Choosing a UPS usually starts with one question: "How many minutes will it last?" The right answer is not a marketing number; it is the result of a short chain of calculation running from the load list to battery capacity and unit efficiency. Here we walk through that chain step by step and work a complete example on the 1000 VA / 800 W online UPS from Keyhan Energy. The unit's specifications are given on the Industrial UPS page.
Want to calculate it right now? UPS Calculator runs this same chain on your own load list and gives the required capacity in kVA and battery amp-hours.
What this page calculates
This calculation answers three questions and claims nothing more:
- Required capacity: Which of the two ceilings, watts or volt-amperes, does your load fall under?
- Backup time: With a given battery bank, how long does the unit run on battery?
- Recovery time: After each outage, how long does the battery take to be ready again?
The procedure follows the same pattern as the Solar calculator — every figure you see either comes from the official product specifications or is calculated from them, and wherever a figure must come from the actual equipment's datasheet, we do not guess it.
Step one: the load list and the difference between watts and volt-amperes
First, list the devices that must be supported and take each one's power rating from its own nameplate. The sum of these ratings is your "useful load".
Why two figures are quoted for a UPS
Two figures are stated for this unit: 1000 VA and 800 W. Their ratio is the output power factor: 800 divided by 1000 gives 0.8. That means both conditions must hold at the same time; a load with a low power factor may hit the VA ceiling before it reaches the watt ceiling.
Calculate the starting load separately
Motors, compressors and pumps draw far more current at start-up than in steady state; this current does not appear in the continuous load list, but the UPS must withstand it. Take its value only from the datasheet of that specific load.
Step two: the backup time formula
Backup time comes from a four-step chain:
- Raw battery energy = battery nominal voltage × amp-hour capacity; the result is in watt-hours.
- Usable energy = raw energy × allowable depth of discharge. This figure is set by the battery manufacturer and differs between lead-acid and lithium; a selection guide is given on the Industrial Battery page.
- Energy delivered to the load = usable energy × the unit's efficiency on battery, which for this product is stated as up to 85 percent.
- Backup time = delivered energy ÷ load power in watts.
A caution: the amp-hour capacity on the label is measured at a specific discharge rate and reference temperature, and decreases as the current rises or the temperature falls. So the output of this formula is a ceiling rather than a guarantee; the acceptance figure must be obtained from a real discharge test.
Step three: worked example on the 1 kVA UPS
We take the battery bank as 12 V, 100 Ah — the ceiling up to which this product's built-in charger is specified — and the load at worst case, i.e. the full rated load of 800 W:
- Raw energy: 12 V × 100 Ah = 1200 Wh
- Energy delivered at maximum efficiency on battery (up to 85 percent): 1200 × 0.85 = 1020 Wh
- Backup time: 1020 ÷ 800 = 1.275 hours, i.e. about 76.5 minutes
But here is what this figure is not. First, 85 percent is the stated efficiency ceiling — not a value guaranteed at every operating point; at light loads or in unfavorable temperatures, real efficiency drops. Second, this calculation assumes the battery is discharged completely, whereas no healthy battery should ever be fully discharged. The reliable figure comes from multiplying these 76.5 minutes by the allowable battery depth of discharge — so the table below is a theoretical ceiling, not a design floor.
| Fraction of rated load | Load power (W) | Theoretical backup time ceiling with a 12 V 100 Ah bank |
| Full load | 800 | 1.275 hours (about 1 hour 16 minutes) |
| Half load | 400 | 2.55 hours (about 2 hours 33 minutes) |
| Quarter load | 200 | 5.1 hours (about 5 hours 6 minutes) |
All three figures in this table are calculated with the same maximum efficiency of 85% and without applying depth of discharge; they are theoretical upper bounds, and the design figure is lower.
The practical point is that the relationship is linear: halving the load doubles the time ceiling. If your goal is to reach a specific backup time, removing unnecessary loads from the UPS output is often cheaper than enlarging the battery bank.
Step four: battery recharge time
The forgotten pillar of UPS selection is recharge time; when outages come back to back, what matters is how full the battery is by the next outage. The built-in charger delivers 15 A for sealed lead-acid and 20 A for lithium iron phosphate, up to 100 Ah.
| Battery type | Charge current | Minimum charge time for 100 Ah |
| 12 V sealed lead-acid (SLA) | 15 A | About 6 hours 40 minutes |
| 12.8 V lithium iron phosphate (LiFePO4) | 20 A | About 5 hours |
These figures come from dividing capacity by current and are the Minimum charging time; actual charging takes longer because current tapers in the final stage. For a larger bank or faster charging, an external charger is the right route: the Battery Charger and, for a lithium bank, the Lithium Battery Switching Chargers.
The real limits of this product
We state plainly where this unit delivers and where it does not, so your time is not wasted.
Where it delivers
- Single-phase useful load up to 800 W with 220 V, 50 Hz output
- Sensitive loads that need a pure sine wave and zero-millisecond transfer time (4 ms transfer to bypass)
- Unstable grids with input voltage swinging between 170 and 265 V
- Environments from −10 to +50 °C and places where noise below 50 dB matters
- Sealed lead-acid or lithium iron phosphate battery bank, with the specified built-in charging current, up to 100 Ah
Where another solution is needed
- Continuous load above 800 W or three-phase supply
- A bank larger than 100 Ah; the built-in charger is specified only up to this ceiling, and charging a larger bank must be reviewed separately
- Power needed for many hours of the day; here the issue is energy generation, not momentary backup, and the right route is a Solar Inverter or Solar Power Plant instead
- Motor loads with heavy inrush current that must be reviewed separately
What we need for an accurate quote
The more complete this list, the more precise our reply will be, without back-and-forth:
- A list of loads with the power of each in watts and the power factor, if available
- Presence of motor or compressor loads and their start-up current
- Target backup time in minutes
- Outage pattern: duration of each outage and interval between outages
- Intended battery type: sealed lead-acid or lithium iron phosphate
- Installation site temperature and space constraints for the battery bank
- Site input voltage and its sag during peak hours
Send this list via the Contact us page. If your purchase also includes batteries, chargers and ancillary equipment, Equipment Supply is the better route.
Frequently asked questions
Exactly how is UPS backup time calculated?
Convert the battery capacity to watt-hours (voltage × amp-hours), multiply by the allowable depth of discharge and then by the unit's efficiency on battery, and divide the result by the load power. This product's efficiency on battery is stated as up to 85%; that figure is a ceiling, so treat the result as a ceiling too.
Why is the actual time usually less than the calculated figure?
Three reasons: battery capacity at high discharge current is less than the label figure, low temperature reduces capacity, and a battery loses capacity as it ages. Add to these the efficiency being below its ceiling at various operating points. So treat the calculated figure as a ceiling and allow a margin.
If I double the battery's amp-hours, does the backup time double?
For light loads, roughly yes, with two caveats: the built-in charging current (15 A for sealed lead-acid and 20 A for lithium iron phosphate) is specified up to 100 Ah, and a separate charging solution must be considered for a larger bank; moreover, the recharge time also doubles.
Lead-acid or lithium iron phosphate?
Both are supported: 12 V sealed lead-acid with 15 A charging and 12.8 V lithium iron phosphate with 20 A charging. The decisive differences are in allowable depth of discharge, cycle life, weight and thermal behavior, which must be read from the datasheet.
What is the basis of the performance classification?
UPS classification is based on the IEC 62040-3 standard, and it is described on the Industrial UPS page. This standard classifies output behavior; it does not replace a real discharge test on your load.
What about software support and warranty?
This UPS's control software is developed in-house; its design and coding are the company's own work. Warranty and support terms are as announced by the company; for the current terms, get in touch Contact us.