Quick answer
Home battery size starts with the loads you must keep running—not the square footage of the house. Add their watt-hours, choose an outage duration, then allow for conversion losses and reserve capacity. If your critical loads require 5kWh of usable energy, a reasonable planning estimate is about 6.2kWh of rated battery capacity when both usable depth of discharge and system efficiency are assumed to be 90%.
That is a planning estimate, not a final electrical design. Inverter surge, 120/240V loads, battery current limits, transfer equipment, local code and the exact product manuals still matter.
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Battery backup sizing formula
Start with energy. For each appliance or circuit, multiply its average power by the hours you expect it to operate:
Load energy (Wh) = average watts × operating hours
Add every critical load, then divide watt-hours by 1,000 to convert the result to kilowatt-hours.
Next, allow for the portion of the battery you plan to use and for conversion losses:
Rated battery kWh = required load kWh ÷ (usable fraction × system efficiency)
Worked example: 5kWh ÷ (0.90 × 0.90) = 6.17kWh of rated battery capacity. Round upward when you need margin for cold weather, aging, unexpected loads or a longer outage.
Use measured energy whenever possible. A refrigerator's nameplate may show its maximum running watts, while its daily energy label or a plug-in meter better captures cycling. Motors and compressors also require a separate startup-surge check.
Step 1: choose critical loads—not every load
A practical backup system protects the loads that matter most. Whole-home operation is possible in some designs, but it usually requires more battery, more inverter capacity and careful load management.
| Load category | Examples | What to record |
|---|---|---|
| Usually critical | Refrigerator, freezer, internet, communications, selected lighting | Average watts, daily kWh and startup surge |
| House-specific | Well pump, sump pump, medical equipment, garage door, security | Voltage, surge, duty cycle and required runtime |
| High demand | Electric water heating, range, dryer, central air conditioning, resistance heat | Whether the load can be excluded, scheduled or supported at all |
Step 2: size inverter power separately
Battery kilowatt-hours determine how long loads can run. Inverter kilowatts determine what can run at the same time. Add the running watts of simultaneous loads, then verify the largest startup event. A refrigerator, pump or air conditioner may briefly need several times its normal running power.
Also separate battery voltage from household output voltage. A “48V” battery system can feed an inverter that produces 120V or split-phase 120/240V AC. If your critical-load list includes a 240V pump, range, dryer or HVAC equipment, the inverter and transfer arrangement must explicitly support that output. Browse current hybrid inverters and off-grid inverters, then verify every model against its manual.
Step 3: choose the outage duration
“How many kWh do I need?” cannot be answered without a time target. A system intended to ride through a two-hour interruption is different from one expected to cover a full night or several cloudy days.
Prioritize refrigeration, internet, lights and essential electronics.
Include cycling loads, nighttime lighting and realistic reserve.
Plan daily energy, recharge opportunities, weather and load shedding.
What battery size should you compare?
The ranges below are shopping landmarks—not promises that a battery will support a particular home. Confirm inverter compatibility, allowable parallel count, current limits, communication protocol, cable requirements and installation rules.
| Rated capacity landmark | Typical configuration concept | Useful next step |
|---|---|---|
| About 5.12kWh | One common 51.2V, 100Ah rack module | Compare server-rack batteries |
| About 10.24kWh | Two matching 5.12kWh modules where the manufacturer permits parallel use | Verify busbars, cables and inverter current |
| About 15.36kWh | Three matching 5.12kWh modules where approved | Check cabinet, clearance and parallel limits |
| About 16kWh | One large-format wall battery such as the RUiXU Lithi2-16 | Confirm wall loading, access and inverter protocol |
Current product paths
- RUiXU RX-LFP48100
- RUiXU RX-LFP48100-H
- SOK SK48V100N
- Epoch self-heating 48V 100Ah
- RICH SOLAR ALPHA 5
- RICH SOLAR ALPHA 5 PRO
Products are shown as current shopping paths, not as a claim that they can be mixed or paired with every inverter.
Can solar make backup last longer?
Solar can extend an outage plan when the array, weather and inverter allow enough energy to replace what the house used. Compare daily consumption with realistic daily solar production—not the array's nameplate watts alone. Clouds, season, temperature, shading and conversion losses all matter.
Battery size and solar-array size solve different problems. The battery covers loads when production is low; the array determines how much energy can be replenished during the available sun window. A larger battery without adequate recharge may simply take longer to refill.
Server-rack vs. wall-mount batteries
Server-rack modules
Modular capacity, front service access and common rack/cabinet layouts can make future expansion easier. Plan for compatible racks, busbars, equal-length cabling, ventilation and manufacturer-approved parallel operation.
Shop server-rack batteries →Wall-mounted batteries
A large-format wall battery can reduce floor-space use and create a clean installation. The wall, fasteners, clearances and service access must support the exact unit, and communication with the inverter must be confirmed.
Shop wall-mount batteries →Installation and transfer equipment
A battery is one component of a backup system. The complete design can include an inverter/charger, disconnects, overcurrent protection, communications, transfer equipment, a critical-load panel, grounding and permitting. Never backfeed a building through an improvised cord or outlet. Hardwired systems should be designed and installed by qualified professionals under the applicable instructions and codes.
Home backup sizing checklist
- List the exact circuits and appliances that must operate.
- Record average watts, daily kWh and startup surge.
- Choose a clear outage-duration target.
- Apply realistic usable-capacity and system-efficiency assumptions.
- Confirm 120V versus 120/240V output needs.
- Verify inverter power, battery current and communication compatibility.
- Check solar recharge limits, installation clearances and parallel limits.
- Review transfer equipment, permits and professional-installation requirements.
Frequently asked questions
How many kWh does a typical home need for backup?
There is no reliable house-wide average for an individual design. The result depends on which loads are backed up, their measured energy use and the outage duration. A critical-load system may be far smaller than a whole-home system with electric heating or air conditioning.
Is a 10kWh battery enough to run a house?
It may cover selected critical loads for a useful period, but runtime must be calculated from usable energy and actual load. It may not support high-demand equipment or a full day of normal household use.
How many 5.12kWh server-rack batteries do I need?
Divide the rated capacity target by 5.12kWh, round up, and then verify that the chosen battery and inverter allow that quantity in parallel. Use only matching, manufacturer-approved modules and connection methods.
Should I size the battery or inverter first?
Size them together. The load list determines both simultaneous power and total energy. An adequate battery with an undersized inverter cannot start the loads, while an oversized inverter does not create additional runtime.
Sources and further reading
- U.S. Department of Energy: Solar and Resilience Basics
- U.S. Department of Energy: Solar Energy and Storage Basics
- U.S. EIA: Measuring Electricity
- National Renewable Energy Laboratory: Resilience and battery-system planning reference
Last reviewed August 2026. This guide provides planning information, not a final electrical design. Follow all product manuals and applicable codes.
