Loading page
About this tool
Estimate home inverter backup hours from battery Ah, bank voltage, connected load and inverter efficiency, with Peukert correction.
The Inverter Backup Time Calculator estimates how many hours a home inverter will run a given load, using the energy balance backup hours = usable battery watt-hours divided by (load watts / inverter efficiency), then correcting it with Peukert's law t = H x (C / (I x H))^k. That second step matters because inverter batteries are rated at the 20-hour rate (C20) but a real household load drains them in three or four hours, where a lead-acid bank delivers materially less than its printed Ah. It is aimed at homeowners sizing a new inverter battery and anyone wondering why a 150 Ah battery does not give the runtime the box promised.
Open Inverter Backup Time Calculator on AltFTool — it loads instantly in your browser.
Enter Battery capacity per block (Ah at C20), Battery voltage (V) and Batteries in series, then pick Battery type — Tubular lead-acid, Flat plate lead-acid, SMF / VRLA sealed or Lithium LiFePO4.
Set Connected load (W), adding appliances with the + preset buttons, and adjust Inverter efficiency (%) and Battery state of charge (%).
Read Realistic backup time, which applies Peukert's law on top of the plain watt-hour sum, then enter Backup you want (hours) to get the rated Ah under Battery size for your target backup.
Shows the realistic runtime, not just the optimistic Wh-divided-by-watts number.
Applies the right depth of discharge and Peukert exponent for tubular, flat plate, SMF and LiFePO4.
Tells you the rated Ah needed to hit the backup duration you actually want.
On a 12 V system with a 300 W load and an 80% efficient inverter, expect roughly 2 hours 50 minutes of real backup. The plain energy sum suggests 3 hours 50 minutes (1440 usable Wh divided by 375 W of DC draw), but the Peukert effect at a 31 A discharge cuts about a quarter of that away.
Backup hours = (battery Ah x bank voltage x depth of discharge) / (load watts / inverter efficiency). For a 150 Ah 12 V tubular battery at 80% depth of discharge feeding a 300 W load through an 80% efficient inverter that is (150 x 12 x 0.8) / (300 / 0.8) = 3.84 hours before Peukert losses.
Three reasons stack up: the inverter itself wastes 10-25% of the energy as heat, lead-acid batteries should only be discharged to about 80% depth to protect cycle life, and Peukert's law means fast discharge yields fewer usable amp-hours than the 20-hour rating. An aged or sulphated battery loses more still.
Yes, because two 12 V 150 Ah batteries in series make a 24 V 150 Ah bank holding twice the watt-hours, and the same load then draws half the current, which also softens the Peukert penalty. Your inverter must be rated for that bank voltage — never wire a 24 V bank to a 12 V inverter.
Add the Inverter Backup Time Calculator widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/inverter-backup-time-calculator"
title="Inverter Backup Time Calculator — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/inverter-backup-time-calculator?utm_source=embed&utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>