Contractor Budget Kit
Guides
Browse tools

SOLAR

Solar Battery Backup Calculator

Size a home backup battery and estimate its storage cost.

JOB INPUTS

Enter your numbers

Loading saved values

kWh used per day during an outage

kWh/j
kWh used per day during an outage

How many days of backup you want

days
How many days of backup you want

Installed cost per usable kWh of capacity

$
Installed cost per usable kWh of capacity
Results update automatically
CALCULATED RESULTUSD
Recommended capacity24.0 kWh
Gross capacity (20% margin)24.0 kWh
Backup needs20 kWh
Estimated cost$25,200.00
Per usable kWh$1,050.00
FORMULA USED20 kWh × 1 days × 1,2 = 24.0 kWh × $1,050.00 = $25,200.00

HOW TO USE IT

How to use the Solar Battery Backup Calculator

  1. 1

    Enter how many kWh you use per day during an outage.

  2. 2

    Choose your days of autonomy.

  3. 3

    Set the installed cost per usable kWh.

  4. 4

    Review recommended capacity and estimated cost.

THE MATH

Battery sizing formula

Capacity = daily needs × days of autonomy × 1.2 (margin)

A 20% margin is applied to usable capacity to account for depth of discharge and aging. Estimated cost is capacity multiplied by installed price per kWh.

PRACTICAL EXAMPLE

A home using 20 kWh/day for 1 day of backup

20 kWh × 1 × 1.2 = 24 kWh usable. At $1,050 per usable kWh, the estimate is about $25,200 — roughly two Powerwall-class units.

FAQ

Frequently asked questions

What does battery storage cost per kWh?

Installed home lithium batteries commonly run $950 to $1,300 per usable kWh in 2026, depending on brand and labor. It is regional.

Should I cover 100% of usage?

No. Most homes size for essential loads — fridge, heating, lights, internet — during an outage, not full 24-hour usage.