Offline
No internet, no account required.
NEC 706
Validated against the standard.
PDF export
Pro-only. Generate professional reports.
$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC 706 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Battery sizing decides how much storage a system needs to ride through the night or an outage. This app sizes a battery bank from the backed-up load, the days of autonomy, and the usable depth of discharge, following good practice and NEC Article 706 for energy storage.
How it works
Battery kWh = daily load kWh × days of autonomy ÷ usable depth of discharge
Lithium systems often allow 80–90% usable depth of discharge; lead-acid much less. Round trip efficiency further trims what you can draw.
Worked example
To back up 10 kWh/day for 1 day at 90% usable depth of discharge: 10 × 1 ÷ 0.90 = 11.1 kWh of battery — so a 13.5 kWh unit gives comfortable margin.
Related calculators
Size generation with the panel sizing calculator and the inverter with the inverter sizing calculator.
Battery sizing FAQ
What is depth of discharge?
Depth of discharge is how much of the battery's capacity you can use. Lithium chemistries commonly allow 80–90%, while lead-acid is kept much shallower to protect cycle life.
What does days of autonomy mean?
It is how many days the bank must carry the load without recharging — one day for typical outage backup, more for off-grid systems in cloudy climates.
Does it consider efficiency?
Yes. Round-trip efficiency and usable depth of discharge are both applied so the nameplate kWh reflects what you can actually draw.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Battery Sizing on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.