Critical-load planning

Home Battery Backup Calculator

Choose the appliances that matter during an outage, set the target runtime and estimate both nominal storage capacity in kWh and minimum continuous output in kW.

1. Choose critical loads

Presets are planning assumptions, not appliance measurements. Replace them with a plug meter or nameplate audit before purchase.

Target backup timeAdditional energy per day

kWh

Additional simultaneous load

2. Storage assumptions

Reserve margin

Round-trip efficiency

Usable depth of discharge

Planning result

5.4 kWh

minimum nominal battery capacity

Minimum continuous output

0.4 kW

Daily critical load

3.7 kWh

Energy delivered in outage

3.7 kWh

Generic 10 kWh units

Generic 13.5 kWh units

Important: motor starting surge, HVAC, electric water heating, EV charging and whole-home loads can require much more inverter power than this simple simultaneous-watt sum. Battery controls may reserve capacity automatically. A qualified installer must complete a load calculation and compatibility review.

Energy sets runtime

Watts × hours gives watt-hours. Add every selected load, scale to the outage duration and account for reserve, conversion losses and usable depth of discharge.

Power sets what runs at once

The inverter must handle simultaneous running watts and short starting surges. Motor loads can dominate even when their daily kWh is modest.

The panel matters too

Backup design can require a critical-load subpanel, transfer equipment, compatible controls and service upgrades that a battery-only price omits.

Battery Backup Sizing Questions

It depends on which loads must run and for how long. A critical-load system for refrigeration, lights, internet and a few outlets can be far smaller than whole-home backup with air conditioning, electric water heating or a well pump. Build a load list before choosing a battery.

Kilowatt-hours measure stored energy and affect runtime. Kilowatts measure how much power the battery and inverter can deliver at once. A system can have enough kWh for an outage but still be unable to start or run a large motor if its kW and surge rating are too low.

Not every nameplate kWh reaches your loads. Conversion and standby losses consume energy, and many owners preserve a reserve for uncertainty or battery protection. Both assumptions are editable because product controls and operating modes vary.

Only if the system is designed for islanded operation with compatible inverters, controls and backup loads. Grid-tied solar without the required backup hardware normally shuts down during an outage. Weather and array size also determine how much recharge is available.

Educational planning tool only. Verify measured loads, surge power, operating modes, fire and electrical requirements, warranty terms and installation design with current product documents and a qualified professional.