How big a battery backup do you need for a power outage?

When the power goes out, the real question is "what do I need to keep running, and for how long". This guide walks you through it in plain steps for a US home, whether you are considering a battery, a battery with solar panels, or a generator. It is written for beginners, including people planning for a parent or for someone who uses a medical device.

The short version

• Start with a list: only the things that must keep running, with their watts and hours per day.
• Motors (fridge, freezer, well pump, sump pump) need a burst of extra power to start. Your battery's inverter must handle that burst.
• The average US customer lost power for about 11 hours in total in 2024, but most of that came from a few big storms. Plan for the outage you actually fear, not the average.
• A battery alone covers short outages. Solar panels stretch it over days. A generator runs as long as you have fuel, but it gives off carbon monoxide, which kills about 100 people a year in the US.
• Anything wired into your home's electrical panel needs a transfer switch or interlock installed by a licensed electrician.
• If someone depends on a medical device, tell your utility and plan a second backup. No single battery should be the only plan.
• Our quiz sizes backup for the share of outages it survives, tested against many years of real weather at your ZIP code.

Step 1: List what must keep running

Go room by room and write down only what you truly need during an outage. Common items:

• Refrigerator and freezer. Ready.gov says a refrigerator keeps food cold for about four hours, and a full freezer holds its temperature for about 48 hours. So the fridge matters most for outages longer than a few hours.
• Medical devices, such as a CPAP machine (a breathing machine for sleep apnea), an oxygen concentrator, or a fridge for insulin. More on these below.
• Lights, internet (modem and Wi-Fi router) and phone charging.
• Well pump (private well), sump pump (basement), and the furnace fan or boiler controls if you heat with gas or oil.

For each item, find its power in watts on the label or in the manual. Then estimate how many hours a day it runs. Watts tell you how fast an item uses electricity (like how fast water leaves a tap). Watt-hours (Wh) tell you how much it uses over time (how much water ends up in the bucket). Watts x hours = watt-hours.

A worked example with example label values (check your own): a 15 W router for 24 hours uses 360 Wh; four 10 W LED lamps for 5 hours use 200 Wh. Our calculation: 15 x 24 = 360 and 4 x 10 x 5 = 200, so 560 Wh a day before the fridge.

A fridge is harder, because its compressor switches on and off. The label's watts are not running all day. The most reliable way to know is to measure it with a plug-in energy meter for a full day, or to use the yearly kWh on its EnergyGuide label divided by 365.

Step 2: Check the starting surge of motors

Your backup source has two power limits:

• Continuous power: how many watts it can supply hour after hour. Everything that may run at the same time must fit under it.
• Surge power: a short burst, for a second or less, when a motor starts.

Motors draw much more current when they start than when they run. White papers from two motor-starter makers put the starting current of a typical induction motor at about 6 times its running current, and say more than 9 times is not unusual. Fridges, freezers, well pumps, sump pumps and furnace fans all have motors. If the surge is too much, the motor will not start, or the inverter (the box that turns battery power into household power) shuts down.

The best number is the one from your appliance's manufacturer or the motor's nameplate (it may say "LRA", locked-rotor amps, which is the starting current). In our quiz, items marked as motors are counted at 4 times their running power at start-up. That is at the low end of the published range, so check your appliance's own figure.

Step 3: Decide how long your outage might last

The US Energy Information Administration (EIA) tracks outages with a figure called SAIDI: the total hours an average customer is without power in a year, not counting momentary flickers.

• 2024: about 11 hours per customer, the most in 10 years. Major events, above all Hurricanes Beryl, Helene and Milton, caused about 80 percent of those hours. Outages from "major events" (big storms) averaged nearly 9 hours, compared with close to 4 hours a year from 2014 to 2023 (EIA, 1 December 2025).
• Everyday outages (without major events) have stayed at around two hours a year since 2013 (EIA).
• 2022: about 5.5 hours per customer, with 1.4 interruptions on average. By state, the average annual interruption time ranged from 10.3 hours in New Hampshire to 19.1 hours in Florida among the five states with the longest outages (EIA, 25 January 2024).

These are averages. They hide the long outages that matter for planning: after a hurricane or ice storm, some homes are off for days while others are not affected. So plan for the longest outage you have had at this address, or that your area's storms, ice or wildfire safety shutoffs make likely. Our quiz offers outage lengths of 4, 12, 24, 48 and 72 hours, or a full week.

Step 4: Battery only, battery plus solar, or generator

Battery only

A battery is silent and gives off no fumes. The usual choices are a portable power station (a battery with outlets built in) or a fixed home battery wired into your electrical panel. Its weakness is that it runs out: a battery that covers 12 hours of your list will not cover three days. Size it from the energy your list uses over the whole outage, with a margin for aging; our quiz also allows for the cold. Our battery size guide explains usable capacity and depth of discharge (how much of the battery you can safely use).

Battery plus solar

Solar panels recharge the battery during the day, so a smaller battery can last through a longer outage, as long as the sun shows up. The catch: storms that cut power often come with days of heavy cloud, and winter days are short. That is why we test every size against real weather, not against an average sunny day. Our guide portable power station or DIY solar system compares the two ways to build it.

Generator

A fuel generator keeps going as long as you can refill it, and can handle large loads. It also has real dangers:

• Carbon monoxide (CO). CO is a gas you cannot see or smell. The Consumer Product Safety Commission (CPSC) says "one generator produces as much CO as hundreds of cars" and that it can kill within minutes. CPSC says an average of about 100 people die in the US each year from CO poisoning from portable generators.
• Where to run it. The CDC says never to use a generator inside your home, garage or carport, and to keep it outside at least 20 feet from doors, windows and vents. CPSC adds that opening doors or windows does not make indoor use safe.
• CO alarms. CPSC advises battery-operated or battery-backup CO alarms on every level of the home and outside sleeping areas, tested monthly. If an alarm sounds, get outside and call 911.

Step 5: Connecting to your house needs an electrician

There are two ways to use any backup source:

1. Plug appliances straight into it, with heavy-duty extension cords. Simple, and no electrician is needed for this part.
2. Feed your home's circuits, so hardwired items like a well pump, furnace or sump pump work. This needs a transfer switch (a switch that moves chosen circuits from the grid to the backup) or an interlock (a plate in your breaker panel that makes it impossible to turn on the main breaker and the backup breaker at the same time).

The National Electrical Code (NEC), the US electrical rulebook adopted by states and cities, covers optional home backup in Article 702. It requires transfer equipment that prevents the utility and the backup source from being connected together (as summarized by IAEI Magazine, the electrical inspectors' journal). Without it, power can flow backward out of your house onto the lines. Electric cooperative DEMCO warns that "a backfed line carries enough voltage to injure or kill a lineman instantly", because crews expect a line they switched off to be dead.

Never plug a generator or power station into a wall outlet to power the house. Have a licensed electrician install a transfer switch or interlock, with the permit your local building department requires.

A fixed home battery has its own rules. UL Solutions notes that residential stationary battery storage must be certified to UL 9540 (the safety standard for energy storage systems) and installed according to its listing, and that installation in sleeping rooms, or in closets or spaces that open directly into them, is prohibited by installation codes. A portable power station is certified as a portable product, not as a home battery.

Medical devices: plan for more than one backup

If someone in your home depends on electricity for their health, the plan needs extra care.

• Tell your utility and fire department. The FDA advises notifying your electric company and fire department that you have a medical device that needs power. Some utilities run programs for this. For example, PG&E in California has a Medical Baseline program that gives enrolled customers extra alerts before planned safety shutoffs. Ask your utility what it offers. Being on a list does not keep your power on.
• Ask the device maker. The FDA says to read the instructions or call the distributor or manufacturer to find out whether your device can run from batteries or a generator.
• Keep a second option. Ready.gov advises alternate plans for power-dependent medical devices and for refrigerating medicines: a spare battery from the device maker, a second power source, or a place you can go.

In our quiz, a medical device is sized like any other item. When you add one, the result carries a clear warning: this is not a life-support system, and it should never be the only supply for a device someone's life depends on. Talk to the prescribing doctor or the equipment supplier as well.

How our quiz sizes backup power

Our quiz tests the system against outages, hour by hour, using many years of real weather records for your location:

• It starts a test outage of your chosen length on every day of the weather record, at four times of day (midnight, 6 am, noon and 6 pm, by solar time), and assumes the battery is full when each outage starts.
• An outage counts as survived if every hour of your list is powered until it ends.
• With solar, it offers three options that survive 90 percent, 97 percent and 99.5 percent of those outages. Each takes the smallest battery that reaches its target with no more panels than it takes to cover one day's use on an average day of the darkest month.
• Without solar, it sizes the battery for the worst outage window of your list, on the coldest day in your location's record, and for the battery's end of life (when it holds about 80 percent of its original capacity).
• It also shows a grid-charged battery (a power station you keep charged from the wall) that holds the whole outage without solar, as a second option.
• Every backup result reminds you that wiring into your house needs a licensed electrician.

Then it builds the system from products in our checked catalog and checks that the parts work together: inverter power and surge, battery current, cold charging, and cable sizes.

What to check before you buy

1. Your must-run list with watts and hours, and the largest motor's starting power against the inverter's surge rating.
2. The battery's usable energy after the inverter's losses, not only the number on the box.
3. Where the battery will sit, and its minimum charging temperature if that spot gets cold. See LiFePO4 batteries in cold weather.
4. Whether any must-run item is hardwired. If so, budget for a transfer switch or interlock and a licensed electrician.
5. Safety listings: UL 2743 for a portable power station, UL 9540 for a fixed home battery.
6. For a generator: a place outdoors at least 20 feet from openings, and CO alarms on every level.
7. For medical devices: the maker's advice, your utility's program, and a second backup.

What can go wrong

• The fridge or well pump will not start. The inverter's surge rating was too low for the motor.
• The battery runs out overnight. The plan counted the label watts for a few hours instead of a full 24-hour day, or forgot the inverter's own losses.
• Cloudy days after the storm. The panels were sized for an average day, not for the gray days that follow storms.
• The battery will not recharge in a cold garage. It is below the minimum charging temperature in its datasheet and has no heater.
• A generator in the garage "with the door open." CPSC says open doors and windows do not give enough ventilation. Outdoors only, at least 20 feet away.
• A "suicide cord" (a cord with plugs on both ends) used to feed the house. It can backfeed the grid and electrocute line workers.

Questions people ask

Can a battery run my whole house? Sometimes, but sizing for the whole house is usually far more expensive than sizing for the items you listed. Central air conditioning and electric heat are especially large loads.

Is a power station enough for a CPAP? It can be, if the station's usable energy covers the CPAP's watt-hours for the nights you plan for. Ask the CPAP maker how to run it from a battery and which settings change its power use. Keep a second plan.

Will solar panels work during an outage? Only if the system is designed for it. Most grid-tied inverters shut down during an outage by design, so they do not send power into lines that crews may be working on (this is called anti-islanding). To keep power during an outage, solar needs a battery and an inverter that can disconnect your home from the grid.

Next step

Tell us your ZIP code, how long an outage you want to cover, and what must keep running. Size my backup system to see the battery-only answer, the solar options at 90, 97 and 99.5 percent of outages survived, and a grid-charged option, with every number explained. Terms like inverter, surge and depth of discharge are in our glossary, and the full method is on our methodology page.

Our results and guides are estimates and do not replace advice from a qualified electrician.