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Backup power calculators
Size a UPS and estimate how long batteries will run your equipment.
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FAQ
Frequently asked questions
What size UPS do I need?
Add up the watts of everything you will plug in, add 20–25% headroom, and divide by the UPS power factor (often 0.9) to get the VA rating. A 450 W load with 25% headroom needs at least about 625 VA and 563 W.
What is the difference between VA and watts on a UPS?
Watts are real power; volt-amperes (VA) are apparent power. A UPS has a limit for both, and the watt rating is VA multiplied by its power factor. Your load must stay under both numbers.
How long will a battery run my equipment?
Multiply battery voltage by amp-hours to get watt-hours, multiply by the usable fraction and inverter efficiency, then divide by the load in watts. A 12 V 100 Ah lithium battery runs a 300 W load for about 3 hours 15 minutes.
Why does a UPS run for less time than calculated?
Lead-acid batteries deliver less energy at high discharge rates (the Peukert effect), age reduces capacity, and inverters lose more at light loads. Treat any runtime calculation as an upper estimate and test with your real load.
Is lithium better than lead-acid for backup power?
Lithium iron phosphate batteries can use about 80–90% of their capacity and last thousands of cycles, while lead-acid batteries should usually be discharged only about 50%. Lithium costs more up front but often less per usable kWh over its life.
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