Home Home Backup Calculator

Free interactive planning tool

Home Backup Calculator

Estimate battery capacity, inverter output, surge demand, generator size, and possible 240V requirements from your essential appliances.

Build my estimate See the methodology

Editable appliance data Separate running and starting watts Real recharge inputs

Start with a real outage plan

Choose the closest starting profile

A profile fills the calculator with a practical first draft. You can change every appliance, wattage, runtime, and assumption before using the result.

Interactive planning tool

Build your home backup estimate

Use appliance labels or measured values whenever possible. The defaults are editable planning examples, not promises about a specific home.

1 Define the outage

Expected outage duration hours Enter 12 for overnight, 48 for two days, or your own duration. Coverage goal This helps interpret the result; it does not change appliance math. Preferred backup route The result will respect this preference where it remains practical. Automatic backup mattersUseful for unattended loads, medical equipment, pumps, and heating.

2 Select and adjust loads

Essentials 5 planning loads

Refrigerator Typical modern refrigerator; compressor cycles.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Freezer Chest or upright freezer; compressor cycles.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Internet and Wi-Fi Modem, router, and small network equipment.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

LED lighting A practical group of efficient lamps.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Phones and small electronics Charging for household communication devices.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Heating and cooling 5 planning loads

Furnace blower Blower only; fuel-fired furnace heat is separate.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Boiler controls and circulator Controls plus one small circulation pump.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Window air conditioner Single-room unit; verify compressor startup demand.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Heat pump Representative central unit; nameplate data is essential.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Central air conditioner Representative condenser and air handler combination.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Water and safety 4 planning loads

Sump pump Intermittent motor load; rainfall can change runtime sharply.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Well pump Representative submersible pump; confirm voltage and locked-rotor data.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Sewage ejector pump Brief but important motor load.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Security system Alarm panel, cameras, and network recorder.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Medical and work 3 planning loads

CPAP machine Humidifier use can materially increase consumption.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Medical equipment Placeholder only; replace with the device label or measured demand.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Home office Laptop, display, and basic office equipment.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Kitchen and comfort 4 planning loads

Microwave Short, high-output cooking load.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Coffee maker or kettle Resistive heating load used briefly.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Television One medium television and streaming device.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

Garage door opener Brief motor load; runtime is small but startup matters.

Quantity Running watts W Starting watts W Hours per day hr Voltage Use pattern

3 Set load and battery assumptions

Simultaneous-load plan Managed mode keeps continuous and cycling loads active, then adds the largest scheduled or intermittent load. AC delivery efficiency % Accounts for inverter and conversion losses. Protected battery reserve % Capacity intentionally left unused for battery protection or uncertainty. Planning buffer % Added to the minimum nominal capacity and continuous output target.

4 Model reliable recharge Recharge available during the outage The calculator uses real recharge inputs and keeps up to one day of load as an autonomy floor.

Your planning estimate Add loads to begin

Select a profile or appliance

The calculator will separate energy capacity, continuous output, motor startup, voltage, and recharge so one number does not hide the real constraints.

Nominal battery planning range 0.0–0.0 kWh Minimum through buffered target

Load energy: 0.0 kWh
Recharge credit: 0.0 kWh
Continuous output: 0 W
Surge capability: 0 W
Generator class: 0 W
Voltage: Not set

Load energy0.0 kWh

Reliable recharge is not included.

Best-fit starting route

Build a load plan first

Choose a profile or select appliances to receive a route based on capacity, output, voltage, duration, and your stated preference.

Review suitable systems Plan the connection

Checks before choosing equipment

Select appliances to generate specific checks.

Planning estimate only. Confirm appliance nameplates, motor data, receptacle and circuit ratings, fuel-specific generator output, transfer equipment, and local requirements.

Read the result correctly

Five outputs must agree before a system fits

A battery capacity number alone cannot tell you whether a backup system can start a pump, support 240V equipment, recharge during a long outage, or connect safely to a home.

01

Energy capacity

Load energy estimates how many kilowatt-hours your appliances consume. The battery range then accounts for delivery efficiency, reserve, recharge, and the selected planning buffer.

02

Continuous output

This is the power the system should sustain while your chosen loads operate together. Managed mode assumes scheduled and intermittent loads can be sequenced.

03

Starting demand

Motor startup is tested as the other planned loads continue running. The tool assumes one motor starts at a time, which still needs confirmation for the actual equipment.

04

Voltage

A 240V well pump, heat pump, or central air conditioner requires a system and connection that can actually supply 120/240V split-phase power.

05

Recovery

Solar and charging are modeled from array size, sun hours, input limits, losses, charging power, and time. The tool does not use a vague percentage discount.

Transparent methodology

How the calculator works

The formulas are deliberately conservative and visible. They narrow the search; they do not replace an appliance label, electrician, installer, or manufacturer design tool.

Energy during the outage

Continuous: running watts × all outage hours.

Cycling: running watts × outage hours × entered daily duty fraction.

Scheduled or intermittent: entered daily use is counted for each full day, plus up to that amount during a partial day. This avoids cutting an eight-hour CPAP plan to four hours merely because the outage lasts 12 hours.

Battery capacity

The tool subtracts modeled recharge from load energy, but never below an autonomy floor equal to the first 24 hours of selected load, or the complete outage if it is shorter.

Minimum nominal capacity = delivered energy need ÷ AC efficiency ÷ usable battery fraction. The top of the range adds your planning buffer.

Output and startup

Conservative mode adds every selected running load. Managed mode keeps continuous and cycling loads together, then adds the largest scheduled or intermittent load.

For each motor, the tool tests the other planned running loads plus that motor’s starting watts. It reports continuous and surge targets separately.

Generator class

The generator class is the larger of the buffered running target and the starting-demand target, rounded upward to a 500W planning class.

Actual output varies by fuel, model, altitude, temperature, power quality, and receptacle. Confirm both running and starting ratings.

From estimate to equipment

Use the result as a decision gate

Small 120V planCompare portable power stationsBest suited to compact essential-load plans without permanent home integration. Expandable or automatic batteryExplore home battery backupFor longer runtime, selected circuits, quieter operation, and automatic response. Long outage resilienceExplore hybrid systemsCombine stored energy with reliable replenishment when outages can last for days. High output or 240VPlan whole-home backupFor major pumps, HVAC, broader panel coverage, and installed load management.

Common sizing questions

Home backup calculator FAQ

Why is the battery result shown as a range?

The lower number is the calculated minimum nominal capacity after reserve and efficiency. The upper number adds the selected planning buffer. Actual systems come in fixed sizes, and cold weather, aging, standby draw, uncertain duty cycles, and future loads can all change the practical requirement.

Does the result guarantee runtime?

No. Runtime changes with actual appliance consumption, cycling, weather, battery state of charge, temperature, system losses, and recharge. Treat the result as a planning range and verify it against product documentation and measured household use.

Can ordinary solar panels keep working in an outage?

Not automatically. A grid-tied solar system usually shuts down when the grid fails unless it has the inverter, isolation, controls, and often storage needed for outage operation. The U.S. Department of Energy notes that solar-plus-storage systems can operate without grid support when they are designed to do so.

What if I do not know starting watts?

Use the appliance label, manual, motor data, manufacturer documentation, or a suitable measurement method. Do not assume starting watts equal running watts for a pump, compressor, blower, or other motor load. A qualified installer can confirm hard-start and locked-rotor requirements.

Should every appliance be counted at the same time?

Use conservative mode when you cannot control overlap. Use managed mode only if you will intentionally sequence scheduled and intermittent loads. Automatic load-management equipment can also shed lower-priority circuits, but its design belongs in the installation plan.

Safety and technical references

Planning values last reviewed July 2026. Calculator outputs are estimates and are not electrical, engineering, medical, or installation advice.