How to Size a Whole-Home Backup Battery System

A whole-home backup system should not be selected only by battery capacity or inverter nameplate power. It need be designed according to the residential monthly electricity consumption and electrical loads, then have the AC power output and desired backup duration.

The sizing process answers three important questions:

1. How much power must the system deliver at one time?

2. How much energy must the battery store?

3. Which loads should remain available during an outage?

For many U.S. homes, a whole-home backup system may require approximately 10–15kW of inverter output and 20–40kWh of battery capacity. However, the correct size can vary significantly depending on air conditioning, electric heating, well pumps, water heating, EV charging and other large household loads.

1. What Is a Whole-Home Backup System?

A whole-home backup system is a solar and battery system designed to supply power to most or all circuits in a home when utility power is unavailable.

A complete system commonly includes:

• A hybrid or battery inverter

• One or more battery modules

• A backup gateway, transfer switch or load hub

• Current transformers or energy-monitoring equipment

• Solar panels, where applicable

• Load-management controls

• Communication and monitoring equipment

During normal operation, the system may use solar energy to power the home, charge the battery and reduce electricity purchased from the utility.

When a grid outage occurs, the backup equipment disconnects the home from the utility. The inverter then supplies electricity from the battery and available solar production.

Whole-home backup does not always mean unlimited power

A system may be connected to the entire electrical panel while still having a maximum output limit.

For example, an 11.4kW inverter cannot supply the full theoretical capacity of a 200A electrical service. If several large appliances operate at the same time, the system may exceed its power limit.

Whole-home systems therefore often use load management to control equipment such as:

• Central air conditioners

• Electric water heaters

• EV chargers

• Electric clothes dryers

• Pool pumps

• Well pumps

• Electric ranges

This type of design is often called managed whole-home backup.

Whole-home backup vs. partial-home backup

A partial-home backup system supplies only selected circuits through a protected-load panel. These circuits may include refrigeration, lighting, internet, medical equipment and essential outlets.

A whole-home backup system keeps most or all circuits connected but may control high-demand loads to prevent overload.

Partial-home backup generally requires less inverter power and battery capacity. Whole-home backup provides more convenience but requires more careful system design.

2. Why Whole-Home Backup Sizing Is Important

Correct sizing is important because inverter power, battery capacity and appliance demand are different measurements.

Inverter power determines what can run

Inverter output is measured in kilowatts.

It determines how much equipment can operate at the same time.

A system with insufficient inverter output may shut down when several appliances operate together, even if the battery still has plenty of stored energy.

Battery capacity determines how long loads can run

Battery capacity is measured in kilowatt-hours.

It determines the approximate length of time the system can support the home's loads.

A large inverter connected to a small battery may operate large appliances but provide only a short backup duration.

Startup surge can exceed normal running power

Motors and compressors often require much more power during startup than during normal operation.

Common surge loads include:

• Central air-conditioning compressors

• Well pumps

• Sump pumps

• Refrigerators

• Freezers

• Workshop equipment

An inverter may have enough continuous power to run an air conditioner but still be unable to start it.

The designer must therefore confirm:

• Appliance running power

• Startup or locked-rotor current

• Inverter surge capability

• Battery discharge capability

• Other loads operating at the same time

Incorrect sizing can create four common problems

The system cannot start major appliances

This often occurs when HVAC or pump startup demand is underestimated.

The battery runs out sooner than expected

This occurs when the design uses nominal battery capacity without accounting for reserve settings and efficiency losses.

The customer pays for unnecessary equipment

Oversizing every component can increase cost without meaningfully improving resilience.

The system requires unexpected load restrictions

A customer may believe the system will run every appliance, only to discover after installation that the EV charger, dryer or air conditioner must be turned off during an outage.

A proper calculation prevents these problems and creates realistic expectations before equipment is purchased.

3. How to Calculate a Whole-Home Backup System

The calculation should be divided into three parts:

1. Continuous inverter power

2. Startup surge power

3. Required battery capacity

Step 1: Identify the Loads That Must Remain Available

Divide household loads into three groups.

Critical loads

These should usually stay on throughout an outage:

• Refrigerator and freezer

• Lighting

• Internet and communications

• Security systems

• Medical equipment

• Garage door opener

• Selected outlets

• Well or sump pump, where required

Important loads

These may operate selectively:

• Central air conditioning

• Furnace blower

• Microwave

• Dishwasher

• Washing machine

• Home office

• Pool circulation pump

Deferrable loads

These can normally be disabled during an outage:

• EV charging

• Electric clothes dryer

• Electric resistance heating

• Large electric water heater

• Hot tub

• Pool heater

• Multiple HVAC systems operating together

This step determines whether the design requires unrestricted backup or managed backup.

Step 2: Calculate Simultaneous Running Power

Add the running wattage of the loads likely to operate at the same time.

Example

The estimated simultaneous load is 8.5kW.

Adding approximately 15%–25% design margin suggests an inverter in the 10–12kW range.

The design margin accounts for:

• Load variation

• Future appliances

• Measurement uncertainty

• Short periods of overlapping demand

Step 3: Check Startup Surge

The designer should review motor and compressor loads separately.

For each major appliance, collect:

• Running watts or amps

• Startup current

• Locked-rotor amps, where available

• Voltage

• Number of phases

• Expected startup duration

The selected inverter must support both the normal continuous load and the short startup demand.

A compatible soft starter may reduce HVAC startup current, but the final result still depends on the air conditioner and inverter specifications.

Step 4: Calculate the Energy Required During an Outage

Use the formula:

Energy in kWh = Load in kW × Operating time in hours

Examples:

• 1kW for 10 hours = 10kWh

• 2kW for 10 hours = 20kWh

• 4kW for 6 hours = 24kWh

The most accurate source is hourly or 15-minute utility interval data.

When interval data is unavailable, estimate each appliance using:

Appliance energy = Appliance power × Daily operating time

Example daily outage energy budget

Step 5: Account for Usable Capacity and Efficiency

A battery's nominal capacity is not the same as the energy delivered to household loads.

The system must account for:

• Minimum battery reserve

• Inverter conversion losses

• Wiring losses

• Temperature

• Battery aging

• Manufacturer operating limits

Use this formula:

Required battery capacity = Backup energy ÷ usable capacity fraction ÷ system efficiency

Assume:

• Backup energy: 15kWh

• Usable battery capacity: 90%

• System efficiency: 90%

Calculation:

15 ÷ 0.90 ÷ 0.90 = 18.5kWh

Add a 20% reserve:

18.5 × 1.20 = 22.2kWh

A reasonable planning target would therefore be approximately 22–25kWh of nominal battery capacity.

Step 6: Confirm Battery Discharge Power

Two batteries with the same capacity may have different power output.

For example:

• Battery A: 20kWh capacity and 5kW maximum discharge

• Battery B: 20kWh capacity and 12kW maximum discharge

Both store the same energy, but Battery B can support larger simultaneous loads.

The battery system must be able to deliver enough power for the selected inverter.

Check:

• Continuous battery discharge power

• Peak discharge power

• Maximum current

• Number of required battery modules

• Low-state-of-charge limits

• Temperature derating

Step 7: Account for Solar Recharging

Solar panels may extend battery runtime during an outage.

For example, if the home uses 20kWh during a day and the solar array produces 15kWh of usable energy, the battery may only need to supply the remaining 5kWh.

However, solar production is affected by:

• Clouds

• Season

• Roof orientation

• Shading

• Snow

• Smoke

• Solar-array size

• Daytime household consumption

Battery capacity should not be based on ideal solar production alone.

For multiday outages, a generator may provide additional resilience without requiring an extremely large battery system.

Step 8: Confirm the Complete Backup Architecture

A whole-home backup system may require:

• Hybrid inverter

• Battery modules

• Backup gateway or transfer switch

• Service-rated disconnect equipment

• Current transformers

• Smart-load controls

• Protected-load panel

• Rapid-shutdown equipment

• Generator interface

• Communication equipment

The exact equipment depends on whether the system is:

• DC-coupled

• AC-coupled

• Partial-home backup

• Managed whole-home backup

• Off-grid

The inverter and battery should not be evaluated as isolated products. They must be verified as one compatible system.

4. Two Typical Whole-Home Backup Project Cases

The following examples illustrate the calculation process. They should be replaced or supplemented with verified Self2Solar customer projects before publication.

Case 1: Medium-Sized Home with Gas Appliances and Central Air Conditioning

Project profile

• Location: California

• Electrical service: 200A

• Home size: approximately 2,000 square feet

• Space heating: natural gas

• Water heating: natural gas

• Cooking: gas

• Central air conditioner: one system

• Existing solar: none

• Backup goal: operate most household circuits for approximately 24 hours

Critical and important loads

The homeowner wants to operate:

• Refrigerator and freezer

• Lighting and outlets

• Internet and office equipment

• Garage door

• Central air conditioning

• Microwave

• Television

• Small household appliances

The homeowner agrees to disable:

• EV charging

• Clothes dryer

• Electric oven

• Other nonessential loads

Power calculation

With design margin, the project requires approximately 9–10kW of continuous inverter power.

Because the central air conditioner has a high startup demand, the system should include:

• An inverter with sufficient surge capability

• A battery bank with adequate discharge power

• A compatible HVAC soft starter, if required

• Load control for the EV charger and dryer

Energy calculation

Estimated outage consumption:

Battery calculation:

13.3 ÷ 0.90 ÷ 0.90 × 1.20 = 19.7kWh

Recommended planning range

• Inverter output: approximately 10–12kW

• Battery capacity: approximately 20–25kWh

• Backup type: managed whole-home backup

• Solar array: sized separately based on annual energy goals and available roof area

Why this system size works

The home uses gas for heating, water heating and cooking, which reduces electrical energy demand.

A 20–25kWh battery can support the estimated daily outage load, while a 10–12kW inverter can operate the selected circuits and central air conditioner when surge requirements are properly managed.

Potential Self2Solar system categories

• Growatt whole-home backup systems

• Sigenergy SigenStor systems

• Fox ESS PowerQ systems

• Solis hybrid systems

The exact product selection should be based on confirmed load data, equipment compatibility and local permitting requirements.

Case 2: Large All-Electric Home with Well Pump and EV Charger

Project profile

• Location: Texas

• Electrical service: 200A

• Home size: approximately 3,500 square feet

• Space heating: electric heat pump

• Water heating: electric

• Cooking: electric

• Central air conditioning: two systems

• Well pump: yes

• EV charger: yes

• Existing solar: 15kW grid-tied system

• Backup goal: support essential loads and selected major appliances during a 24-hour outage

Load-management strategy

Operating every large load at the same time would require a very large inverter and battery system.

The project therefore uses managed whole-home backup.

The following loads remain available:

• Refrigerator and freezer

• Lighting and outlets

• Internet and office equipment

• One HVAC system

• Well pump

• Electric water heater on a controlled schedule

• Kitchen appliances used selectively

The following loads are disabled during an outage:

• EV charging

• Clothes dryer

• Second HVAC system

• Pool heater

Power calculation

If all these loads operate together, total demand could reach:

14.6kW

Instead of designing for all loads simultaneously, the system prevents the water heater and HVAC compressor from operating at the same time.

Managed simultaneous demand is reduced to approximately:

10–12kW

Because the well pump and HVAC system both have startup surge, the project requires significant surge capacity.

Energy calculation

Battery calculation:

25 ÷ 0.90 ÷ 0.90 × 1.20 = 37.0kWh

Recommended planning range

• Inverter output: approximately 12–18kW

• Battery capacity: approximately 35–45kWh

• Backup type: managed whole-home backup

• Existing solar integration: AC-coupled or system-specific retrofit design

• Load controls: required for EV charging, water heating and multiple HVAC systems

Why load management is essential

Without load management, the project may require substantially more inverter power and battery discharge capability.

By preventing selected large appliances from running together, the homeowner can maintain essential comfort and water service without installing an unnecessarily oversized system.

Potential Self2Solar system categories

• AC-coupled battery systems

• High-capacity Sigenergy systems

• Expandable Growatt battery systems

• FranklinWH retrofit systems

• Multi-inverter or modular hybrid systems

The final system must be checked for compatibility with the existing solar inverter and utility interconnection requirements.

Final Recommendation

Whole-home backup sizing should always answer three separate questions:

1. What is the maximum continuous load?

2. What is the maximum startup surge?

3. How much energy is required for the desired backup duration?

The selected inverter, battery bank, gateway and load-management equipment must satisfy all three requirements as one complete system.

For a more accurate recommendation, provide Self2Solar with:

• Twelve months of utility bills

• Hourly or 15-minute interval usage data

• Electrical-service size

• Existing solar inverter model

• Major appliance nameplate information

• HVAC specifications

• Well-pump information

• Desired backup duration

• Loads that can be disabled during an outage

Self2Solar can then help compare suitable whole-home backup configurations based on the project's power, energy and compatibility requirements.