"How long will my UPS last on battery?" is one of the most searched UPS questions in India, and it is also one of the most consistently miscalculated, usually because people either skip the efficiency factor entirely or use a formula meant for a completely different unit system. Here is the actual formula, explained properly.

The Formula

Backup Time (hours) = (Battery Capacity in Ah × Battery Voltage × Battery Efficiency) ÷ Load in Watts

Each term matters:

• Battery Capacity (Ah) — the ampere-hour rating printed on the battery, representing how much current it can deliver over time

• Battery Voltage (V) — the nominal voltage of the battery bank (commonly 12V per battery, multiplied by the number of batteries in series for the UPS's DC bus voltage — e.g., 4 × 12V batteries in series = 48V)

• Battery Efficiency — accounts for real-world losses in the inverter and battery itself; a reasonable planning assumption is 0.7-0.8 (70-80%), not 1.0

• Load in Watts — your actual connected load drawing power during the outage, not the UPS's rated capacity

Why the Efficiency Factor Is Not Optional

This is the single most common mistake in DIY backup time calculations: using the raw Ah × V figure without discounting for efficiency, which overstates actual runtime by 20-30%. Real losses come from the inverter's own conversion efficiency, internal battery resistance (which increases output as the battery's state of charge drops), and a planning safety margin against optimistic-case figures. Always apply a realistic efficiency factor — 0.75 is a reasonable default for planning purposes if the manufacturer has not published a specific figure for your unit.

Worked Example 1: A Single 12V/7Ah Battery, Small Home UPS

A typical small home UPS load — router, modem, a couple of lamps — drawing roughly 60W, backed by a single 12V, 7Ah battery:

Backup Time = (7 × 12 × 0.75) ÷ 60 = 63 ÷ 60 = 1.05 hours (~63 minutes)

Worked Example 2: A 4-Battery Bank (48V), Small Office UPS

A small office UPS with 4 × 12V, 42Ah batteries in series (48V system), supporting a 10-PC office load of approximately 1,200W (see our worked sizing example in How to Calculate UPS Capacity):

Backup Time = (42 × 48 × 0.75) ÷ 1,200 = 1,512 ÷ 1,200 = 1.26 hours (~76 minutes)

Worked Example 3: A Larger Battery Bank for Extended Backup

A data center server rack load of around 3,000W, requiring at least 2 hours of independent backup before the DG set is guaranteed to be running and stable, using a 48V system:

Rearranging the formula to solve for required Ah:

Required Ah = (Load × Backup Time) ÷ (Voltage × Efficiency) = (3,000 × 2) ÷ (48 × 0.75) = 6,000 ÷ 36 = 166.7 Ah

This would typically be achieved using either large-format batteries or multiple parallel battery strings reaching this combined Ah capacity — see Extend UPS Backup with External Battery Banks for how this is practically configured when the standard internal battery compartment is not large enough.

The Most Common Mistake: Confusing UPS VA Rating With Actual Backup Time

Many buyers assume a higher kVA-rated UPS automatically means longer backup time. It does not — kVA rating describes how much load the UPS can support, not how long the battery lasts. Backup time is purely a function of battery Ah capacity, voltage, and your actual load, independent of the UPS's maximum rated capacity. A 10 KVA UPS running a 2 KVA load with a small battery will have a shorter backup time than a 3 KVA UPS running the same 2 KVA load with a larger battery bank. Always check battery Ah rating and backup time at your specific load separately from the kVA capacity rating.

A Second Common Mistake: Ignoring Battery Age and Temperature

The Ah rating on a battery's nameplate reflects its capacity when new, under standard test temperature (typically 25°C). Real-world backup time degrades as batteries age — a battery at 70% of its original capacity (a reasonable threshold for replacement consideration) delivers roughly 70% of the calculated backup time, not the nameplate figure. High ambient temperatures, common in Indian installations without climate control, also accelerate this capacity degradation over the battery's life. See VRLA vs Lithium-Ion UPS Batteries for how battery chemistry affects temperature tolerance and aging behavior differently.

A Quick Reference Table

(All figures use 0.75 efficiency factor; actual results vary by manufacturer and battery condition.)

The Bottom Line

Backup time calculation is straightforward once you use the correct formula and apply a realistic efficiency factor — the two most common sources of error are skipping the efficiency discount and confusing kVA capacity with battery runtime, both of which lead to overly optimistic expectations that show up as a disappointing surprise during an actual extended outage.

Paradyne's technical team calculates exact backup time for your specific load and battery configuration as part of every quote, so you know precisely what runtime to expect before you buy.