Home Battery Backup for Power Outage: How It Works

A home battery backup for power outage use, paired with the right inverter and switching equipment, is what keeps lights on when the grid goes down. A standard grid-tied inverter must shut off in an outage to protect utility workers, so many solar owners are surprised the first time their panels stop powering the house.

Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.

Why Grid-Tie Solar Shuts Off

Grid-tied inverters are certified to UL 1741 / IEEE 1547, which require anti-islanding: if the grid disappears, the inverter stops exporting within seconds. Otherwise it could energize lines that crews believe are dead. So even on a sunny day, a grid-only solar system goes dark in a blackout.

How Home Battery Backup for Power Outage Works

A backup-capable system adds:

A battery (AC- or DC-coupled)
An inverter that can form its own grid (hybrid or battery inverter)
An automatic transfer switch or backup gateway that disconnects the home from the utility

Once islanded, the battery powers your backed-up loads, and solar can keep recharging it.

System	Works in outage?
Grid-tie solar only	No
Grid-tie + backup battery with gateway	Yes, for backed-up loads
Hybrid inverter with battery	Yes
Off-grid	Yes (no grid at all)

Whole-Home vs Partial Backup

Partial / critical loads: a subpanel with essentials such as fridge, lights, internet, outlets, furnace fan
Whole-home: everything, which needs enough kW output and capacity for big loads like AC, well pumps and ranges

More kW and kWh cost more.

How Long Will It Last?

Hours ≈ usable kWh ÷ average load (kW). Example: 13.5 kWh at a 1 kW average ≈ 13 hours, longer with solar recharging during the day.

Microinverters and Outages

Microinverter systems also need an approved backup solution (e.g., a system controller) to run in an outage.

Generators

A generator can extend outages; some battery systems accept generator input. Never backfeed through an outlet.

Worked Example: Runtime by Load

Using hours ≈ usable kWh ÷ average kW, here’s how long one 13.5 kWh battery lasts with no solar recharge (illustrative):

Backed-up loads	Average load	Runtime (13.5 kWh)	Runtime (27 kWh)
Fridge, lights, Wi-Fi	0.3 kW	about 45 h	about 90 h
Plus furnace fan and outlets	0.6 kW	about 22 h	about 45 h
Plus well pump and microwave	1.0 kW	about 13.5 h	about 27 h
Whole home with central AC	2.5 kW	about 5.4 h	about 11 h

With sun, solar can refill the battery during the day, so a well-sized system can run critical loads through multi-day outages. See how many batteries to power a house.

Worked Example: Can the Battery Start My AC?

Capacity (kWh) is how long; power (kW) and surge are what you can run at once. Motors briefly draw several times their running current at startup, known as locked-rotor amps (LRA). Tesla lists Powerwall 3 at 11.5 kW continuous and 185 LRA motor start.

Example load	Running	Starting (example)	Fits 11.5 kW / 185 LRA?
Fridge	0.15 kW	small	Yes
Well pump, 240V	1.0 kW	example 50 LRA	Yes
3-ton central AC	3.5 kW	example 80–100 LRA	Usually, check the label
AC + range + dryer together	3.5 + 3 + 5 kW	—	About at limit

Check the LRA on your AC’s data plate. A soft-start kit can cut startup current. See Powerwall 3 vs Enphase vs FranklinWH.

AC-Coupled vs DC-Coupled Backup

	AC-coupled	DC-coupled
How it connects	Battery has its own inverter on the AC side	Solar and battery share a hybrid inverter
Best for	Adding to existing solar	New installs
Efficiency	Extra conversion when charging from solar	Fewer conversions
Example	Enphase IQ Battery 5P	Powerwall 3 with its built-in solar inverter

See grid-tie vs hybrid vs off-grid solar.

Step-by-Step: Getting Outage-Ready

List the loads you need in an outage.
Estimate average kW and peak/starting kW.
Choose partial or whole-home backup.
Size kWh for the outage length you want to cover.
Confirm the inverter and gateway can island your home.
Install with a permit; the backup gateway or transfer switch must be inspected.
Test it: turn off the main breaker on a sunny day and confirm backup works.

FAQ

Do solar panels work during a power outage?

Not grid-tie only. With a backup battery or hybrid inverter, yes.

Can I add a battery to existing solar for backup?

Usually, via an AC-coupled battery and gateway.

How long does a home battery last in an outage?

Divide usable kWh by your average load.

Will my battery charge from solar during an outage?

Yes, if the system is designed for backup: once the gateway islands the home, the inverter can form a local grid and solar recharges the battery.

Is a battery better than a generator for outages?

A battery is quiet, switches automatically and needs no fuel, but it holds a fixed amount of energy. A generator runs as long as you have fuel. Many homes use both.

What is a critical loads panel?

It’s a subpanel holding only the circuits you want backed up, so a smaller battery can cover them. See critical loads panel.

Should I get a generator or a battery?

Compare them in generator vs battery backup for home.

Sources: NFPA 70 (NEC) 2023 Articles 702, 705 and 706; UL 1741 / IEEE 1547 anti-islanding requirements; UL 1741 and IEEE 1547; Tesla Powerwall 3 datasheet; Enphase IQ Battery 5P datasheet. Accessed Oct 2026.