"How long will my UPS run?" is the question we hear most. The answer depends on the load, the number and capacity of the batteries, UPS efficiency, temperature and battery age. For an exact answer use our UPS calculator – it works from the manufacturers' discharge tables. Below we explain how it all fits together.

1. Every UPS has a fixed battery voltage

Each UPS runs on a set DC battery voltage, for example 24V, 36V, 72V, 96V, 192V or more. 12V batteries are connected in series to form a string:

  • 24V = 2 batteries
  • 72V = 6 batteries
  • 96V = 8 batteries
  • 192V = 16 batteries

The number of batteries per string is set by the UPS and cannot be changed. To get more runtime you either use higher-capacity (Ah) batteries or connect several strings in parallel.

2. A rough calculation

Energy needed (Wh) = load (W) × required time (hours) ÷ UPS efficiency (≈0.9)

Example: 500 W load for 2 hours → 500 × 2 ÷ 0.9 ≈ 1110 Wh.

On a 72V UPS (6 × 12V) that is 1110 ÷ 72 ≈ 15.4 Ah. That is only the theory – in practice you must allow for:

  • Fast discharge reduces capacity. Battery Ah is rated over a 20-hour discharge. Over 1–2 hours you get roughly 60–80% of it, and over 10–15 minutes even less.
  • Age and temperature. Add a 20–25% reserve so the system still meets the target after two or three years.

In this example a realistic choice is 6 × 12V 26–40Ah batteries. The calculator gives the exact answer for a specific UPS and battery model.

A common mistake: "100Ah × 12V = 1200 Wh, so it will run 600 W for 2 hours." Because of inverter losses and fast discharge you will actually get about 1–1.3 hours.

3. Don't forget the charger

Large battery banks can take many hours to recharge. The ideal charging current for lead-acid batteries is about 10% of capacity (≈10A for 100Ah). Long-run UPS models for external batteries have stronger chargers – check this when choosing.

4. Which battery to choose

  • Lead-acid (AGM, Gel) – the standard choice for UPS systems, with the best price per Ah.
  • Pure lead – longer life and better performance at high temperatures, for critical systems.

Several batteries need a cabinet or rack and properly sized connecting cables.

5. Golden rules

  • Never mix batteries of different capacity, brand or age in one string.
  • Replace a whole string at once.
  • Keep batteries at 20–25°C – heat shortens their life more than anything else.
  • Every 6–12 months, test how long the system really runs.

See batteries and UPS systems for external batteries, or read how to choose a UPS.