How Many kWh of Battery Do You Need for Whole-Home Backup? (Worked Examples)

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"Whole-home backup" can mean anything from keeping the fridge and lights on for a night to running central air for a week. The battery size for those two goals differs by a factor of ten or more. This guide shows how to size a battery from your own loads, step by step, and works through three example houses.

Two numbers, not one

Every home battery has two separate limits:

1. Energy (kWh): how long it lasts. This is the battery's capacity.
2. Power (kW): how much it can run at once, and how big a motor it can start. This is the inverter's continuous and surge rating.

A large battery with a small inverter can run your essentials for days but may not start your central AC. A big inverter with a small battery can start the AC but only run it for a couple of hours. Size both.

Step 1: Know your baseline

Your utility bill shows monthly kWh. The U.S. Energy Information Administration reports that in 2022 the average U.S. residential customer bought 10,791 kWh per year, about 899 kWh per month. That works out to roughly 29.6 kWh per day. Your home could be far above or below that. Electric heat, electric water heaters, pools and EV charging push it up.

You almost never need to back up your full average day. In an outage most people skip laundry, the dryer, the oven and EV charging. So the useful number is your outage day, built from the loads you will actually run.

Step 2: List outage loads and their daily energy

For each appliance: watts × hours per day = Wh per day. For cycling appliances like fridges and pumps, use annual kWh from the EnergyGuide label, or measure with a plug-in meter. Our other guides walk through the trickier ones:

• Refrigerator runtime during an outage
• Sump pump runtime on a power station
• Can a power station run a gas furnace blower?
• Power station for a 240V well pump
• CPAP runtime on a power station

The example numbers below come from the worked examples in those guides. They are illustrations for the math, not measurements of any specific home. Use your own appliance labels or a plug-in meter for real numbers.

Step 3: Add the inverter's idle draw

The inverter uses power all day just by being on. BLUETTI lists 30W for its AC500 with outputs on; Jackery lists 50W standby for the Explorer 5000 Plus. We did not find published idle/self-consumption figures for the EcoFlow DELTA Pro Ultra X or Tesla Powerwall 3. We use 30W × 24h = 0.72 kWh/day as an example.

Step 4: Convert to battery size

For battery systems rated in DC capacity (most power stations, including EcoFlow, Jackery, Anker and BLUETTI):

Battery kWh needed = outage kWh per day × days ÷ (depth of discharge × inverter efficiency)

We assume 90% depth of discharge and 85% inverter efficiency, so the divisor is 0.765. Adding idle draw on top of that is slightly conservative, which is the right direction for backup planning.

Important: Tesla rates Powerwall 3 in AC energy. Its datasheet lists 13.5 kWh AC nominal battery energy (at 25°C, beginning of life). That is energy after conversion, so you do not apply the inverter-loss factor again. You would still keep a backup reserve. When comparing a "6,144Wh" EcoFlow battery with a "13.5 kWh" Powerwall, remember they are measured differently. See our DELTA Pro Ultra vs Powerwall 3 comparison.

Worked example A: essentials, gas heat, city water

Battery (DC-rated) needed: 10.06 ÷ 0.765 = 13.2 kWh for 1 day, 26.3 kWh for 2 days, 39.5 kWh for 3 days.

Worked example B: essentials plus a well pump and CPAP

Add to example A:

Battery needed: 17.1 kWh for 1 day, 34.2 kWh for 2 days, 51.3 kWh for 3 days. The well pump also sets a power requirement: 240V output and enough surge to start it.

Worked example C: summer, with central air

Take example B, remove the furnace blower (3.84), and add a central AC drawing an example 3,500W for 8 hours of compressor runtime (28 kWh). That wattage is an example within the 3,000-3,500W running range Champion Power Equipment's generator wattage chart lists as typical for a 3-ton central AC; the 8 hours is an assumption. Real values depend on tonnage, efficiency and weather, so check your condenser's nameplate.

• Subtotal: 12.36 − 3.84 + 28.0 = 36.52 kWh
• Plus idle: 37.24 kWh per outage day
• Battery needed: 48.7 kWh for 1 day, 97.4 kWh for 2 days, 146 kWh for 3 days

Central air turns a modest backup into a large one. It also needs a big inverter: the AC compressor's starting current is the critical spec. Tesla lists Powerwall 3's load start capability as 185 LRA; EcoFlow lists DELTA Pro Ultra X at 12kW continuous; for the DELTA Pro 3, EcoFlow says it can power a 3-ton central AC, noting the test used a soft starter.

Turning kWh into hardware

Battery modules and nominal sizes from manufacturer spec pages:

Example A for 2 days (26.3 kWh DC-rated) would be, on paper, 5 DELTA Pro Ultra X batteries (30.7 kWh), or roughly 2 Powerwall 3 units on an AC basis (10.06 kWh × 2 days = 20.1 kWh AC, before reserve). These are capacity matches only; inverter power and installation requirements still need to fit.

The site's owner bought an EcoFlow DELTA Pro Ultra X with 4 batteries and is setting it up now (24.576 kWh nominal). With our 0.765 factor that is about 18.8 kWh of usable AC energy: a little under 2 days of example A, or about half a day of example C. We'll add real measurements once it's running.

Solar changes the math

With solar charging, the battery only needs to cover nights and cloudy stretches, not every hour. How much a given array produces depends on location, tilt, shading and season. The PVWatts calculator (pvwatts.nlr.gov, from the National Laboratory of the Rockies, formerly NREL) estimates monthly production for your ZIP code. Use it before you size around solar.

The site's owner has ordered 22 CW Energy 450W panels (9.9 kW rated) planned for the DELTA Pro Ultra X. They are not installed yet, so we make no production claims here. Rooftop or permanent arrays must be designed and installed by a licensed electrician, with permits and NEC rules including rapid shutdown.

Tax credit note

The IRS says the federal Residential Clean Energy Credit (Section 25D, 30% of qualified costs, including battery storage of at least 3 kWh) is not available for property placed in service after December 31, 2025. That does not mean every incentive is gone. State programs and utilities may still offer rebates or rate plans. Check with your state energy office and your utility.

Installation

Any system that powers your home's circuits, whether a smart panel, a transfer switch or a grid-tied battery, must be installed by a licensed electrician, with permits and inspection where required. Portable stations used with extension cords avoid that, but cover far less.

FAQ

Is 10 kWh enough for whole-home backup? For essentials over one day, maybe; in example A, about 10 kWh of AC energy covered a day. For central air or electric heat, no.

Should I size for 1 day or 3 days? Look at your area's outage history and whether you will have solar or a generator to recharge. Longer, storm-driven outages argue for more capacity or a recharge source.

Can I start small and add batteries? Most systems above are expandable. Check the maximum per inverter before you buy.

Do I need to cover my peak power or my daily energy? Both. Daily energy sizes the battery. Peak and surge size the inverter.

Product links: Check price EcoFlow DELTA Pro Ultra X · Check price EcoFlow DELTA Pro Ultra · Check price EcoFlow DELTA Pro 3 · Check price Jackery Explorer 5000 Plus · Check price Anker SOLIX F3800 · Check price BLUETTI AC500