Home batteries are sold by the kilowatt-hour, and every extra kWh costs real money. Buy too small and the lights go out halfway through an outage. Buy too big and you have paid thousands of dollars for capacity that sits at full charge most of the year. Sizing is the single decision that determines whether a battery feels like a smart purchase or an expensive regret.
The good news is that the math is not complicated. You need three things: a clear job for the battery, an honest look at your electricity use, and an understanding of the difference between capacity and power. Here is how to work through it.
Start With the Job You Want the Battery to Do
Batteries get bought for three different reasons, and each one points to a different size.
- Backup power. You want the fridge, some lights, the internet and maybe the furnace blower to keep running when the grid goes down. This is a runtime question, not a whole-home question.
- Solar self-consumption. You have panels and your utility no longer pays retail rates for exports, so you want to store the afternoon surplus and use it after sunset. Size follows your evening load, not your total daily use.
- Rate arbitrage. You are on a time-of-use plan and want to charge cheap and discharge during expensive peak hours. Size follows the length of the peak window.
Many homeowners want all three. That is fine, but decide which one matters most, because it sets the floor for your capacity.
Step 1: Find Your Real Daily Usage
Pull twelve months of utility bills, or download your interval data if your utility offers it. According to the U.S. Energy Information Administration, the average American residential customer used roughly 865 kWh per month in 2024, which works out to about 29 kWh per day. State averages vary enormously, from around 6,200 kWh a year in Hawaii to nearly 14,800 kWh in Louisiana.
Your own number is what matters. Divide your highest-use month by the number of days to get your worst-case daily figure, and your lowest-use month for the best case. If you have never done this exercise, our DIY home energy audit walkthrough shows how to read your meter and spot where the kilowatt-hours are going.
Here is the important part: almost nobody needs to back up their entire daily load. A whole-home 29 kWh day would require a very large and very expensive battery bank. Backup sizing works from a much smaller list.
Step 2: Build a Critical Loads List
Write down what genuinely has to run during an outage and estimate the daily energy each item consumes. Approximate figures for a typical home:
- Refrigerator or freezer: roughly 1 to 2 kWh per day
- LED lighting for the rooms you actually use: under 1 kWh per day
- Internet router, modem and phone charging: well under 1 kWh per day
- Furnace or air handler blower: roughly 2 to 5 kWh per day in heating season
- Well pump or sump pump: roughly 1 to 3 kWh per day depending on cycles
- Medical equipment such as a CPAP: usually under 1 kWh per night
Add them up and most households land somewhere between 5 and 12 kWh per day for essentials only. Central air conditioning, electric ovens, clothes dryers and electric water heaters are the loads that blow the budget, which is why installers usually leave them off the backup panel entirely.
Step 3: Turn Daily Load Into Battery Capacity
Once you have a daily critical load, multiply by how many days of autonomy you want:
- 8 kWh per day of essentials with a one-day outage target means roughly 8 to 10 kWh of usable capacity.
- The same 8 kWh with a two-day target means roughly 16 to 20 kWh, or two battery units.
If you have solar panels, the arithmetic changes in your favor. A working array recharges the battery every sunny day, so a single 10 to 13.5 kWh unit can carry essential loads through a multi-day outage rather than a single night. Just make sure your system is configured for islanding, since a grid-tied array without battery-capable inverters shuts down when the grid does. Our guide to how home battery storage works covers that wiring distinction in more detail.
Capacity Is Not the Same as Power
This is where sizing goes wrong most often. Capacity, measured in kWh, is how much energy the battery holds. Power output, measured in kW, is how much it can deliver at any instant.
A 13.5 kWh battery rated for 5 kW continuous output can run a fridge, lights and a blower comfortably. Ask it to start a 4-ton air conditioner or an electric range at the same time and it will trip on overload even though it is fully charged. Motor-driven appliances also draw a surge on startup that can be several times their running wattage.
So check two numbers on the spec sheet: continuous kW and surge kW. If you want to back up an AC compressor or a well pump, either choose a higher-power unit, stack two batteries, or fit a soft-start kit on the compressor.
Usable kWh, Depth of Discharge and Efficiency
Manufacturers quote nominal capacity, but you rarely get all of it. Two adjustments matter:
- Depth of discharge. Modern lithium iron phosphate batteries typically allow 90 to 100 percent usable capacity, while older chemistries reserve more. Look for the “usable” figure, not the nominal one.
- Round-trip efficiency. Charging and discharging loses energy, usually around 5 to 10 percent. A battery advertised at 90 percent efficiency returns about 9 kWh for every 10 kWh you put in.
Build a modest cushion into your estimate rather than sizing to the exact number. Capacity also fades slowly over the warranty period, typically to around 70 percent of original after ten years.
What the Capacity Will Cost
Installed pricing in 2026 generally falls between roughly $700 and $1,300 per kWh, with a typical 10 to 13.5 kWh system landing somewhere around $10,000 to $16,000 before any incentives. Costs vary widely with brand, chemistry, electrical panel work and local labor rates, so treat these as planning ranges and get at least three quotes.
One important change: the 30 percent federal Residential Clean Energy Credit (Section 25D), which previously covered home battery storage, was terminated for systems placed in service after December 31, 2025. Homeowners who commissioned a system before that date can still claim it on their return. Third-party-owned systems such as leases and power purchase agreements may still access the commercial Section 48E credit, and many states and utilities run their own storage rebates or demand-response payments. Verify current federal rules with the IRS and check your state energy office and utility before budgeting.
Sizing for Bill Savings Instead of Backup
If outages are rare where you live and the goal is a lower bill, size to your peak window rather than your outage. Find the hours your utility charges the most, add up how much electricity you consume in that window on a typical weekday, and buy roughly that much usable capacity.
For many households that is 5 to 10 kWh, which is smaller and cheaper than a backup-oriented system. Our explainer on time-of-use electricity rates shows how to identify those windows, and if you export solar, how net metering pays for exported power will tell you whether storing surplus beats selling it.
The Bottom Line
Do not start with a battery model and work backwards. Start with the job, then the load list, then the runtime you want, and let those three numbers tell you the capacity. Most homeowners sizing for essential backup land in the 10 to 20 kWh range, while those chasing time-of-use savings often do well with less. Check the kW output rating as carefully as the kWh capacity, use the usable figure rather than the nominal one, and leave a small cushion for efficiency losses and long-term degradation.
Incentives, electricity rates and equipment prices change frequently and differ by state, province and utility. Confirm the current numbers locally before you sign anything.
