How long will the battery actually hold?
Most backup calculators multiply amp-hours by volts and divide by the load. That overstates the answer, often by half, because a lead-acid battery rated at the 20-hour rate does not deliver its printed capacity when you discharge it in twenty minutes. This one derates for the discharge rate and shows you the working.
Real power in watts, not the UPS rating. Work it out.
DC bus 192 V
Rated at C20. Loses capacity sharply at high discharge rates.
90% is typical for a double-conversion UPS on battery. Larger units run higher.
| Bank | Backup |
|---|---|
| 16 × 12 V 26 Ah | 39 min |
| 16 × 12 V 42 Ah | 72 min |
| 16 × 12 V 65 Ah | 2 h 6 min |
| 16 × 12 V 100 Ah | 3 h 36 min |
| 16 × 12 V 150 Ah | 5 h 53 min |
| 16 × 12 V 200 Ah | 8 h 20 min |
Capacity and runtime are two different purchases
The UPS rating sets how much load you can connect. The battery bank sets how long it holds. A 10 kVA unit can run for eight minutes or for two hours on the same electronics — the difference is entirely the bank behind it, and it is usually the larger part of the invoice. Decide the runtime you need first, then let the bank follow from it.
Backup time questions
Anything not covered here, call us — most of it is faster to answer on the phone.
How do you calculate UPS battery backup time?
Multiply the bank capacity in amp-hours by the DC bus voltage to get watt-hours, then reduce it by the depth of discharge, the inverter efficiency and the rate factor, and divide by the load in watts. The rate factor is the part people miss: a lead-acid battery rated at the 20-hour rate delivers only about half its printed capacity if you discharge it in an hour.
Why does my UPS run for less time than the calculation says?
Three usual reasons. The bank is rated at C20 and you are discharging it far faster, so you never had the full capacity available. The bank has aged and lost capacity while still reading a healthy float voltage. Or the room runs hot — VRLA life and capacity both fall away above 25 °C. A discharge test is the only way to separate these.
How many batteries do I need for 30 minutes of backup?
It depends on the load and the DC bus voltage your UPS runs, because the number of batteries in series is fixed by the UPS and only the amp-hour rating is yours to choose. As a rough guide, a 10 kVA unit at full load on a 192 V bus needs around 100 Ah cells for 30 minutes. Halve the load and the same bank runs well past an hour.
Can I just add more batteries in parallel for longer backup?
Sometimes, but check with us first. Extra parallel strings raise the charging current demand, and the UPS charger has to be rated for it or the bank never reaches full charge. Parallel strings also need to be matched in age and capacity, or the newer string carries the older one and degrades to match it.
Is it better to have more batteries or higher capacity batteries?
Higher capacity cells in a single string, usually. Fewer parallel strings means fewer connections, less imbalance between strings and a simpler bank to test. Parallel strings become the practical answer only when the cell size you would otherwise need is too heavy to handle or not made.