Modular Battery Systems for Whole-Home Backup

A modular battery system for whole-home backup lets you match power and storage to how your house actually uses electricity—then expand when loads grow. Unlike a single fixed pack, modular stacks combine a power module with add-on battery modules, so you can phase investment instead of oversizing on day one.

This guide covers US split-phase sizing (120 V/240 V), essential-load versus whole-home planning, and two BLUETTI modular paths: the Apex 300 with B300K or B500K expansion batteries for flexible backup, and the professionally installed EnergyPro 13K with EnergyPack 500 batteries for higher-output whole-home storage.

Key Takeaways

• Whole-home backup requires both enough kWh (runtime) and enough kW (motor starts for A/C, well pumps, dryers).

• Essential-load backup capacity varies significantly by household. Rather than assuming one universal kWh range or runtime, calculate the energy consumption and operating time of the refrigerator, lighting, communications equipment and other selected loads.

• Homes with larger electrical appliances may require substantially more storage capacity and output, but the final system should be based on measured household loads, appliance startup requirements and a professional system design.

• The Apex 300 with one B300K provides 5,529.6 Wh of nominal capacity and 3,840 W of rated output. One B500K provides a larger 5,120 Wh expansion option, giving the Apex 300 + B500K configuration 7,884.8 Wh of nominal capacity.

• For professionally installed whole-home storage, the EnergyPro 13K provides up to 13.2 kW of output and supports two to four EnergyPack 500 batteries, expanding from 9.6 kWh to 19.2 kWh.

Why Modular Designs Fit Whole-Home Backup Planning

Choosing a home battery is not just about preparing for the next outage. It's about deciding how much of your home keeps running during an outage. So, home backup does not have to mean "every circuit, all day, no planning." Most households choose one of two strategies:

1. Essential-load backup — power selected circuits longer during outages.

2. Broad whole-home backup — support major 240V appliances with higher kW and kWh.

Modular systems help in both cases because you can add battery modules as outage experience, budget, or loads change—within product limits and installer design rules.

How to Size a Modular Battery System for a U.S. Home

Once you've decided on your backup strategy, the next step is sizing your battery system. For most U.S. homes with 120V/240V split-phase electrical service, that comes down to two key measurements:

• Battery capacity (kWh) determines how long your system can power your appliances.

• Power output (kW) determines what appliances it can run at the same time.

Central air-conditioning startup demand can be substantially higher than its normal running demand. Check the equipment nameplate, compressor specifications, locked-rotor current and installer measurements rather than relying on one general surge figure.

As a planning framework, the following backup tiers describe the types of loads involved rather than prescribing universal capacity or runtime ranges:

Note: Actual requirements vary depending on your home's electrical loads and backup plan.

Essential vs. Whole-Home Load Lists

Of course, your backup strategy also determines which appliances you'll power during a blackout. Essential backup focuses on what you rely on every day, while whole-home backup extends to larger and more energy-intensive household loads.

Essential loads (prioritize these first)

• Refrigerator/freezer (~0.15 kW steady)

• Wi-Fi router and lighting

• Phone/laptop charging and TV

• Garage door opener, security system

• Medical devices (CPAP, oxygen concentrators)

Whole-home / high-demand loads (need higher kW + kWh)

• Central HVAC or heat pump (multi-kW when running)

• Electric well or sump pumps

• Microwave, dishwasher, electric oven

• Washer and electric dryer

• Level 2 EV charging (often deferred during outages)

Trying to run every appliance at once drains any battery quickly. Smart circuit planning—whether via a backup sub-panel or app-based load shedding—extends runtime materially.

Portable Modular Stack: Apex 300 + B300K

If you're looking for modular backup without a permanent wall install, BLUETTI Apex 300 pairs with expansion batteries such as B300K and B500K.

A good fit when you need:

• Backup for selected essential loads

• A movable option for direct plug-in loads

• Versatile power for RV weekends or job sites after an outage

Important limitation: One Apex 300 provides 3,840 W of rated output. Its suitability should be judged against the combined running and startup demand of the selected backup loads—not against the home's 200 A service rating. It should not be presented as a substitute for a higher-output professionally installed whole-home inverter when large HVAC systems or multiple high-demand appliances must operate together.

Adding B300K or B500K batteries to one Apex 300 increases storage capacity, while the rated output of that single power module remains 3,840 W. Approved multi-unit Apex 300 configurations can increase both capacity and total output, scaling from 2.7 kWh to approximately 100 kWh and from 3.8 kW to approximately 11.5 kW with the required system components.

Installed Modular Stack: EnergyPro 13K + EnergyPack 500

When the goal is professionally installed whole-home modular backup with higher output, the BLUETTI EnergyPro 13K with EnergyPack 500 batteries is the current installed option.

A good fit when you need:

• Selected 120 V and 240 V household circuits, including appropriately sized pumps, HVAC equipment and other high-demand loads

• Extended outage support

• A single-vendor modular stack sized with a professional installer

Actual circuit coverage depends on the home's load calculation, appliance startup requirements, electrical-panel design and installed EnergyPack 500 capacity. A 13.2 kW inverter does not mean that every household appliance can necessarily operate at the same time.

Warning: EnergyPro 13K installation requires qualified system design, permits and professional electrical installation, including appropriate grounding, overcurrent protection, transfer equipment and utility interconnection compliance. Budget installation labour separately from hardware.

Smart Load Management and Circuit Planning

Whether you choose a portable modular stack or an installed EnergyPro 13K system, circuit planning determines real-world performance:

• Sub-panel / backup panel — permanently assigns which loads receive backup power; changing circuits later can be costly.

• Essential-only strategy — keeps fridge, communications, and lighting online far longer than "everything on."

• A/C soft starters — may reduce compressor startup demand on compatible motor loads when installed according to the HVAC equipment and soft-starter manufacturers' requirements. They are not suitable for every compressor or variable-speed system.

• Solar + battery — most conventional grid-tied solar systems shut down during an outage unless approved equipment isolates the home from the utility and establishes a local source capable of supporting the solar system.

Installation, Codes, and Safety

• Use a qualified electrician for any panel tie-in, transfer switch, or 120 V/240 V wiring.

• Panel, smart-distribution or transfer-equipment installation may require a temporary interruption of household power. Confirm the expected duration with the installer.

• PV systems tied to buildings may require rapid shutdown (RSD) and other NEC compliance measures—verify with your installer.

• Site batteries in an approved location that meets the manufacturer's requirements for temperature, clearances, ventilation, moisture protection, mounting and service access. Outdoor-rated equipment still requires installation within its approved environmental and mounting conditions.

Final Thoughts

Choosing the right modular battery system comes down to three decisions: your backup strategy, the appliances you want to keep running, and the battery capacity (kWh) and power output (kW) your home requires.

For flexible backup of selected direct plug-in loads or appropriately integrated circuits, the BLUETTI Apex 300 can be paired with B300K or B500K batteries. Adding batteries to one Apex 300 increases capacity while rated output remains 3,840 W; approved multi-unit configurations can increase both capacity and output.

For professionally installed whole-home storage, the EnergyPro 13K supports two to four EnergyPack 500 batteries, providing 9.6–19.2 kWh of capacity with up to 13.2 kW of output.

FAQ

Can a modular battery run my central air conditioner during an outage?

Sometimes, but the system must support the air conditioner's measured running demand and startup requirements. Smaller portable modular stacks may require strict load prioritisation and, where compatible, a professionally selected soft starter. Higher-output installed systems such as EnergyPro 13K may support larger 240 V loads when the complete battery, inverter and circuit configuration has been properly sized.

Do I need a battery if I already have solar?

For conventional outage backup, generally yes. Most standard grid-tied solar systems shut down during a blackout unless the home has approved equipment that isolates it from the grid and establishes a local electrical source. Some specialised solar-inverter configurations may provide limited backup without a conventional battery, so confirm the capability of the installed system.

How long will a ~15 kWh modular system run my house?

It depends on how much electricity your home uses. A 15 kWh system may power essential loads such as a refrigerator, lighting, Wi-Fi, and small electronics for many hours, while running energy-intensive appliances like central air conditioning will reduce runtime significantly. Instead of estimating based only on battery capacity, calculate the total power consumption of the appliances you plan to back up.

Is it better to buy all modules now or expand later?

Modularity is designed for phased expansion when firmware, enclosure limits, and compatibility allow. Confirm expansion rules with your installer or product documentation before assuming future module counts.

Disclaimer

General information only. Not electrical, tax, or safety advice. Whole-home battery systems require professional design and installation. Follow manufacturer manuals, utility interconnection rules, and local electrical codes.