Home battery backup: the three classes, what each actually runs, and how to size yours

“Home battery backup” is one search that covers three very different purchases: a box you plug the refrigerator into, a system on wheels that an electrician connects to a few circuits, and a battery bolted to the garage wall that takes over the whole house. The right one depends on one number, kilowatt-hours per outage day, and that number depends on what you decide must stay on. This page is how to find it, what each class delivers, and where the honest boundaries are. The sizing calculator does the arithmetic for your own list.

First, the number: kilowatt-hours per outage day

Every appliance page on this site publishes a sourced draw and a duty cycle. Added up for the loads most households call essential:

Load	Daily watt-hours at the outlet	Source
Refrigerator, 18 cu ft	about 1,250 Wh	ENERGY STAR annual figure, refrigerator page
CPAP with humidifier, 8 h	about 425 Wh	ResMed user guide, CPAP page
Router, phones, a few LED lights	about 700 to 900 Wh	nameplates
Sump pump in steady rain	about 6,700 Wh	Zoeller nameplate at 25% duty, sump pump page
Well pump, 1/2 HP	about 2,800 Wh	Franklin motor data at a heavy 10% duty, well pump page

The three-day plan runs these rows for four house sizes. The first three rows come to roughly 2.5 kWh a day, about 3 kWh of usable battery once the inverter’s loss is included. Add the pump that applies to your house and the number doubles or triples. For scale, the average American household bought 10,260 kWh in 2023 according to EIA, about 28 kWh a day; running the house as if nothing happened is ten times the essentials, and that gap is the whole difference between the classes.

The three classes

Portable power stations, 1 to 4 kWh. A single box with outlets. You carry it to the appliance, or leave it plugged in as a UPS so the refrigerator never sees the outage. No installation, no permit, and the class most households actually need: a 2 kWh unit is a day of essentials or a night of a cycling sump pump. The appliance pages rank them, and so do the 1 kWh and 2 kWh roundups. Their limit is the inverter, usually 1,800 to 2,600 W, and 120 V only, which rules out a well pump or a central air conditioner.

Expandable systems, 4 to 12 kWh. The same idea with extra batteries stacked underneath and a larger inverter, some with 240 V output. The Anker SOLIX F3800 and EcoFlow DELTA Pro Ultra on the well pump page are this class, ranked on the expandable roundup: 6,000 to 7,200 W, enough to start a well pump, expandable to tens of kilowatt-hours, and connectable to a few circuits through a transfer switch an electrician installs. Days of essentials plus one big motor load, still on wheels.

Installed home batteries, 10 kWh and up. Wall-mounted, permanent, wired to the panel through a gateway that disconnects the house from the grid automatically. Tesla’s Powerwall 3 datasheet lists 13.5 kWh, up to 11.5 kW continuous and a 185 A locked-rotor start capability; FranklinWH’s aPower 2 lists 15 kWh, 10 kW continuous, 15 kW for ten seconds, the same 185 A start, and a 15-year warranty. Both state UL 9540. This is the class that runs the air conditioner and the well pump together, and it is the class with an installer, a permit and an inspection attached. The whole-house page covers it.

Which class, in one table

If your essentials list is	And the largest single load is	Class
Fridge, CPAP, phones	Nothing with a motor over 1,500 W	Portable, 1 to 2 kWh
The above plus a sump pump or chest freezer	A 120 V pump	Portable, 2 to 4 kWh, or expandable
The above plus a well pump	A 240 V motor	Expandable with 240 V output, through a transfer switch
The whole house, including central air	A central AC compressor	Installed, sized by an electrician

The boundary that decides more than capacity

A 240 V load, a well pump or a central air conditioner, cannot be run from a 120 V portable unit no matter how large, and a motor’s start-up draw decides whether any battery works before its watt-hours matter. Franklin’s 1/2 HP well pump motor starts at about 7,400 VA; Powerwall 3 and aPower 2 both quote a 185 A locked-rotor start, which at 240 V is about 44,000 VA, the reason installed systems handle a central air conditioner that no portable unit can. The mechanism is in running versus surge watts.

Battery or generator

For hours to a couple of days, a battery is silent, produces no carbon monoxide and needs no fuel run. For a week with no grid and no sun, a generator or a generator-charged battery is the only thing that keeps working. The battery versus standby generator page puts the two on the same scale.

What connects it to the house

A portable unit plugs into an appliance. Anything that feeds a circuit, from an expandable system on a transfer switch to an installed battery on a gateway, is a licensed electrician’s project with a permit behind it, and the transfer switch page explains conceptually what that work is and why the shortcut is dangerous. Nothing on this site is a wiring instruction.

Incentives, plainly

The federal residential clean energy credit, which covered battery storage of 3 kWh or more at 30%, ended for systems placed in service after December 31, 2025 under 2025 legislation. State and utility programs continue in some places; the DSIRE database at dsireusa.org lists them by state, and the IRS page for Form 5695 has the federal rules. We do not compute anyone’s credit.