The power goes out, and for a second or two, you sit there in the dark. But then, because you planned ahead and invested in a home battery, it kicks in, and the lights come back on. The refrigerator keeps humming, and your Wi-Fi barely misses a beat.
So far, so good. But what happens if the outage stretches into the next morning? Or the next day?
A home battery might keep your essentials running for a day or two, but add an air conditioner or another power-hungry appliance to the mix, and that runtime can evaporate fast. We’ll explain how long your battery should last below so you’ll know what to expect.
How Home Battery Runtime Actually Works
There are two things to consider when thinking about battery runtime: capacity and power output.
Home battery capacity is measured in kilowatt-hours (kWh), which tells you how much energy the battery can store. Power output, measured in kilowatts (kW), tells you how much electricity it can deliver at once.
Both are important, but capacity is the starting point for figuring out how long your backup power will last.
Here’s a simple way to estimate runtime:
Battery runtime (hours) = Usable battery capacity (kWh) / Average power consumption (kW)
Say you have a home battery with 13.5 kWh of usable capacity. If your household is drawing an average of 500 watts (0.5 kW), you’d have roughly 27 hours of backup power. Double that consumption to 1,000 watts (1 kW), and your estimated runtime drops to 13.5 hours.
Those are idealized estimates. Your actual runtime will depend on how much charge the battery has when the outage starts, how much energy is lost during conversion, and how your electricity use changes throughout the day.
The takeaway? The more electricity you use, the faster your battery drains. That’s why keeping a refrigerator and a few lights running is a very different proposition than powering your entire house.
What Determines How Long Your Battery Will Last?
The amount of time your battery will last depends on how much power you’re actually using during an outage. A few things can affect how quickly your battery drains:
- Battery Capacity: The more energy your battery can store, the longer it can provide backup power. Some systems let you connect multiple batteries for additional capacity.
- Appliances and Equipment: Your AC and other high-demand appliances can eat through stored energy quickly, especially if several are running at once.
- Essential-Load vs. Whole-Home Backup: A battery powering only critical circuits generally lasts longer than one supplying electricity to the entire house.
- Starting Charge: A battery that’s only 60% charged when the outage starts won’t provide nearly as much backup time as one that’s fully charged.
How Long Can a Home Battery Power Your House?
It’s hard to really gauge what a battery would look like in your house without an example. For this one, we’ll use a home battery with 13.5 kWh of usable capacity. Depending on what you’re running, that could last well over a day or run down in just a few hours.
| Usage Scenario | Average Power Draw | Estimated Runtime |
| Basic essentials: Refrigerator, Wi-Fi, and a few LED lights | 300 watts | 45 hours |
| Moderate use: Essentials plus a TV, fans, and additional electronics | 750 watts | 18 hours |
| Heavy use: Essentials plus air conditioning and other household loads | 2,500 watts (AC alone can require 500 to 4,000 watts) | 5.4 hours |
These are just examples, and the estimates assume the battery starts fully charged and supplies its entire usable capacity, without accounting for additional energy losses. Actual runtime will vary as appliances cycle on and off throughout the day.
If you’re trying to make it through an overnight outage, powering the essentials might be plenty. If you want to keep your AC running during a multiday California power shutoff, you’ll need a lot of battery capacity.
Can Solar Extend Your Battery’s Runtime During an Outage?
If you want more out of your battery, a solar-plus-battery setup can be a great option. Your panels can generate electricity during the daylight hours and recharge your battery while the grid is down. That can make a huge difference during those power shutoffs that last for days.
There’s a catch, though. Not every solar system can work during an outage. Most standard grid-tied solar systems automatically shut down when utility power goes out. To keep generating electricity, you’ll need a system configured to safely operate independently of the grid.
And even with the right setup, solar recharging isn’t guaranteed to keep your battery full. Cloud cover can reduce production, and your panels won’t generate electricity at night. If you use more energy than the panels can replace during the day, your battery will eventually run down.
For longer outages, the hope is to generate enough solar energy during the day to cover your household’s needs and replenish the battery for overnight use.
How Can You Make Your Home Battery Last Longer?
If the power’s only out for a couple of hours, you might not need to change much about how you use electricity. But if you’re facing a multiday outage, conserving battery power can buy you some valuable extra time.
Here’s what you can do to help stretch your battery’s charge:
- Stick to the Essentials: Prioritize your refrigerator, necessary medical equipment, basic lighting, and other appliances you can’t comfortably go without.
- Limit AC: AC can be one of the biggest drains on your battery. Raising the thermostat a few degrees or only cooling certain areas can help limit consumption.
- Hold Off on Energy-Heavy Chores: Save the laundry, dishwasher, and other high-demand appliances for when grid power returns.
- Unplug Unnecessary Electronics: Devices left plugged in can continue drawing small amounts of electricity, even when you’re not using them.
- Keep an Eye on Your Battery: Many home battery systems have apps that show how much charge remains and how fast you’re using it. Check yours periodically so you can adjust your electricity use before the battery runs low.
If your system lets you set backup priorities or reserve a certain percentage of battery capacity for outages, review those before the next power shutoff so you know how to use them if you need to.
How Much Battery Backup Do You Need for Your Home?
Before settling on a battery size, think about what you’d need to keep running during an outage:
- Essential Appliances: Make a list of what you need to power, including any high-demand equipment like air conditioning.
- Outage Duration: Consider how long you might be without electricity, especially during an extended PSPS event.
- Solar Availability: If you have compatible solar panels, daytime charging could reduce the amount of battery storage you need.
- Household Energy Use: Review your electricity consumption to get a better idea of how much backup capacity you’ll need.
A single battery may be enough, or you might need multiple units to cover your household’s needs. An electrician can help you calculate your backup loads and choose a system that’s appropriately sized for your home.
FAQ: How Long Does a Home Battery Last?
From battery capacities to average appliance usage, homeowners often have a number of questions about preparing for the next outage. We’ll answer some of the most common ones our clients have below.
How long will a 10-kWh battery last during an outage?
A 10-kWh home battery could provide roughly 20 hours of backup power if your household averages 500 watts of electricity use. At 1,000 watts, that drops to roughly 10 hours. These are ideal estimates that assume all 10 kWh are usable, the battery starts fully charged, and there are no energy losses (which isn’t always the case).
Can a home battery run an air conditioner?
Yes, a home battery could run an air conditioner if the system has enough power output to handle the AC’s electrical demands, including startup. Central air conditioning can drain a battery fairly quickly, though. You may need multiple batteries or a larger backup system if you want to keep the AC going for an extended period.
Can a home battery last through a multiday power outage?
Possibly, if your electricity consumption is low enough or you have sufficient storage capacity. A compatible solar system can help by recharging the battery during daylight, provided there’s enough sunlight and the system is configured to work during outages.
How long does it take to recharge a home battery?
Home battery charging time depends on the battery’s capacity, its current charge level, and how much charging power is available. A 10-kWh battery getting a steady 2 kW of charging power would take about five hours to recharge from empty under ideal conditions. Actual charging time might look a little different due to energy losses and charging limitations.
Will my home battery automatically turn on when the power goes out?
Yep, it usually will, as long as the system is designed and configured for backup power to automatically switch over when the grid goes down. That switchover time depends on the equipment, so you may notice a brief interruption. If you want that switch to be automatic, make sure you let your installer know, since not every battery installation includes this functionality.
What happens when a home battery runs out of power during an outage?
Once your home battery runs out of juice, it’ll stop supplying backup electricity. Any circuits relying on the battery will lose power until another power source becomes available. If you have a compatible solar system, sunlight may allow the battery to recharge and resume supplying power, depending on the system’s configuration.
Can you add more batteries to extend backup runtime?
Yep, you sure can. Many home battery systems allow you to add more batteries for additional storage capacity. This can extend how long your home stays powered during an outage, assuming your electricity use stays about the same. Expansion options will depend on your existing equipment, though, so your electrician will need to confirm compatibility and system capacity.
Get the Right Battery Backup for Your California Home
The last thing you want during a power outage is to find out that your battery falls woefully short of meeting your household’s needs. Alpha Omega Electric can help you figure out how much backup power your home needs and install a system that’s sized accordingly.
Contact us today to discuss your home battery backup options and get ready for the next California power outage.
Sources & References
- California Public Utilities Commission (CPUC) – “Public Safety Power Shutoff FAQs”
Explains California’s Public Safety Power Shutoffs (PSPS), outage preparedness, and battery backup considerations.
https://www.cpuc.ca.gov/consumer-support/psps/public-safety-power-shutoff-faqs - EnergySage – “How Many Watts Does an Air Conditioner Use?”
Provides air conditioner wattage estimates and explains how cooling equipment affects household electricity consumption.
https://www.energysage.com/electricity/house-watts/how-many-watts-does-an-air-conditioner-use - U.S. Energy Information Administration (EIA) – “Kilowatthour”
Defines kilowatt-hours (kWh) and explains how electrical energy consumption is measured.
https://www.eia.gov/tools/glossary/index.php?id=Kilowatthour
