UPS Backup Time Calculator
A 1,000 VA UPS with a 150 Ah, 12 V tubular battery runs a 300 W desktop PC for about 2 h 33 min. The UPS can deliver at most 600 W (1,000 VA × 0.6 power factor), so this load uses 50% of it. Backup hours = Ah × volts × usable depth × inverter efficiency ÷ load watts.
How this result was calculated
Capacity check: 1,000 VA × 0.6 power factor = 600 W, and 300 W is 50% of that, so the UPS can run it. Backup: 150 Ah × 12 V × 50% usable × 85% inverter efficiency = 765 Wh; 765 Wh ÷ 300 W = 2.55 hours, about 2 h 33 min.
Key facts
- UPS backup time in hours = battery Ah × battery volts × usable depth of discharge × inverter efficiency ÷ load in watts; the VA rating sets how big a load the UPS can carry, not how long it runs.
- A line-interactive home UPS delivers about 0.6 W per VA, so a 1,000 VA (one kVA) UPS can run loads up to about 600 W; an online UPS delivers closer to one W per VA.
- A 1,000 VA UPS on a 150 Ah, 12 V tubular battery gives about 2 h 33 min at 300 W (a desktop PC), about 1 h 55 min at 400 W, and about 8 h 30 min for a 60 W router and CCTV load on a 100 Ah battery.
- The calculator refuses a load above VA × power factor instead of showing a shorter backup, because a real UPS trips on overload rather than running the load for less time.
Formula
P_max = VA × PF (load must be ≤ P_max); t = (Ah × V × DoD × η) ÷ P
- P_max = the most watts the UPS can deliver
- VA = UPS rating in volt-amperes, PF = UPS power factor
- t = backup time in hours
- Ah = battery capacity, V = battery bank voltage
- DoD = usable depth of discharge, η = inverter efficiency
- P = load in watts
UPS backup time for a PC
A desktop PC with a monitor is about 300 W when working and more when gaming, so on a 1,000 VA UPS with a 150 Ah tubular battery you get about 2 h 33 min, enough to ride through a normal loadshedding slot or to save and shut down. A small office UPS with its built-in sealed battery of a few Ah gives only minutes; enter its Ah to see how few.
A gaming PC at 600 W needs a bigger UPS, not just a bigger battery: a 1,000 VA line-interactive unit can only deliver about 600 W. A 1,500 VA UPS delivers about 900 W, and on two 200 Ah batteries at 24 V it runs the gaming PC for about 3 h 24 min.
Why the load must fit the UPS rating first
A UPS has two limits. The VA rating with its power factor sets the largest load it can carry at any moment; the battery sets how long it can carry it. This tool checks the first limit before the second: if the load is more than VA × power factor, it shows an error instead of a backup time, because the UPS would trip on overload and run the load for no time at all.
Line-interactive home UPS units (the usual square-wave or modified sine wave units in Pakistan) are built for a power factor of about 0.6, so a 1,000 VA unit delivers about 600 W. Online and better pure sine wave units manage closer to one W per VA. Change the power factor under Advanced to match the label on your UPS.
Worked examples
Desktop PC and monitor (300 W) on a 1,000 VA UPS
1,000 VA × 0.6 = 600 W, so the PC uses 50% of the UPS. 150 Ah × 12 V × 50% × 85% = 765 Wh, ÷ 300 W = about 2 h 33 min.
Wi-Fi router and CCTV (60 W) on a 650 VA UPS with a 100 Ah battery
650 VA × 0.6 = 390 W, and the router with a small CCTV recorder is only 15% of that. 100 Ah × 12 V × 50% × 85% = 510 Wh, ÷ 60 W = about 8 h 30 min, enough for several loadshedding slots in a day.
Gaming PC (600 W) on a 1,500 VA UPS with two 200 Ah batteries
1,500 VA × 0.6 = 900 W, so the gaming PC is 67% of the UPS. Two 200 Ah batteries in series make a 24 V bank: 200 Ah × 24 V × 50% × 85% = 2,040 Wh, ÷ 600 W = about 3 h 24 min.
One kVA UPS with a 150 Ah battery at 400 W
A PC, monitor and a fan come to about 400 W, which is 67% of the 600 W the UPS can deliver. 765 Wh ÷ 400 W = about 1 h 55 min.
UPS backup time for common loads on a 1,000 VA UPS with a 150 Ah, 12 V tubular battery
| Load | Share of UPS | Backup time |
|---|---|---|
| 10 W | 2% | 76 h 30 min |
| 60 W | 10% | 12 h 45 min |
| 100 W | 17% | 7 h 39 min |
| 150 W | 25% | 5 h 6 min |
| 250 W | 42% | 3 h 4 min |
| 300 W | 50% | 2 h 33 min |
| 400 W | 67% | 1 h 55 min |
| 600 W | 100% | 1 h 16 min |
Common mistakes
- Reading VA as watts. A 1,000 VA line-interactive UPS delivers only about 600 W; a load above VA × power factor trips the UPS, and this tool rejects it for that reason.
- Expecting hours from a small office UPS. Its built-in sealed battery holds a few Ah, so backup is minutes; long backup needs an external tubular or lithium battery.
- Entering the PC's power supply rating as the load. A PC with a supply rated for several hundred watts usually draws far less; use a plug-in meter or the appliance load calculator.
- Draining the battery fully. Tubular batteries should be used to about 50%; many UPS units cut off later than that, which gives longer backup but a shorter battery life.
- Adding the Ah of batteries in series. Two batteries in series on a 24 V UPS double the volts, not the Ah.
Safety: Fit the battery with a fuse and correctly sized cable, keep lead-acid batteries ventilated, and have a licensed electrician connect a UPS to the house wiring.
Frequently asked questions
Which UPS do I need for a desktop computer?
A desktop PC with a monitor is about 300 W, so a 1,000 VA line-interactive UPS (600 W) runs it at 50% load. For long loadshedding backup pair it with an external tubular battery; a 150 Ah battery gives about 2 h 33 min.
How long will a UPS last for a PC?
About 2 h 33 min for a 300 W PC on a 150 Ah, 12 V tubular battery, and about 1 h 55 min at 400 W with a fan added. A small office UPS with its own sealed battery lasts only minutes at this load.
What is the UPS backup time for a Wi-Fi router during loadshedding?
A router or fibre modem is about ten watts, so almost any UPS carries it; the battery decides the hours. A router with a small CCTV recorder, about 60 W, runs about 8 h 30 min on a 100 Ah battery, and a router alone runs several times longer.
How much backup does a UPS give for CCTV?
A DVR with a few cameras is a few tens of watts. On the 650 VA UPS with a 100 Ah battery in the example, 60 W of router and CCTV gives about 8 h 30 min; enter your own camera count and watts for an exact figure.
What is the backup time of a 1,000 VA (one kVA) UPS?
It depends on the battery and the load, not the VA. With a 150 Ah tubular battery a one kVA UPS gives about 2 h 33 min at 300 W and about 1 h 55 min at 400 W. The 1,000 VA rating only limits the load to about 600 W.
What is the UPS battery backup calculation formula?
Backup hours = battery Ah × battery volts × usable depth of discharge × inverter efficiency ÷ load watts, after checking that the load is no more than VA × power factor. For a tubular battery use about 50% usable and about 85% efficiency; for lithium about 90% usable.
Which UPS is right for a gaming PC?
A gaming PC can draw 600 W or more, which is beyond a 1,000 VA line-interactive UPS. A 1,500 VA unit delivers about 900 W, and with two 200 Ah batteries at 24 V it gives about 3 h 24 min. Choose a pure sine wave model for a PC power supply.
Sources
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