GoSolarIndex

Battery Backup Sizing Calculator

Determine the exact battery capacity (kWh) and continuous inverter output (kW) required to keep your home operational during a power outage. Pick your essential circuits to see required storage across 12, 24, 48, or 72 hours and match real commercial battery hardware.

24 Hours

Select Your Backup Loads (5 selected)

·

Critical Survival & Medical

Household Essentials & Connectivity

Heavy Motors, HVAC & Well Pumps

Adjust Inverter Efficiency & Battery Reserve Margin
85%

AC-to-DC conversion loss in inverters typically ranges from 10% to 15%.

15%

Prevents deep-discharging the battery to 0% to extend LiFePO4 cell longevity.

Recommended Battery Capacity (24-Hour Outage)
3.5 kWh(3.0 kWh usable energy after inverter losses)
Daily Consumption
2.5 kWh
per 24-hr cycle
Continuous Draw
320 W
all items running
Max Motor Surge
970 W
starting surge
Reserve Buffer
15%
headroom added

Real Hardware Matching Your Requirements

Your power requirement fits within a heavy-duty portable power station:

Anker SOLIX F3800
Capacity: 3,840 Wh · Inverter: 6,000W
Weight: 132.3 lbs$2,200
Anker SOLIX F3800 Plus
Capacity: 3,840 Wh · Inverter: 6,000W
Weight: 135.6 lbs$2,600
Bluetti AC500 (inverter, sold bundled with B300K batteries)
Capacity: 5,529.6 Wh · Inverter: 5,000W
Weight: 66.2 lbs$2,099

The Dual-Sizing Principle: Energy (kWh) vs Power (kW)

The most frequent mistake homeowners make when selecting a backup battery is confusing energy capacity (kWh) with power output (kW). A successful battery installation requires sizing both metrics correctly:

1. Capacity (Kilowatt-Hours, kWh)

Think of kilowatt-hours as the size of a water bucket. It dictates how long your system can supply energy. A 13.5 kWh battery can run a steady 500W load (refrigerator, lights, Wi-Fi, CPAP) for roughly 23 to 24 hours before running completely empty.

2. Power Output (Kilowatts, kW)

Think of kilowatts as the width of the bucket's spigot. It dictates what appliances can run simultaneously. If your well pump (1,000W) kicks on while your microwave (1,000W) and coffee maker (1,200W) are running, your battery must deliver 3.2 kW continuous power without tripping its internal circuit breaker.

If your system has plenty of kWh but insufficient kW, the battery will instantly shut down when high-draw motors attempt to start. Conversely, if your system has high kW but low kWh, it will power heavy loads with ease, but deplete its charge within a couple of hours.

The Cycling Load Problem: Why Refrigerator Draw Is Misunderstood

Motor-driven compressor loads—specifically refrigerators, freezers, and air conditioners—do not draw power continuously. Sizing battery capacity using the nameplate wattage on the back of your refrigerator will grossly overestimate the required kilowatt-hours.

A standard residential refrigerator draws roughly 150 watts while its compressor motor is actively running. However, once the refrigerator interior reaches temperature, the compressor cycles off, dropping the power consumption to negligible levels (5–10W for control boards and sensors). Over a standard 24-hour period, a refrigerator compressor runs approximately one-third of the time (an 8-hour cumulative duty cycle).

Actual Daily Fridge Energy = 150W running × 8 hours = 1,200 Wh/day (1.2 kWh/day)
Effective Continuous Average = 1,200 Wh ÷ 24 hours = 50 Watts

Our calculator uses the 50W 24-hour average draw when calculating required storage capacity (kWh), but checks the 150W running draw and 800W motor starting surge against inverter power ratings (kW).

Motor Inrush Surge: Sump Pumps, Well Pumps & AC

Inductive motors demand a momentary spike of electrical current—known as locked-rotor amperage (LRA) or inrush current—when starting from a complete standstill. This starting surge lasts only 100 to 300 milliseconds, but can exceed the normal running current by 300% to 600%:

  • 1/3 HP Sump Pump: 800W running draw requires an instantaneous 1,300W to 1,800W surge to spin the impeller under load.
  • 1/2 HP Submersible Well Pump: 1,000W running draw requires a 2,100W to 3,000W surge to lift column water from the aquifer.
  • 3-Ton Central Air Conditioner: 3,500W running draw can surge up to 15,000W to 22,000W without a soft-start capacitor.

Modern lithium iron phosphate (LiFePO4) home batteries such as the Tesla Powerwall 3 and FranklinWH aPower 2 feature robust peak surge ratings (15 kW to 15.4 kW) specifically engineered to start high-surge residential motors without nuisance tripping.

Critical Loads Subpanel vs Whole-Home Backup

When configuring a home battery installation, electricians offer two fundamental wiring architectures:

Critical Loads Subpanel (Partial Home Backup)

An electrician installs a secondary electrical panel alongside your main service panel. Only 6 to 12 designated breaker circuits are relocated into this subpanel: refrigerator, internet router, medical devices, sump pump, kitchen convenience outlets, and gas furnace controls.

Advantage: Can be reliably supported by a single 10 kWh to 15 kWh battery, lowering turnkey installation costs by $8,000 to $15,000.

Whole-Home Backup with Smart Energy Management

A gateway or whole-home automatic transfer switch sits between the utility meter and your entire 200A main breaker panel. Every circuit in the house remains energized when the grid fails. Smart energy management software automatically sheds non-essential heavy loads (EV charging, electric clothes dryer, pool pumps) to prevent depleting the batteries prematurely.

Advantage: Maximum comfort with zero circuit selection compromises; typically requires two or more stacked battery units (20 kWh to 30 kWh+).

Frequently Asked Questions

What size battery do I need to run a house during a blackout?

To back up critical circuits (refrigerator, Wi-Fi, lighting, medical equipment, and furnace fan) during a 24-hour blackout, most American homes need 10 kWh to 15 kWh of usable battery storage. For whole-home backup including central air conditioning, electric cooking, or a well pump, homes typically require 20 kWh to 30 kWh (usually two stacked home batteries like dual Tesla Powerwalls or FranklinWH units) to ensure sufficient continuous wattage and surge output.

What is the difference between battery kWh and kW?

Kilowatt-hours (kWh) measures total energy capacity (the volume of energy stored, determining how long the battery lasts), while kilowatts (kW) measures power output (the rate at which electricity flows out of the battery inverter). A 13.5 kWh battery with a 5 kW inverter can run modest appliances for hours, but cannot start a large central AC compressor that demands 7 kW continuous or 15 kW surge.

Can a home battery run a central air conditioner?

Yes, but it depends on the battery inverter output and whether a soft-starter is installed. A standard 3-ton central AC draws 3,000 to 3,800 watts while running, but requires an instantaneous locked-rotor surge of 15,000 to 22,000 watts to start. Advanced batteries like the Tesla Powerwall 3 (11.5 kW continuous / 15.4 kW peak) can start many central AC units directly, or pairing an inexpensive aftermarket soft-starter can reduce starting surge by 65%.

Why does inverter efficiency matter in battery sizing?

Home batteries store energy as direct current (DC). Inverters convert this DC power into 120V/240V alternating current (AC) for household appliances. This conversion produces heat, resulting in a 10% to 15% efficiency loss (85% to 90% round-trip efficiency). If your appliances consume 10 kWh of AC electricity, the battery must supply roughly 11.5 kWh of internal DC energy.

Data sources

Last updated 2026-09-24

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Written by Aditya Patil