direct sun is not.
- Backup subpanelThe chosen circuits are moved to a critical load panel so they can be isolated from the grid during an outage.
- PermittingEnergy storage has its own permitting requirements, and in some jurisdictions its own inspection.
- Installation and commissioningPhysical mount, electrical tie-in, then configuration: backup reserve, time-of-use schedule, and how it behaves when the grid drops.
- Utility approvalStorage usually needs its own interconnection sign-off, separate from any existing solar approval.
What moves the price
We do not publish a national average and present it as your price. These are the variables that separate two quotes for the same job — run your own numbers with the cost guides and calculators, and check what incentives you actually qualify for.
| Factor | Why it moves the number |
|---|---|
| Usable capacity | Quoted capacity and usable capacity differ. Ask which number you are being sold. |
| Power rating | Higher continuous and surge ratings cost more and are what make heavy appliances work. |
| Backup scope | A few critical circuits is far cheaper than whole-home backup. |
| Electrical work | Adding a critical load panel and rewiring circuits onto it is meaningful labour. |
| Retrofit vs new | Adding storage to existing solar may require an inverter change depending on the original architecture. |
| Permitting | Storage permits and inspections are separate from solar permits in many jurisdictions. |
What to ask before you commit
Take this list to the conversation. A professional who answers these directly is telling you a lot; one who deflects is telling you more.
- What is the usable capacity, not the nameplate capacity?
- What is the continuous power rating, and what is the surge rating?
- Exactly which circuits will be on the backup panel?
- Will this start my air conditioner, or my well pump?
- What does the warranty guarantee — years, cycles, or retained capacity — and which comes first?
- What happens to the battery if I lose my solar production on a cloudy week?
Common questions
Will a home battery power my whole house?
Only if it was designed and paid for to do that. By default a battery powers a defined set of circuits on a backup subpanel, chosen during installation. Whole-home backup requires a much higher power rating and costs considerably more.
How long will a battery run my house?
It depends entirely on what you are running. The same battery might run a fridge, lights and internet for a day or more, and a central air conditioner for a couple of hours.
Can I add a battery to solar I already have?
Usually yes. How cleanly depends on the existing inverter architecture — some systems accept AC-coupled storage easily, others need an inverter replacement.
Is a battery better than a generator?
Different tools. A battery is silent, needs no fuel and works instantly; a generator runs indefinitely as long as you can feed it. For long multi-day outages a generator often wins on pure runtime.