Quick answer
Battery Backup is a home battery's ability to keep power flowing to some or all of your circuits when the utility grid goes down. The battery system disconnects the house from the grid within moments and runs it as a small island until grid power returns.
With solar attached, panels can recharge the battery each day of a long outage; without a battery, a standard grid-tied solar system shuts off.
Quick facts
The key facts about battery backup, with sources:
- Runtime formula
- Usable kWh ÷ average load in kW = hours
- Shortest possible runtime
- Usable kWh ÷ maximum continuous kW; about 1.2 hours on a Powerwall 3 at full output 2
- US average outage time, 2024
- 11 hours per customer, the most in 10 years 5
- South Carolina, 2024
- Nearly 53 hours, the longest of any state 5
Key takeaways
- Battery backup keeps chosen circuits on when the grid fails, by cutting the home off and running it as an island.
- Runtime = usable kWh ÷ average load in kW. What runs at once depends on the kW rating.
- Solar can refill the battery each day of a long outage; a battery alone is a fixed tank.
- US customers averaged 11 hours without power in 2024; South Carolina nearly 53.
- A licensed electrician sets which circuits are backed up. This is never DIY work.
What battery backup means
Most solar homes stay tied to the grid. When the grid fails, a plain solar system must shut off so it does not send power onto lines that crews are fixing. Battery backup changes that. The battery and a switch let your home keep running on its own.
It is a job a battery does, not a separate product. The same solar battery that stores midday surplus can hold a reserve for outages.
Outages are getting longer. The EIA reports US customers averaged 11 hours without power in 2024, nearly twice the yearly average of the decade before 5.
Sources: [5]
What happens when the power goes out
- The grid fails. The system controller senses it and opens a switch that isolates your home from the utility lines, so no power flows back to line workers.
- The battery inverter starts forming its own AC supply, which is the "island".
- Backed-up circuits keep running, either the whole house or a critical load panel of chosen circuits.
- If solar is connected, the panels keep producing and recharge the battery during daylight.
- When the grid returns and is stable, the controller reconnects and the battery goes back to its normal mode.
Tesla lists three islanding devices for Powerwall 3 (Gateway 3, Backup Switch, Backup Gateway 2); Enphase uses the IQ System Controller 2 1.
Backup set-ups compared
| Set-up | What stays on | Trade-off |
|---|---|---|
| Partial (critical loads) | A subpanel of chosen circuits | Smaller battery; you choose loads in advance |
| Whole-home | Every circuit, within the battery's power limit | Needs more kW, often more units, and load management |
| Backup-only battery | Chosen circuits, with the battery held full | Most runtime, but no daily bill savings |
| Generator instead of battery | Depends on generator size | Fuel and exhaust; see the battery vs generator guide |
Worked example: how long backup lasts
An example only. The battery capacities and power ratings are from the data sheets; the 500 W average load is an assumption standing in for a refrigerator, lights, internet and a few outlets. Your own load is the real input.
| Battery | At a 500 W average load (assumed) | At the battery's maximum continuous output |
|---|---|---|
| Enphase IQ Battery 5P, 5.0 kWh usable, 3.84 kVA 1 | 5.0 ÷ 0.5 = about 10 hours | 5.0 ÷ 3.84 = about 1.3 hours |
| Tesla Powerwall 3, 13.5 kWh nominal, 11.5 kW 2 | 13.5 ÷ 0.5 = about 27 hours | 13.5 ÷ 11.5 = about 1.2 hours |
| FranklinWH aPower 2, 15 kWh usable, 10 kW 4 | 15 ÷ 0.5 = about 30 hours | 15 ÷ 10 = about 1.5 hours |
Real runtime will be somewhat shorter: conversion losses, any reserve the system keeps, and cold or heat all reduce it. Enphase notes its usable figure includes a 2% safety-critical limit and 3% held for the electronics overnight 1. Run your own loads through the battery sizing calculator.
Where backup earns its keep: long storm outages
A battery alone is a fixed tank. Paired with solar, it refills each sunny day. That matters for the outages that do the most damage: EIA found that hurricanes Beryl, Helene and Milton accounted for 80% of the hours US customers spent without power in 2024.
Major events drove nearly nine of the 11 hours in 2024. Routine outages averaged about two hours a year 5. So backup is mostly about storms, which also cloud the sky.
Sizing for multi-day outages is therefore a balance of three things: enough kWh to get through the night, enough panel output to refill by evening, and a load you are willing to trim to.
Sources: [5]
Battery backup: strengths and limits
What it does well
- Switches on by itself within moments, with no one at home needed.
- No fuel to store and no exhaust, so it can sit by the house.
- Refills from solar during long outages.
- Does daily bill-saving work when the grid is up.
Where it falls short
- Hours of storage, not days, unless solar refills it.
- The kW rating limits what can run at once, especially AC and pumps.
- Cloudy storm days refill it slowly.
- Needs a permit, an inspection and, often, panel work.
Limits to plan around
Not every setup supports backup. Enphase says backup is not supported for 208 V single-phase service on the IQ Battery 5P 1. Ask your installer which service voltage your home has.
Heat cuts output. Tesla says Powerwall 3 may be de-rated above 40°C (104°F) 2. And on a Powerwall 3 system, any loss of AC power triggers the solar rapid shutdown devices; the islanding device then restores a local AC supply 2. Ask your installer how quickly solar resumes in backup on your design.
What drives the cost of backup
Backup adds hardware beyond the battery: a gateway or backup switch, and often a critical load panel or a new main panel. NLR's home battery cost model counts the pack, the battery inverter, other hardware, labor, permits and interconnection, sales and profit 7. HyreSolar does not print prices without dated quotes.
The biggest swing is how much you back up. Whole-home backup needs more kW, often more units, and sometimes load management. Partial backup costs less.
Warranty and backup. Tesla's US warranty gives unlimited cycles for backup use, along with solar self-use and time-of-use, with 70% capacity at 10 years 3.
How battery backup gets set up
A licensed electrician does every wiring step. The usual order:
- Choose what stays on: list must-run loads, such as fridge, lights, well pump, medical gear.
- The installer checks the kW and surge needs and picks partial or whole-home.
- They design the islanding switch and, if partial, the critical load panel.
- Permit and, for grid-tied systems, utility review; see permission to operate.
- Install, inspect, then test a simulated outage with you present.
- Set the backup reserve and any storm mode in the app.
Keeping backup ready
- Raise the reserve before a forecast storm. Some systems, such as FranklinWH, can fully charge ahead of severe weather 4.
- Keep the system online for alerts and updates; Tesla expects owner-supplied internet 2.
- Know which outlets are backed up. Label them.
- After each outage, check the app for overloads and how fast charge fell.
Warning signs and when to call a pro
- Backup did not switch on, or dropped out when a motor started.
- Backed-up circuits flicker or lights dim hard.
- The battery drains much faster than the runtime math says.
- Breakers near the battery trip or feel warm.
- Call your installer. Do not rewire or bypass anything to get power back.
Safety rule
Never back-feed a home through an outlet or skip the islanding switch. It can electrocute line workers. Backup wiring is for a licensed electrician only.
Rules for battery backup
Equipment listing. The battery system must be listed to UL 9540 under IRC R328 and NFPA 855 8. Islanding hardware and inverters carry their own listings; Powerwall 3 lists UL 1741 and IEEE 1547, the US grid-connection standard 2.
Placement. Units are capped at 20 kWh each and 40 or 80 kWh in total by location, and kept out of bedrooms 8.
Local edition. South Carolina enforces the 2021 building codes 9. Your installer files the permit under whatever code your area uses.
Battery backup vs a portable generator
| Battery backup | Portable generator | |
|---|---|---|
| Fuel | Stored power, refilled by solar or grid | Gasoline or other fuel |
| Runtime | Usable kWh ÷ load, plus solar refills | As long as fuel lasts |
| Starts | By itself, within moments | By hand, in most cases |
| Carbon monoxide | None | CPSC: nearly 100 US deaths a year; run it outside, 20 ft from the house 6 |
| Noise | Under 50 dB(A) typical for Powerwall 3 2 | Varies by model |
Misconceptions
- Myth My solar panels will keep the lights on in a blackout.
- Reality A standard grid-tied system shuts off. Backup needs a battery and an islanding switch.
- Myth A big battery can run everything.
- Reality kWh sets how long; the kW rating sets what runs at once.
- Myth The battery will last as long as the outage.
- Reality Without solar refills, it lasts its usable kWh ÷ your load, and no longer.
Battery backup in SC, GA and VA
South Carolina customers averaged nearly 53 hours without power in 2024, the longest of any state, after Hurricane Helene's winds and flooding (EIA) 5. A one-night battery does not cover that; solar recharging and a trimmed load do.
If your battery may charge from the grid ahead of a storm, check your utility's rider: on Dominion Energy SC's Solar Choice rider, storage counts only if it charges solely from onsite renewables 10. Georgia and Virginia utilities set their own terms.
Is battery backup worth it for you? What to settle on the quote
- Worth a close look if outages are long or frequent, or someone at home relies on powered medical gear.
- Which circuits are backed up, written on the single-line diagram.
- The power rating and surge rating against your largest motor loads.
- Whether solar keeps charging the battery during an outage on this design.
- The backup reserve the installer will set, and how to change it before a storm.
- That a licensed electrician performs and permits all wiring; backup work is not a DIY job.
Next steps
- Battery sizing toolHours of backup for your own loads
- Home battery storageCompare backup options and quotes
- Battery or generator?
Questions about battery backup
How long will a solar battery power my house?
Divide the battery's usable kWh by your average load in kW. A 13.5 kWh battery at a 0.5 kW average load lasts about 27 hours before losses; at full output it lasts little over an hour. With solar, it can recharge each day, which stretches backup through multi-day outages.
Do solar panels work during a power outage?
Not on a standard grid-tied system without a battery: the inverter shuts down for line-worker safety. With a battery and an islanding device, the panels can keep running and recharging the battery. Ask the installer to confirm solar charging in backup on your design.
How fast does battery backup switch on?
Within moments, through the system controller or gateway. Many homes see no interruption on backed-up circuits; check the switchover spec for your model if you run medical or sensitive equipment. Ask the installer to show it during the outage test at turn-on.
How many batteries do I need for backup?
Enough kWh for the hours you want and enough kW for the loads you will run at the same time. The sizing calculator does both checks; for whole-home backup, power is usually what forces a second unit. Long outage areas may also need more kWh.
Is battery backup better than a generator?
It depends on the outage. A battery starts by itself, is quiet and makes no exhaust, and solar can refill it. A generator runs as long as it has fuel. CPSC says nearly 100 people die each year from portable generator carbon monoxide, so generators must run outside, at least 20 feet from the house.
Can I run my air conditioner on battery backup?
Sometimes. The battery's continuous kW and motor-start (LRA) ratings must cover the AC along with everything else on. Powerwall 3 and aPower 2 each list 185 LRA. A single small unit may not. Your installer should check your AC's nameplate.
What should I back up in an outage?
Start with what you cannot do without: refrigerator, some lights, internet, a few outlets, a well pump if you have one, and any medical equipment. Add big loads like AC only if the power rating allows. A critical load panel groups those circuits.
Should I charge my battery fully before a storm?
Usually, yes, if outages are likely. Many systems have a storm mode that charges to full ahead of severe weather. Check whether grid charging affects your utility credits first; some solar riders only count storage that charges from onsite solar.
Sources
- Enphase Energy, IQ Battery 5P data sheet (DSH-00010-11.0-EN-2025-05-06), retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- Tesla, Powerwall Limited Warranty (USA), effective 21 August 2023, retrieved .
- FranklinWH, aPower 2 data sheet (2025), retrieved .
- US EIA, Today in Energy: Hurricanes in 2024 led to the most hours without power in the United States in 10 years (1 December 2025), retrieved .
- US CPSC, As Winter Storms Continue, CPSC Urges Families to Take Steps to Prevent Carbon Monoxide Poisoning and Fires (19 January 2024), retrieved .
- National Laboratory of the Rockies (NLR, formerly NREL), Annual Technology Baseline 2024: Residential Battery Storage, retrieved .
- UL, 2021 Residential ESS Requirements (IRC R328 and NFPA 855 Chapter 15 summary, 2021), retrieved .
- SC Building Codes Council, code adoption, retrieved .
- Dominion Energy South Carolina, Residential Solar Choice rider (Rider to Rate 5), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Capacities, power ratings, islanding devices and limits from the Enphase, Tesla and FranklinWH data sheets and Tesla US warranty; outage figures from EIA; generator CO figure from CPSC; code limits from UL's IRC R328 / NFPA 855 summary; cost structure from the NLR ATB.
The runtime example divides published capacity by a stated, assumed load and nothing else.
Suggest a correction. We fix errors and say what changed.