Most homes need around 5–10 kWh of battery capacity for essential backup, 10–20 kWh for broader partial-home backup, and 20–40 kWh or more for whole-home backup. The right size depends mainly on what you want to run and how long you need it to run.
In this guide, I’ll help you calculate a realistic battery size for your home. We’ll look at appliance usage, backup duration, usable battery capacity, inverter power, HVAC loads, solar charging, and several real-world examples.
Key Takeaways
- Around 5–10 kWh can cover basic household essentials during many outages.
- Around 10–20 kWh provides more flexibility for partial-home backup.
- Whole-home backup often requires 20–40 kWh or considerably more.
- Battery capacity should be based on your outage loads, not house size alone.
- kWh determines runtime, while kW determines how much equipment can run together.
- HVAC systems, well pumps, and electric heating can greatly increase battery requirements.
- Solar panels can extend backup runtime by recharging batteries during longer outages.
- Always check usable battery capacity instead of relying only on advertised capacity.
How Much Battery Capacity Do You Need for Home Backup?
There is no single battery size that works for every house. However, the following ranges give you a useful starting point before calculating your actual loads.
| Backup Goal | Suggested Battery Capacity | Typical Loads | General Backup Target |
| Minimal emergency backup | 2–5 kWh | Wi-Fi, phones, lights, small electronics | Several hours |
| Essential home backup | 5–10 kWh | Refrigerator, freezer, lights, Wi-Fi, electronics | Overnight to around 24 hours |
| Partial-home backup | 10–20 kWh | Essentials, selected outlets, microwave, furnace blower, pumps | Around 12–24 hours |
| Whole-home backup | 20–40 kWh | Broader household loads with load management | Around 12–24 hours |
| High-demand home | 30–60+ kWh | HVAC, electric cooking, pumps, water heating | Depends heavily on usage |
| Multi-day backup | 40–100+ kWh or solar-assisted | Selected or whole-home loads | Two days or longer |
These are planning estimates rather than guaranteed runtimes. Your actual requirement depends on appliance consumption, weather, HVAC use, battery losses, inverter output, and whether you can recharge during the outage.
For many homeowners, starting somewhere between 10 and 30 kWh makes sense. However, someone backing up only refrigeration and communications may need much less, while an all-electric home can require considerably more.
Why Your Normal Daily Electricity Use Is Not Your Backup Requirement
Your normal electricity consumption can help establish a baseline, but you usually should not size your battery around your full everyday usage.
During an outage, your priorities change.
You probably will not run the clothes dryer, dishwasher, electric oven, pool equipment, and EV charger exactly as you would on a normal day. You may also reduce air-conditioning use and turn off unnecessary lighting.
That means a household normally using 25 or 30 kWh each day might need only a fraction of that amount for emergency backup.
This is why I recommend creating a separate outage load calculation.
Instead of asking, “How much electricity does my house normally use?” ask, “Which appliances do I absolutely want available when the grid goes down?”
That number gives you a much better starting point.
First Decide What You Actually Want to Back Up
Before calculating battery capacity, decide what level of backup you actually want. This step can easily change your battery requirement by tens of kilowatt-hours.
Essential Loads Only
Essential backup is usually the most affordable approach.
You keep the important equipment running while leaving high-consumption appliances turned off.
Typical essential loads include:
- Refrigerator
- Freezer
- Wi-Fi router
- LED lights
- Phones
- Laptops
- CPAP or essential medical equipment
- Television
- Selected electrical outlets
A carefully managed essential-load setup may use only a few kilowatt-hours over several hours.
For many homes, 5–10 kWh gives considerably more useful emergency coverage than people initially expect.
Partial-Home Backup
Partial-home backup keeps more of your normal lifestyle available without attempting to power everything.
You might include:
- Refrigerator and freezer
- Lights
- Wi-Fi
- Television
- Computers
- Microwave
- Furnace blower
- Sump pump
- Well pump
- Selected kitchen outlets
- Limited air conditioning
At this level, I would pay much closer attention to both battery capacity and inverter output.
A pump or air conditioner may not consume huge amounts of energy continuously, but it can demand substantial power when starting.
That is why 10–20 kWh is a common practical range for broader home backup.
Whole-Home Backup
Whole-home backup requires more careful planning.
It does not necessarily mean every electrical device runs continuously during an outage.
You can still manage heavy appliances to extend battery runtime.
Major loads may include central air conditioning, heat pumps, electric cooking, electric water heaters, pumps, clothes dryers, and other 240V equipment.
Once several of these loads are included, battery requirements can quickly move toward 20–40 kWh or substantially higher.
kWh vs. kW: Battery Capacity and Power Are Not the Same
This is one of the most important concepts to understand when sizing a home battery. A battery can store plenty of energy and still be unable to run certain appliances if its power output is too low.
| Rating | What It Means | Why It Matters |
| kWh | Stored energy | Determines how long your equipment can operate |
| Continuous kW | Sustained power output | Determines how much equipment can run together |
| Surge power | Short burst of extra power | Helps start motors and compressors |
| Usable kWh | Energy actually available | Gives a better estimate of real runtime |
Think of kWh as the size of your energy tank.
Think of kW as how quickly energy can leave that tank.
For example, a battery might contain 15 kWh of usable energy. That does not automatically mean it can deliver 15 kW of power at once.
If its inverter can provide only 5 kW continuously, your combined appliance load must remain within that limit.
This distinction becomes especially important when running air conditioners, heat pumps, well pumps, and other equipment with motors.
How to Calculate the Battery Capacity You Need
You do not need complicated engineering software to make a useful estimate. A simple appliance list and a few calculations can get you surprisingly close.
Step 1: List the Appliances You Need During an Outage
Start with the equipment you actually want to keep running.
For every appliance, write down:
- Running watts
- Expected hours of use
- Starting watts where applicable
- Whether the appliance runs continuously or cycles
You can usually find wattage information on the appliance label or owner’s manual.
Use this basic formula:
Energy used in kWh = watts × operating hours ÷ 1,000
For example, a 100-watt device running for five hours uses:
100 × 5 ÷ 1,000 = 0.5 kWh
Repeat this calculation for each appliance.
Step 2: Calculate Daily Backup Energy
Now add your appliance consumption together.
Suppose your outage setup looks approximately like this:
- Refrigerator: 1.5 kWh
- Wi-Fi equipment: 0.5 kWh
- LED lighting: 0.5 kWh
- Phones and laptops: 0.5 kWh
- Television and small appliances: 0.5 kWh
- Other essential loads: 1 kWh
Your estimated total would be:
1.5 + 0.5 + 0.5 + 0.5 + 0.5 + 1 = 4.5 kWh per day
These are only example figures. Your refrigerator, lighting habits, electronics, and other equipment may consume more or less.
Step 3: Choose How Long You Need Backup
Next, decide how long the battery should support your home without recharging.
You might be planning for:
- Four to six hours
- Overnight
- 12 hours
- 24 hours
- 48 hours
- Several days
If your essential loads require 5 kWh per day and you want two full days of backup without solar charging:
5 kWh × 2 days = 10 kWh
You would need at least 10 kWh of usable stored energy before accounting for reserves and system losses.
Another useful formula is:
Required energy = average load in kW × backup hours
If your home averages a continuous 500-watt load during an outage, that is 0.5 kW.
For 12 hours:
0.5 × 12 = 6 kWh
Step 4: Account for Usable Capacity and System Losses
The advertised battery capacity is not always the amount of AC electricity you can actually use.
You may see two specifications:
- Nominal or total capacity
- Usable capacity
Usable capacity is the more useful number for backup planning.
Some battery systems reserve part of their energy to protect the battery. The inverter also consumes some energy while converting stored DC power into AC electricity for your home.
If a manufacturer already gives you a usable-capacity rating, use that figure rather than reducing capacity twice.
If you only know nominal battery capacity, you can estimate:
Required rated capacity = required AC energy ÷ usable fraction ÷ system efficiency
Suppose you need 9 kWh of delivered energy, the battery allows 90% usable capacity, and you assume roughly 90% system efficiency.
9 ÷ 0.90 ÷ 0.90 = about 11.1 kWh
A battery around 11–12 kWh would therefore be a more realistic starting point than exactly 9 kWh.
Step 5: Add a Practical Reserve
I would avoid sizing a battery so precisely that you expect it to reach empty exactly when the outage ends.
Real life is rarely that predictable.
A reserve of around 10–20% can provide useful breathing room for:
- Longer outages
- Unexpected appliances
- Hotter or colder weather
- Battery aging
- Longer refrigerator cycles
- Higher HVAC demand
- Reduced solar production
You do not necessarily need a huge reserve.
You just want enough flexibility that one unexpected load does not ruin your backup plan.
Step 6: Verify Inverter Power and Starting Surge
Once your kWh requirement looks right, check whether the system can actually power your appliances.
Look at:
- Continuous inverter output
- Maximum surge output
- 120V and 240V support
- Central AC starting requirements
- Heat-pump requirements
- Well-pump startup power
- Simultaneous appliance loads
This step matters because motors can briefly require much more power when starting than they use while running.
A battery system might have enough stored energy to run your AC for hours but still fail to start the compressor if the inverter cannot provide the required surge.
Typical Home Backup Appliance Energy Use
Appliance consumption varies between models, households, and climates. Still, these ranges can help you build an initial estimate before checking your actual equipment.
| Appliance | Typical Power Consideration | Approximate Daily Energy | Sizing Note |
| Refrigerator | Cycles on and off | 1–2 kWh | Compressor startup matters |
| Wi-Fi router | Low continuous load | 0.2–0.7 kWh | Usually runs continuously |
| LED lighting | Depends on number of lights | 0.2–1 kWh | Easy load to reduce |
| Television | Moderate | 0.2–0.8 kWh | Depends on runtime |
| Phones/laptops | Low | 0.1–0.5 kWh | Usually minor |
| Microwave | High but brief | 0.2–1 kWh | High running wattage |
| Furnace blower | Moderate to high | 1–5+ kWh | Depends on climate and runtime |
| Well pump | High intermittent load | Variable | Startup surge matters |
| Sump pump | Intermittent | Variable | Weather greatly affects runtime |
| Central AC | High | Several kWh or more | Often one of the largest loads |
| Heat pump | High and variable | Several kWh or more | Weather strongly affects use |
| Electric water heater | High | Several kWh | Can greatly increase battery size |
Do not treat these numbers as exact appliance ratings.
Use them as planning ranges, then replace them with measurements or manufacturer specifications whenever possible.
Battery Capacity Examples for Real Homes
Examples make battery sizing much easier to understand. Here are four common situations showing how quickly requirements can change.
Example 1: Essential Backup for One Day
Suppose you want to run:
- Refrigerator
- Wi-Fi
- LED lights
- Phones
- Laptop
- Television
- A few small appliances
Your total outage consumption might be around 4–5 kWh over 24 hours.
After allowing for conversion losses and some reserve capacity, approximately 5–7 kWh of usable storage could be a reasonable starting point.
Moving up to 8–10 kWh would provide more flexibility if your outage lasts longer than expected.
Example 2: Partial-Home Backup for 24 Hours
Now suppose you also want to run a furnace blower, microwave, additional outlets, a sump pump, and more household electronics.
You may use somewhere around 8–12 kWh over the day.
A battery system providing roughly 10–15 kWh of usable capacity could make more sense.
However, you would also need to confirm that the inverter has enough output to handle the largest simultaneous loads.
Example 3: Whole-Home Backup With Managed HVAC
Imagine you want most household circuits available while also operating central air conditioning periodically.
Your home might consume 15–25 kWh during a carefully managed outage day.
In that case, 20–30 kWh of usable battery capacity becomes much more realistic.
You could extend runtime by temporarily avoiding the dryer, electric oven, EV charger, and other heavy loads.
Example 4: High-Demand All-Electric Home
An all-electric house can be a completely different situation.
You may have:
- Heat pump
- Electric water heater
- Electric range
- Well pump
- Central air conditioning
- Multiple refrigerators
- Large electronics load
A home like this may require 30–60 kWh or more for a full day of broad backup.
Cold winter weather or very hot summer weather can push consumption considerably higher.
This is why whole-home battery sizing should always be based on actual loads rather than a generic recommendation.
How Long Will a 5 kWh, 10 kWh, 20 kWh, or 30 kWh Battery Last?
Runtime depends on your average electrical load. The following table shows simple theoretical examples assuming the listed battery capacity is fully usable.
| Usable Battery Capacity | 500W Average Load | 1,000W Average Load | 2,000W Average Load |
| 5 kWh | About 10 hours | About 5 hours | About 2.5 hours |
| 10 kWh | About 20 hours | About 10 hours | About 5 hours |
| 20 kWh | About 40 hours | About 20 hours | About 10 hours |
| 30 kWh | About 60 hours | About 30 hours | About 15 hours |
The formula is simple:
Battery capacity in kWh ÷ average load in kW = estimated runtime
For example:
10 kWh ÷ 0.5 kW = 20 hours
Actual runtime will normally be lower because of inverter losses, reserved battery capacity, temperature, battery condition, and changing appliance loads.
Your refrigerator also does not draw the same amount of power every minute, so real household consumption will constantly move up and down.
How Much Battery Capacity Is Enough for 24 Hours?
For 24 hours of essential backup, roughly 5–10 kWh may be enough if you carefully manage appliances.
If you want broader partial-home coverage, 10–20 kWh gives you much more flexibility.
Whole-home backup can easily require 20–40 kWh or more, particularly if you want central air conditioning, pumps, electric heating, or cooking appliances available.
All-electric homes may need even more.
The easiest way to think about it is this:
Your required battery size depends less on the number of rooms in your house and more on what electrical equipment remains running during those 24 hours.
How Much Battery Capacity Do You Need for Two or Three Days?
Multi-day outages change the equation because stored energy can disappear quickly if there is no way to recharge the battery.
For example, a home using 10 kWh of backup energy each day would theoretically need:
- About 20 kWh for two days
- About 30 kWh for three days
Then you would still need to consider efficiency losses and reserve capacity.
That can make very long battery-only backup expensive.
For longer outages, three strategies usually make more sense:
- Reduce unnecessary loads.
- Recharge the batteries with solar.
- Use another backup charging source when appropriate.
This is why I consider recharge capability almost as important as battery capacity when planning for multi-day outages.
A smaller battery that receives reliable solar charging every day can sometimes provide more practical resilience than a much larger battery with no recharge source.
How Solar Panels Change the Battery Capacity You Need
Solar panels can dramatically change the way you size a backup battery. However, solar does not automatically mean unlimited backup power.
Home Backup Without Solar
Without solar or another charging source, your battery contains a fixed amount of stored energy.
Once that energy is gone, you must wait for grid power to return or recharge the battery another way.
That makes battery capacity especially important.
If you regularly experience long outages, you may need considerably more storage.
Home Backup With Solar
A properly designed solar-plus-battery system can recharge during daylight hours while the grid is down.
That can substantially extend your backup duration.
Imagine your household uses 8 kWh during an outage day.
If your solar system successfully replaces most of those 8 kWh during daylight, your battery does not need to hold several days of energy at once.
Instead, the battery mainly helps carry your home through nighttime and periods when solar production is low.
Why Solar Does Not Guarantee Unlimited Backup
Do not assume your solar panels will always refill the battery completely.
Solar production changes because of:
- Cloud cover
- Snow
- Shade
- Winter conditions
- Shorter daylight hours
- Panel orientation
- Household daytime consumption
- Charging limits
Your home may also consume some solar electricity directly before anything reaches the battery.
For reliable outage planning, use conservative solar estimates rather than assuming perfect sunny-day production.
Should You Size the Battery From Your Electric Bill?
Your electricity bill is a useful starting point, but I would not use it as the only sizing method.
Look for your monthly electricity consumption in kWh.
Then calculate:
Monthly electricity use ÷ number of billing days = average daily consumption
For example:
900 kWh ÷ 30 days = 30 kWh per day
That tells you your normal average use.
Now create a second calculation containing only appliances you would actually run during an outage.
You may discover that your 30 kWh normal day becomes a 10 kWh backup day once nonessential appliances are removed.
That second number is usually much more useful for battery sizing.
Does Home Size Determine Battery Capacity?
Not very reliably.
A larger house does not automatically need a larger battery.
A 3,000-square-foot home with gas heating, efficient appliances, and modest electrical usage could consume less electricity than a smaller all-electric house.
The smaller home could have central AC, a heat pump, electric water heating, an EV charger, and electric cooking.
Those appliances matter far more than floor area.
Use your actual electrical loads rather than choosing battery capacity from square footage alone.
Do HVAC, Heat Pumps, and Well Pumps Require More Battery Capacity?
Large motor-driven appliances can affect both battery runtime and inverter sizing. If your home relies on HVAC or pumps during outages, calculate them separately rather than hiding them inside a general household estimate.
Central Air Conditioning
Central AC is usually one of the most demanding loads you can place on a home battery.
It consumes significant energy while operating and may require substantial starting power when the compressor turns on.
Running central AC for several hours can consume more energy than your refrigerator, lights, Wi-Fi, and electronics combined.
If air conditioning is important during an outage, expect your required battery size to rise quickly.
Heat Pumps and Electric Heating
Heat pumps can be efficient, but their electricity use varies dramatically with weather.
A mild day may require modest energy.
Extreme cold or heat can lead to much longer operating periods.
Systems using electric resistance backup heat can create especially heavy loads.
If you’re sizing batteries for winter backup, calculate heating needs using colder-weather conditions rather than annual averages.
Well and Sump Pumps
Pumps create another sizing challenge.
They may operate for relatively short periods, so their total energy use can remain manageable.
However, their startup power can be much higher than their normal running wattage.
Your inverter therefore needs enough surge capacity even when the battery has plenty of stored energy available.
What Happens If Your Home Battery Is Too Small?
An undersized battery may still be useful, but you will have less flexibility during an outage.
Common problems include:
- Backup power ending sooner than expected
- Frequent load shedding
- Limited HVAC operation
- Reduced ability to handle longer outages
- Greater dependence on daytime solar
- Batteries reaching their minimum charge level quickly
- Constant decisions about which appliances to turn off
The solution is not always buying the largest battery possible.
Sometimes reducing unnecessary loads during outages gives you much more value from the capacity you already have.
Can a Home Battery Be Too Large?
Yes, you can buy more battery capacity than you realistically need.
A larger battery gives you longer runtime, but every additional kilowatt-hour costs money.
If your outages usually last only a few hours, buying enough storage for several days may provide little practical benefit.
Solar matters too.
Installing a massive battery bank behind a relatively small solar array may mean the batteries rarely receive a full recharge from solar.
Future needs should still be considered.
If you expect to add a heat pump, electric vehicle, home office, or other large loads, some additional capacity can make sense.
A modular system often gives you the best balance because you can start with your current requirement and expand later.
One Large Battery or Multiple Smaller Batteries?
Home battery systems are increasingly modular, which means you may be able to combine several batteries instead of installing one extremely large unit.
Things to consider include:
- Maximum system capacity
- Battery compatibility
- Inverter compatibility
- Available installation space
- Future expansion options
- Continuous output limits
- Surge power
- Installation cost
- Solar charging capacity
One important point is that adding another battery does not always increase inverter output.
Some systems let additional modules increase stored energy while the maximum power output remains unchanged.
Always verify both specifications before assuming two batteries provide twice the operating power.
Common Home Battery Sizing Mistakes
Battery sizing becomes much easier when you avoid a few common mistakes.
- Choosing capacity from house square footage
- Confusing kW with kWh
- Using total capacity instead of usable capacity
- Reducing usable capacity twice
- Ignoring inverter losses
- Ignoring motor startup requirements
- Assuming appliances run continuously
- Using normal daily consumption instead of outage consumption
- Forgetting HVAC requirements
- Assuming perfect solar production every day
- Ignoring winter and summer energy differences
- Forgetting future electrical loads
- Buying huge capacity without enough charging capability
- Focusing on battery capacity while ignoring inverter output
The biggest mistake is usually looking for one universal battery size.
There isn’t one.
Your appliances and desired runtime determine the answer.
A Simple Home Battery Sizing Checklist
Before choosing a home backup battery, check each of these points.
- List your critical appliances.
- Find their running wattage.
- Check motor starting or surge requirements.
- Estimate daily operating hours.
- Calculate total daily backup kWh.
- Decide how many backup hours or days you need.
- Check usable battery capacity.
- Allow for conversion losses.
- Include a practical reserve.
- Confirm continuous inverter output.
- Confirm surge output.
- Check whether you need 120V and 240V power.
- Estimate solar recharge capability.
- Check system expansion limits.
- Consider future appliances.
- Review battery warranty coverage.
- Confirm installation location requirements.
- Use proper transfer and isolation equipment.
- Follow applicable electrical and fire-safety requirements.
Taking ten minutes to work through this checklist can prevent you from spending thousands of dollars on the wrong size system.
Final Words
For most homeowners, somewhere around 10–30 kWh is a useful range to investigate for home backup, but your actual requirement could be much lower or considerably higher.
A household wanting only refrigeration, Wi-Fi, lights, and electronics may be comfortable with 5–10 kWh. Add HVAC, pumps, cooking equipment, electric water heating, and other large loads, and 20–40 kWh or more can become necessary very quickly.
My recommendation is to calculate your critical loads first. Then verify the battery’s usable kWh, continuous inverter power, surge capability, and recharge options. That approach gives you a backup system designed around your home instead of a generic number.
Related FAQs
Here are quick answers to some of the most common questions homeowners have when comparing home battery capacity.
How Many kWh of Battery Backup Do I Need for a House?
Around 5–10 kWh may cover essential loads, while 10–20 kWh works better for broader backup. Whole-home systems often require 20–40 kWh or more depending on HVAC, pumps, and other large appliances.
Is 10 kWh Enough to Power a House?
A 10 kWh battery can provide useful home backup if you focus on refrigerators, lights, Wi-Fi, electronics, and selected outlets. It may not provide long whole-home runtime if you frequently use central AC, electric heat, or other high-demand appliances.
Is a 20 kWh Battery Enough for Home Backup?
A 20 kWh battery can provide substantial backup for many homes and may cover essential or partial-home loads for a full day. Runtime will be much shorter if you continuously operate HVAC, electric water heating, or other heavy loads.
How Long Will a 10 kWh Battery Power a House?
A 10 kWh usable battery could theoretically support a 500-watt average load for about 20 hours or a 1,000-watt load for about 10 hours. Real runtime will be slightly lower because of system losses and reserved capacity.
How Much Battery Storage Do I Need for 24 Hours?
Around 5–10 kWh may cover carefully managed essentials for 24 hours, while broader household backup can require 10–20 kWh. Whole-home backup with HVAC may require 20–40 kWh or significantly more.
How Many Batteries Do I Need for Whole-Home Backup?
The number depends on each battery’s usable capacity. If you need 30 kWh and each battery provides 10 kWh usable capacity, you may need approximately three units, assuming the system supports that configuration.
Can a 5 kWh Battery Run a Refrigerator and Lights?
Yes. A 5 kWh battery can generally provide useful runtime for a refrigerator, LED lighting, Wi-Fi, and several small electronics. Exact runtime depends on refrigerator cycling, lighting use, battery losses, and other connected appliances.
How Much Battery Capacity Do I Need to Run an Air Conditioner?
The answer depends heavily on the AC’s size, efficiency, startup requirements, and operating hours. A central air conditioner can consume several kilowatt-hours during extended use, so AC backup often pushes home battery requirements into the 15–30 kWh range or higher.
Do I Need More Battery Capacity If I Have a Heat Pump?
Usually, yes. A heat pump can become one of your home’s largest outage loads, particularly during very hot or cold weather. Calculate its expected runtime separately and make sure your inverter can handle both normal operation and startup demand.
Can Solar Panels Recharge My Home Battery During an Outage?
They can if your solar-plus-storage system is designed to operate while disconnected from the utility grid. Available charging will still depend on sunlight, weather, solar array size, household consumption, and the battery’s charging limits.
Should I Use Usable or Total Battery Capacity When Sizing?
Use usable battery capacity whenever the manufacturer provides it. Usable capacity represents the energy actually available for your home and gives you a more realistic runtime estimate than total nameplate capacity.
How Much Extra Battery Capacity Should I Add as a Buffer?
Adding roughly 10–20% of practical reserve capacity can help cover unexpected loads, system losses, weather changes, battery aging, and slightly longer outages. The exact buffer depends on how conservative you want your backup plan to be.
Blake Cooper writes practical guides on generators, portable power stations, and home backup systems. He focuses on simplifying power topics and helping homeowners make informed buying decisions.
