How to use the UPS backup calculator
- 1 Add up the load in watts of fans, lights and other devices on the UPS.
- 2 Enter the battery capacity in Ah.
- 3 Choose the voltage and number of batteries.
- 4 Choose the battery type and read the backup time.
Worked example
A 400 W load on one 200 Ah, 12 V tubular battery. The battery stores 2,400 Wh, of which 1,200 Wh is safely usable. After inverter losses, backup is about 2 hours 33 minutes. To last a 4-hour outage, keep the load to about 255 W.
Making batteries last
- Do not drain fully: lead-acid and tubular batteries last longest if you use only about half their capacity each time.
- Top up water in tubular batteries with distilled water and keep terminals clean.
- Match the charger: long, frequent outages need a UPS that can fully recharge the battery between them.
- Lithium batteries cost more but allow deeper use, charge faster and last many more cycles.
To keep power on longer and save on bills, look at a hybrid system with the Solar System Calculator. Check the watts of each appliance with the Appliance Running Cost Calculator.
Features
Real usable capacity
50% for lead-acid and tubular, 80% for lithium.
Inverter losses
85% efficiency included.
Outage planning
Maximum load for 2 or 4 hours.
When to use it
Loadshedding
Plan what to run during outages. Add panels with the Solar System Calculator.
Buying batteries
Choose between 150 Ah, 200 Ah and lithium. Check watts with the Appliance Running Cost Calculator.
Adding solar
Size batteries for a hybrid system. See your bill with the Electricity Bill Calculator.
Saving battery life
Keep discharge within safe limits. Running an AC? Check the AC Tonnage Calculator.
Related Bills and Energy tasks
Frequently asked questions
How do I calculate UPS backup time?
Backup hours = battery Ah × voltage × usable share × inverter efficiency ÷ load in watts.
How much load do fans and lights use?
A ceiling fan uses about 75 to 100 W (inverter fans 30 to 40 W), an LED bulb 10 to 15 W, a TV 80 to 150 W and a router about 10 W.
Why only 50% for lead-acid batteries?
Discharging lead-acid and tubular batteries much below half shortens their life.
Are lithium batteries worth it?
They cost more but give more usable capacity, last longer and charge faster.
Can I run an AC on a UPS?
Normal UPS systems cannot run an AC for long. Inverter ACs need large solar or lithium systems.
How long does a battery last?
Tubular batteries often last 2 to 4 years with care; lithium batteries can last much longer.
Why is my real backup shorter?
Old batteries, high heat and motors starting can reduce backup.
Should I use two batteries?
A 24 V system with two batteries is more efficient for loads above about 600 W.
What size UPS do I need?
Choose an inverter rated at least 20% above your total load, for example a 1,000 W UPS for 700 to 800 W of load.
How long does a battery take to charge?
Usually 8 to 12 hours for a tubular battery from mains power, faster for lithium.
Why does my UPS beep and shut down early?
When battery voltage falls too low, the UPS switches off to protect the battery. Old batteries, loose connections or too much load can make this happen sooner than expected.
How should I look after a tubular battery?
Check the water level every month, top up with distilled water, keep terminals clean and avoid deep discharge. A well-kept battery gives longer backup and lasts years longer.
Results are estimates based on the official rates linked above. Confirm important figures with the issuing authority or a qualified adviser before you act on them.
Built and reviewed by Hamza Farooq, founder and lead developer
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