UPS Battery Backup Hours Calculator

A UPS battery backup time calculator for fan and lights with presets for the classic home UPS setup. Use the free UPS Battery Backup Hours Calculator below — enter your values and get instant results, right in your browser. Total load (W) = fans × 80 + bulbs × 12. Usable energy (Wh) = Ah × voltage × 0.7 (lead-acid usable depth) × 0.9 (efficiency). Backup hours = usable Wh ÷ total load. Example: 150Ah × 12V × 0.63 = 1,134 Wh ÷ 208W = 5.45 hours.

How to use the UPS Battery Backup Hours Calculator

The classic Pakistani home UPS runs fans and lights through load shedding — but will it last the full outage? This UPS battery backup time calculator for fan and lights is pre-set for the standard setup: an 80W ceiling fan and 12W LED bulbs on a 12V battery. Enter your battery size and how many fans and bulbs are on the UPS, and get the realistic backup time. It is the quickest way to decide between a 100Ah, 150Ah, or 200Ah battery at the shop.

  1. Enter your UPS battery capacity in Ah — 150Ah is the most common home size.
  2. Select 12V for a single battery or 24V if your UPS uses two batteries in series.
  3. Enter how many fans are wired to the UPS (each counts as 80W).
  4. Enter how many bulbs are on the UPS (each counts as 12W).
  5. Click Calculate to see the total load and the backup time in hours.

Formula

Total load (W) = fans × 80 + bulbs × 12. Usable energy (Wh) = Ah × voltage × 0.7 (lead-acid usable depth) × 0.9 (efficiency). Backup hours = usable Wh ÷ total load. Example: 150Ah × 12V × 0.63 = 1,134 Wh ÷ 208W = 5.45 hours.

Worked example

A home UPS has a 150Ah 12V battery running 2 fans (160W) and 4 bulbs (48W) — 208W total. Usable energy = 150 × 12 × 0.7 × 0.9 = 1,134 Wh. Backup = 1134 ÷ 208 = 5.45 hours, comfortably covering a typical 4-hour load-shedding slot. Adding a third fan drops it to 3.9 hours.

Frequently asked questions

About 5.4 hours. The load is 208W and a 150Ah 12V lead-acid battery delivers roughly 1,134 usable watt-hours, so 1134 ÷ 208 = 5.45 hours.

Not advisable. A fridge draws 150W running but 800–1,000W at compressor start, which trips small UPS units and drains the battery fast. Keep fridges on solar or the grid, and reserve the UPS for fans and lights.

For 2 fans + 4 bulbs (208W): 100Ah ≈ 3.6 hours, 150Ah ≈ 5.4 hours, 200Ah ≈ 7.3 hours. Match the battery to your area's longest load-shedding slot, plus one hour of margin.

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