Solar Battery Backup Hours Calculator

A solar battery backup time calculator for home load shedding that estimates hours from your bank and load. Use the free Solar Battery Backup Hours Calculator below — enter your values and get instant results, right in your browser. Usable energy (Wh) = Ah × voltage × usable depth × 0.9 (inverter efficiency). Backup hours = usable Wh ÷ load (W). Example: 200Ah × 12V × 0.7 × 0.9 = 1,512 Wh ÷ 300W = 5.04 hours.

How to use the Solar Battery Backup Hours Calculator

Load shedding hits and the real question is: how long will my batteries last? This solar battery backup time calculator for home load shedding takes your battery bank's amp-hours, voltage, and chemistry, divides by the load you actually run during outages, and gives the backup time in hours and minutes. Use it before buying batteries to check they cover your area's outage pattern — or after, to see which appliances to switch off to stretch the night.

  1. Enter your battery bank capacity in Ah (add up all batteries; series strings keep the same Ah).
  2. Select the bank voltage — 12V, 24V, or 48V — from the dropdown.
  3. Select your battery type: lead-acid gives 70% usable capacity, lithium 90%.
  4. Enter the load in watts you run during load shedding (e.g. 2 fans + 4 bulbs ≈ 208W).
  5. Click Calculate to see backup hours and minutes; switch off loads and recalculate to see the gain.

Formula

Usable energy (Wh) = Ah × voltage × usable depth × 0.9 (inverter efficiency). Backup hours = usable Wh ÷ load (W). Example: 200Ah × 12V × 0.7 × 0.9 = 1,512 Wh ÷ 300W = 5.04 hours.

Worked example

A 200Ah 12V tubular battery bank running 300W (3 fans + 5 bulbs) during load shedding: usable energy = 200 × 12 × 0.7 × 0.9 = 1,512 Wh. Backup = 1512 ÷ 300 = 5.04 hours (302 minutes). Switching off one fan (saving 80W) stretches it to 6.9 hours.

Frequently asked questions

Two fans draw ~160W. A 12V 200Ah lead-acid battery holds about 1,512 usable Wh, giving 1512 ÷ 160 = 9.45 hours. With lithium (90% usable) the same battery gives about 12.2 hours.

Lead-acid batteries lose capacity with age — a 3-year-old 200Ah battery may hold only 120–140Ah. Deep discharges below 50% also permanently damage them. If runtime has halved, the battery likely needs replacement, not a bigger inverter.

No. Backup time depends only on battery energy (Ah × voltage) and your load. A bigger inverter only lets you run heavier loads at once — it actually wastes slightly more energy at idle.

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