How Much Battery Backup Do I Need for My House?

It depends on what you want to run and for how long. A small essential-loads system might need roughly 4 to 10 kWh. A whole-home system with air conditioning or electric heat often needs 30 kWh or more.

The table below shows planning ranges built from the worked examples in this guide. They are not manufacturer specs, so treat them as a starting point.

System type	Typical continuous load	Daily energy	Runtime goal	What it can run
Essential loads	0.5 to 2 kW (surge 3 to 5 kW)	About 5 to 10 kWh	Overnight to 1 day	Fridge, lights, Wi-Fi, phones, sump pump, medical devices
Expanded circuits	3 to 7 kW	About 10 to 25 kWh	1 day, longer with solar	Above, plus a well pump, gas furnace fan, microwave
Whole home	8 to 20+ kW	30+ kWh	About 1 day, or less if AC or heat runs	Central AC, electric range, dryer, electric heat

Expert Insight: Don’t start with the biggest battery on the market. Start with a list of what you cannot live without. That list sets the price.

What is a home backup battery?

A home backup battery stores electricity and sends it to your house when the grid goes down. It works with an inverter, which turns the battery’s DC power into the AC power your outlets use.

Most systems sit on a dedicated panel, often called a critical-loads or backup panel. When the grid fails, an automatic transfer device disconnects your home from the utility and the battery takes over. This disconnect protects line workers from backfeed. With solar, the panels charge the battery during the day. Without solar, the battery recharges from the grid once power returns.

What is the difference between battery capacity in kWh and inverter output in kW?

kWh is the size of the fuel tank. kW is how fast you can pull fuel out. Both must be big enough, and one does not make up for the other.

A 13 kWh battery with a 3 kW inverter can run a fridge for a long time. It may still fail to start a well pump or air conditioner, because those need a large burst of power for a second or two. A small battery with a big inverter has the opposite problem. It can start the motor but runs dry fast.

You will also see nominal capacity and usable capacity. Nominal is the number on the box. Usable is what you can actually draw after depth-of-discharge limits and losses. Always compare usable kWh.

How do I calculate the battery size for my home?

Use this formula: required battery kWh = average backup load in kW × backup hours ÷ inverter efficiency ÷ usable depth of discharge, then add a planning reserve. The AC energy you need is the load times the hours. Inverter losses and depth of discharge mean the battery must be larger than that number.

homeowner writing appliance wattages on a notepad next to a utility bill at a kitchen table

Calculator worksheet

List your critical loads. Write down each appliance and its running watts from the nameplate or a plug-in watt meter.
Estimate hours of use. Count how many hours each runs in your outage window.
Find daily kWh. Watts × hours ÷ 1,000 for each item, then add them up.
Divide by inverter efficiency. Use 0.90 if the spec sheet doesn’t say.
Divide by usable depth of discharge. Use 0.90 as a planning figure, or the value the manufacturer states.
Add a reserve. A 20 percent cushion is a common planning choice. It covers cold weather, aging, and surprise loads.

For a published example, a 2 kW load for 12 hours is 24 kWh. At 90 percent efficiency, that needs about 26.7 kWh of battery before other design factors. Another sizing example says a home using 30 kWh a day, wanting one day of backup, with lithium iron phosphate batteries at 80 percent depth of discharge and 95 percent round-trip efficiency, needs roughly 40 kWh installed.

How do I estimate daily energy use from my bills?

Pull 12 months of utility bills and find the highest-use month. Divide its kWh by about 30 to get a daily figure. This shows what the whole house uses, which is the upper limit for a whole-home system. For essential circuits, the appliance list above gives a much smaller number.

If you want to cross-check your math, the home battery backup sizing calculator at TheCalcs runs similar inputs for kWh and runtime.

What size battery backup do I need for my refrigerator, lights, internet, and sump pump?

For these five loads, about 4 kWh covers one night and about 8 kWh covers a full day. These numbers use the assumptions below, so you can redo them with your own appliances.

Assumptions: 90% inverter efficiency, 90% usable depth of discharge, 20% reserve. Appliance numbers are typical planning values. Measure your own.

Load	Assumption	12-hour kWh	24-hour kWh
Refrigerator	150 W while running, about 10 hours per day	0.75	1.5
LED lights	100 W total, 4 to 5 hours	0.4	0.5
Modem and router	25 W, always on	0.3	0.6
Phone charging	A few phones and a tablet	0.1	0.1
Sump pump	800 W, 1.5 hours or 3 hours	1.2	2.4
Total AC energy		2.75	5.1

Now the math:

Overnight: 2.75 ÷ 0.90 ÷ 0.90 = 3.4 kWh. With 20 percent reserve, about 4.1 kWh.
24 hours: 5.1 ÷ 0.90 ÷ 0.90 = 6.3 kWh. With reserve, about 7.6 kWh.
Three days, no recharge: 15.3 ÷ 0.81 = 18.9 kWh. With reserve, about 22.7 kWh.

Running watts add up to about 1.1 kW. That is a modest continuous load, but the sump pump and fridge both surge on startup, so the inverter needs more than 1.1 kW. More on that next.

What about medical equipment?

Check the device’s label for watts. A CPAP machine is often a low draw, while oxygen concentrators draw more. Add them to the table above with the real hours of use. If a device is life-critical, ask your doctor and installer about extra reserve and a backup plan beyond the battery.

How do I account for starting surges from HVAC systems, well pumps, and refrigerators?

Find the starting (surge) rating on each motor’s nameplate or manual, then make sure the inverter’s surge rating covers the largest motor plus whatever else is running at that moment. Motors can pull several times their running watts for a moment when they start.

Planning habits that help:

Check the nameplate for locked-rotor amps (LRA) on compressors and pumps.
Add up only the loads that could start together, not everything in the house.
Ask about soft-start kits for air conditioners and pumps. They reduce the starting burst.
Look at both the continuous and the surge rating on the inverter spec sheet. Surge ratings usually last only seconds.

For the example above, a 3 kW continuous inverter with a higher surge rating is a reasonable target. Confirm with your installer, since sump pumps vary a lot.

Expert Insight: This is why a big-kWh battery can still fail to start a well pump. The battery has the energy, but the inverter cannot deliver the burst.

How much battery storage do I need for whole-home backup versus essential circuits?

Essential circuits usually need a fraction of the battery that whole-home backup needs. Backing up only selected circuits can be substantially smaller than a system meant to run the entire house, because you size from the critical-load profile instead of total household use.

Here is what happens when you add big loads to the 24-hour example. Same assumptions: 90% efficiency, 90% depth of discharge, 20% reserve.

Add-on	Assumption	Extra AC kWh	Extra battery kWh
Well pump	1 hp, about 1 kW, 1.5 hours; large surge	1.5	About 2.2
Central AC	About 3 kW for 8 hours	24	About 35.6
Electric resistance heat	About 5 kW for 8 hours	40	About 59.3

A well pump is a small energy add but a big surge problem. Air conditioning and electric heat are the opposite: they eat energy and power both.

If you only need to protect food, internet, lights, and a sump pump, do size an essential-circuits system.

If you have a well, do check the pump’s surge rating before choosing the inverter.

If you want central AC or electric heat, do price a whole-home system, or plan to run only one zone, or use a generator for the big loads.

If you use gas heat, do remember that the furnace blower and controls still need power.

How do battery efficiency, depth of discharge, reserve, and degradation affect sizing?

Each one makes the battery you buy larger than the energy you use. Efficiency is the share of stored energy that survives the trip through the inverter. Depth of discharge is how much of the battery the system lets you use. Reserve is the cushion you add on purpose.

Efficiency: Lithium systems are often rated in the 90 to 95 percent range. Use the spec sheet figure.
Depth of discharge: Many lithium iron phosphate batteries allow 80 percent or more. Some systems also hold back a backup reserve for storm mode.
Degradation: Capacity falls over the years. Read the warranty for the guaranteed end-of-life capacity and add margin if you want the same runtime in year 10.
Temperature: Cold garages can limit output on some batteries. Check the operating range.

Briggs & Stratton explains these steps in its article How to Right-Size Your Battery Storage System.

Solar, generator, and grid recharging: how long can the battery last?

A battery without a recharge source is a fixed supply. Solar, a generator, or the returning grid decides whether you survive a multi-day outage.

rooftop solar panels on a suburban house under a clear blue sky

To estimate daily solar recharge, use this formula: array watts × peak sun hours × system losses. For a 5 kW array, 4.5 peak sun hours, and 25 percent losses (0.75): 5 × 4.5 × 0.75 = about 16.9 kWh per day. Peak sun hours range across the US, often from about 3 in cloudy northern winters to 6 or more in the Southwest. Look up your own location.

That 16.9 kWh would cover the 5.1 kWh essential-loads day with room to spare. Cloudy storm weather can cut it sharply, so don’t count on full output.

Key points:

Standard grid-tied solar inverters shut off during an outage. You need a battery-ready or hybrid inverter to keep solar running.
A generator can recharge the battery or share load. It must connect through a proper transfer arrangement installed by an electrician.
Grid recharge costs money. At the US average of about 17 cents per kWh (per the U.S. Energy Information Administration), a full 10 kWh recharge costs about $1.70. Your state’s rate may be roughly 12 to 40 cents.
Plug-in solar rules vary by state and change quickly. Confirm with your utility before connecting anything. Never use extension cords to power a house, and never bypass a microinverter’s grid shut-off.

Can I build a DIY battery backup for my house safely?

For wiring into your house, no. DIY batteries involve high fault currents, thermal risks, and grid isolation problems that can hurt people. A household battery installation should be designed and inspected by qualified professionals.

Here is what makes it risky:

Backfeed: Without a proper transfer switch or listed backup panel, your system can send power onto utility lines and endanger line workers.
Fault current: Large battery banks can deliver enormous current in a short circuit. Wrong fuses or breakers can cause fires.
Thermal risk: Batteries need correct charging limits, a battery management system, and safe placement.
Inverter compatibility: Mixing parts that don’t communicate can create faults or void warranties.
Permits and code: Most areas require permits and inspection. Insurance claims can be refused for unpermitted work.

A safe DIY option is a portable power station used with a few plugged-in devices, such as a router or a lamp, using the unit’s own outlets. Plug each device directly into the unit. Don’t backfeed an outlet. For anything connected to your panel, hire a licensed electrician.

What affects cost, and what should I check before I buy?

Battery price is only part of the bill. Permits, panel upgrades, a new backup subpanel, wiring, and installation delays can add up. Get written quotes that list every line.

Pre-purchase checklist

Load measurements: Use a plug-in watt meter on key appliances. Don’t rely on guesses.
Surge ratings: Confirm the inverter can start your largest motor.
Transfer equipment: Ask what automatic transfer device or backup panel is included.
Permits: Ask who pulls them and how long your jurisdiction takes.
Warranty terms: Check years, throughput limits, and end-of-life capacity.
Installer support: Ask who services the system after installation, and how fast.
Emergency recharge: Plan for solar, generator, or grid recharge.
Replacement plan: Ask what happens when the battery nears end of life.

For another set of inputs to compare with your own math, try the Hyre Solar battery sizing tool.

What do owners say?

Owners like

Fridges, internet equipment, lights, medical devices, and selected outlets keep running automatically during outages.
Quiet operation, automatic switchover, app-based monitoring, and the ability to recharge from solar.
Reliable operation over the long term when the system is correctly installed and maintained.

Common complaints

Installation and permitting delays.
Batteries going offline or needing service after installation.
Poor post-installation support, slow warranty responses, and trouble finding qualified technicians.
Advertised capacity not matching usable backup energy after efficiency losses and reserve settings.
Essential-load systems that can’t support central air conditioning, electric heating, or well pumps.

Summarised from owner reviews and long-term user reports; individual experiences vary.

Frequently Asked Questions

How much battery backup do I need for my house calculator?

Use the worksheet above: battery kWh = average backup load in kW × hours ÷ inverter efficiency ÷ depth of discharge, plus a 20 percent reserve. Online tools such as the TheCalcs backup sizing calculator follow the same logic. Check that any calculator you use includes surge loads and usable capacity.

How much battery backup do I need for my house?

For a fridge, lights, Wi-Fi, phones, and a sump pump, about 4 kWh covers one night and about 8 kWh covers a full day. Adding a well pump adds about 2 kWh. Adding central AC or electric heat adds dozens of kWh.

How much battery storage do I need for my house?

Check your highest-use month on your utility bills and divide by 30 for daily kWh. A home using 30 kWh a day that wants one day of whole-home backup needs roughly 40 kWh installed in one published lithium iron phosphate example. Backing up only critical circuits cuts that number sharply.

What size battery backup do I need for my house?

You need two sizes: battery kWh for runtime and inverter kW for power. Match the inverter’s continuous rating to your running loads and its surge rating to your largest motor start. Then size the battery for your outage length, with efficiency, depth of discharge, and reserve built in.

What is a home backup battery?

It is a rechargeable battery with an inverter that powers your home when the grid fails. It can charge from the grid or from solar panels. An automatic transfer device disconnects your house from the utility during an outage.

DIY battery backup for house: is it safe?

Wiring a battery bank into household circuits is not a safe DIY job. Isolation from the grid, fault currents, battery heat risks, and local code all need a licensed electrician and inspection. Use portable power stations for small loads, plugged in directly, with no extension-cord tricks to feed the house.

Sources

Wattfolk Editorial Team

The Wattfolk Editorial Team researches plug-in solar, home batteries and ways to cut electricity costs. Every guide is built from manufacturer datasheets, certification listings, government energy data, state laws and verified owner feedback, with the assumptions behind every savings estimate shown so readers can check the numbers for their own home.