How Much Backup Power Does a Home Actually Need?

The average U.S. residential customer buys about 28.4 kWh of electricity per day from the grid. That does not mean the average home needs 28 kWh of batteries for every day of backup.

During an outage, most households can reduce or shut off some of their largest loads. Laundry, EV charging, electric cooking and parts of the heating or cooling system may be far less important than refrigeration, communications, lighting or a critical pump.

That makes the first sizing question surprisingly simple:

What actually needs to stay on when the grid is down?

Once that is clear, a backup system has to pass several different tests. Battery capacity alone cannot tell you whether it will work.

What We Found

The average residential utility customer purchased about 28.4 kWh/day in 2024, but a deliberately lean backup load can be far smaller.
Our illustrative Core Essentials profile uses about 2.8 kWh/day; a broader Household Essentials profile uses about 4.9 kWh/day.
A system can have enough battery energy and still fail because it lacks sufficient continuous power, starting power or the required voltage.
Berkeley Lab found that heating and cooling can move three-day solar-plus-storage needs from relatively modest battery capacities into tens of kilowatt-hours.
Recharging during an outage can reduce the amount of energy that must be stored in advance.

28.4 kWh

Average daily utility purchases

2.8 kWh

Core Essentials per day

4.9 kWh

Household Essentials per day

40–90 kWh

Modeled median storage in some HVAC-heavy scenarios

Critical Loads Four Tests HVAC Outage Duration Sizing Process

Critical Loads

Start With What You Need, Not What the House Normally Uses

Whole-home electricity use includes many loads that can be reduced during an outage. Critical-load planning usually produces a much smaller—and more useful—number.

The U.S. Energy Information Administration reports that residential customers purchased an average of about 865 kWh per month in 2024, or roughly 28.4 kWh per day. Homes with onsite solar may consume more electricity than their utility purchases indicate, so even that figure is not a perfect measure of total household use.

More importantly, normal use is not the same thing as emergency need. NREL resilience guidance notes that a building’s critical load can differ from ordinary load in magnitude, timing and peak demand.

Backup level	What the household is trying to maintain
Core essentials	Refrigeration, communications, basic lighting and essential electronics
Household essentials	Core loads plus selected cooking, freezer and other important household equipment
Comfort backup	Essentials plus meaningful heating or cooling
Near whole-home	Most routine household functions with active load management
True whole-home	Relatively few behavioral changes, including major electrical loads

Two Illustrative Essential-Load Profiles

To make the difference concrete, we built two transparent Solar Waypoint examples. They are planning benchmarks, not claims about what every household uses.

Profile	Included loads	Estimated daily energy
Core Essentials	Refrigerator, Wi-Fi/router, laptop, four LED lights, basic device charging	2.76 kWh/day
Household Essentials	Core profile plus separate freezer, four hours of TV and 15 minutes of microwave use	4.93 kWh/day

Refrigerator and freezer values use EIA’s 2020 existing-housing-stock averages. Actual appliances, especially newer efficient models, can use less.

The Core Essentials profile rounds to about 2.8 kWh/day. The broader Household Essentials profile is close to 5 kWh/day.

Neither includes HVAC, a well pump, sump pump, electric water heating or medical equipment. Those household-specific loads can change the answer dramatically.

System Sizing

A Backup System Has to Pass Four Different Tests

A large battery can still be the wrong backup source if it lacks enough inverter power, motor-starting capability or the voltage required by the equipment.

Test	What it tells you	Common unit
Energy	How long the selected loads can operate	kWh
Continuous power	How much equipment can operate at the same time	kW / W
Starting power	Whether motors and compressors can start	Surge / peak W
Voltage	Whether the source can supply the required circuits or equipment	120V / 240V

Our Core Essentials profile demonstrates the difference. Using conservative sizing values from Generac, the refrigerator, router, laptop and four LEDs could total roughly 845W if operating together. Generac’s illustrative refrigerator sizing value adds another 2,200W of starting demand for the compressor.

That creates a conservative momentary example near 3,000W, even though the same profile consumes only about 2.8 kWh over an entire day.

Energy Is Only Half the Answer

A four-hour outage may require only about half a kilowatt-hour of energy in our Core Essentials model. The backup source can still need several kilowatts of momentary output to start a compressor. Actual appliance labels and manufacturer specifications should override generic sizing values.

Pumps and 240V Equipment Need Special Attention

A sump pump, furnace blower or private well can change the power requirement much faster than it changes average daily energy consumption. Generac’s representative guide lists a ½-hp sump pump at about 1,050W running with another 2,200W of starting demand, and a ½-hp furnace blower at about 800W running with another 2,350W at startup.

Private wells vary even more. DOE research notes that residential well pumps commonly range from about ½ to 3 horsepower.

Some pumps, HVAC systems and other large household equipment also require 240V. A power station that provides only 120V cannot operate those circuits regardless of how much battery energy remains.

Medical equipment should also be treated separately. If electrical equipment is medically essential, use its actual manufacturer power requirements and plan appropriate redundancy rather than relying on a generic appliance estimate.

Heating & Cooling

Heating and Cooling Can Multiply the Storage Requirement

Essential-only backup can be relatively modest. Trying to maintain normal indoor temperature during a multi-day outage can move a household into a completely different storage class.

Berkeley Lab modeled roughly 1,000 homes in each of ten U.S. locations during a challenging three-day outage. The modeled systems paired rooftop solar with battery storage, and batteries began fully charged.

When heating and cooling were excluded, Berkeley found that 15 kWh of battery storage paired with solar could cover the modeled backup requirements of most homes in every location studied.

That is not a claim that a stand-alone 15 kWh battery can run most houses for three days. Solar was replenishing the batteries and the study used defined building, weather and outage assumptions.

Adding HVAC changed the result dramatically. In half of the study locations, median storage requirements for baseline homes rose to roughly 40–90 kWh.

The HVAC Effect

In Berkeley’s modeled three-day solar-plus-storage outage, 30 kWh or less of battery storage was sufficient for only about 6% of baseline Phoenix homes but about 90% of baseline Los Angeles homes. Climate, building efficiency and HVAC equipment can overwhelm simple national battery-size rules.

Reducing the Load Can Beat Buying More Batteries

Berkeley also found that changing the building or the way HVAC operates during an outage can materially reduce storage needs. In Dallas–Fort Worth, efficiency improvements combined with a 5–6°F thermostat adjustment reduced modeled median storage requirements by roughly 50 kWh. In Phoenix, efficient heat-pump upgrades reduced the modeled median by roughly 30 kWh compared with less-efficient equipment.

Those are specific research scenarios, not universal savings estimates. The broader lesson is useful: when HVAC dominates the load, reducing demand can sometimes change required storage more than adding another battery.

Outage Duration

How Long You Want Backup Changes the Answer

A system sized for several hours of essential loads can be relatively small. Supporting the same loads for three full days requires several times more energy unless the system can recharge.

Illustrative profile	4 hours*	24 hours	72 hours
Core Essentials	~0.5 kWh	~2.8 kWh	~8.3 kWh
Household Essentials	~0.8 kWh	~4.9 kWh	~14.8 kWh

*Four-hour values prorate average daily energy use. Figures represent estimated load energy, not exact required battery nameplate capacity.

A real battery needs additional room for inverter losses, idle consumption, usable-capacity limits, reserve settings and uncertainty. The battery may also be below 100% state of charge when the outage begins.

Recharging During the Outage Can Reduce the Stored-Energy Target

A simple duration calculation assumes no energy comes back into the system. Solar, generator charging or temporary grid restoration can reduce the cumulative amount that has to come from the initial battery charge.

A Useful Planning Framework

Critical-load energy during the target outage − conservatively estimated replenishment + losses and reserve ≈ stored-energy target.

Solar should not be treated as guaranteed daily production. Weather, season, shading, array size and charge limits all affect what is actually available during an outage.

Our analysis of the changing economics of backup power looks more closely at how adding replenishment can change the cost of long-duration backup.

Your Home

A Practical Way to Size Backup Power

Three homes of similar size can need very different systems. An apartment backing up refrigeration and electronics may fit comfortably within a portable system. A house with a well or sump pump can need much higher inverter output. An all-electric home trying to maintain normal HVAC during a multi-day outage may require tens of kilowatt-hours of storage plus substantial 240V power capability.

1. List What Must Stay On

Start with refrigeration, communications, lighting and any household-specific critical equipment. Add comfort loads only after deciding how important they really are during a prolonged outage.

2. Check Running Power and Voltage

Use appliance labels, manuals or manufacturer specifications where possible. Pay particular attention to pumps, blowers, air conditioners and other major equipment.

3. Check Motor Starting Requirements

A compressor or pump can briefly need far more power at startup than during normal operation. Your backup source must be capable of handling that momentary load.

4. Estimate Energy Use

For equipment with reasonably predictable use:

Watts × hours of use ÷ 1,000 = kWh

For cycling equipment such as refrigerators, annual energy-consumption data can be more useful than pretending the rated wattage runs continuously.

5. Choose the Outage Duration

Decide whether you are planning for four hours, overnight, one full day or a prolonged outage. Avoid sizing for an undefined idea of “emergency backup.”

6. Account for Recharging, Losses and Reserve

Credit solar or another charging source conservatively, then leave room for conversion losses, idle draw, reserve capacity and uncertainty.

Finally, compare the result against both sides of the specification sheet: battery energy in kWh and the system’s continuous output, surge capability and voltage.

Bottom Line

There is no useful national answer such as “every home needs a 10 kWh battery.” Our lean illustrative profiles use about 2.8–5 kWh per day, but pumps, starting loads, 240V equipment and especially heating or cooling can change the requirement dramatically. Decide what must stay on first, then size the energy, power, voltage and outage duration around those loads.

For broader context on generators, permanent batteries, portable systems and grid conditions, see our State of Home Backup Power in America 2026. You can also see whether U.S. power outages are getting worse when deciding how much outage duration is worth planning for.

Research Notes

Methodology

This analysis combines U.S. Energy Information Administration residential electricity data, Berkeley Lab solar-plus-storage resilience research, NREL resilience-sizing guidance and representative manufacturer equipment-sizing information.

Our Core Essentials profile includes a refrigerator, Wi-Fi/router, laptop, four LED lights and basic device charging for an estimated 2.76 kWh/day. Household Essentials adds a separate freezer, four hours of television use and 15 minutes of microwave use, producing 4.93 kWh/day.

Refrigerator and freezer energy values use EIA’s 2020 existing-housing-stock averages of 617 and 570 kWh/year. EIA’s 2024 RECS detailed consumption-and-expenditure tables are scheduled for release in 2027, so the 2020 survey remains the latest detailed federal appliance-consumption dataset available for these estimates.

Four-hour values prorate average daily energy consumption and therefore may differ from actual short-outage use. The profiles intentionally exclude HVAC, wells, sump pumps, electric water heating and medical equipment because those loads vary substantially by household.

Berkeley Lab HVAC findings refer to modeled three-day outages in ten U.S. locations using rooftop solar plus battery storage. They are not stand-alone battery runtime guarantees or universal battery-size recommendations.

Primary Sources

Sources

Citing This Research

Publishers, manufacturers and journalists may cite the Solar Waypoint Core Essentials and Household Essentials profiles with attribution to Solar Waypoint and a link to this page. These are transparent illustrative load profiles, not national household averages or engineering recommendations for a particular home.

Research Updates

Update History

Sep. 5, 2026: Published with critical-load profiles, outage-duration calculations, current federal consumption data and Berkeley Lab residential backup research.