🔋 How to calculate UPS runtime properly
The vendor calculators from APC, Eaton & Co. are product selectors: they use the maximum nameplate rating and tell you which model to buy. This calculator uses your measured load and shows you the formula.
The formula, short and honest
Battery energy: E = blocks × volts × Ah. Only part of it is usable: E_usable = E × depth of discharge × efficiency × ageing × Peukert. Runtime is then t = E_usable / load.
Example: two blocks of 12 V, 9 Ah give 216 Wh. With 50 % depth of discharge and 90 % efficiency, 97.2 Wh remain usable — once the battery set has aged (factor 0.8) only 77.8 Wh are left. At 380 W that means 12.3 minutes instead of the 15.3 minutes you calculate with fresh cells.
Five wrong assumptions that cost you runtime
- „1000 VA sind 1000 W" — nein. VA ist Scheinleistung, W ist Wirkleistung. Der Leistungsfaktor deiner Last (0,6 bis 1,0) entscheidet über die Umrechnung.
- Netzteil-Nennleistung als Verbrauch — ein 750-W-Netzteil zieht selten mehr als 250–400 W. Wer damit rechnet, kauft die dreifache USV.
- Batterie-Werbung als nutzbare Energie — „845 VAh" ist keine Wh-Angabe. Ohne Entladetiefe und Wirkungsgrad ist sie wertlos.
- Lineare Entladung — je höher die Last, desto stärker sinkt die nutzbare Kapazität. Die Peukert-Korrektur fängt das ab.
- „Möglichst lange laufen" — das Ziel ist ein sauberes Herunterfahren oder die Überbrückung bis zum Generator, nicht drei Stunden Betrieb.
What you need the UPS calculator for
You are standing in front of the rack and want to know whether the existing UPS still carries the new server. Or you are planning a purchase and want to know which model realistically bridges 15 minutes. Both questions come down to two numbers — measured real power and battery capacity — as long as you do not forget depth of discharge, efficiency and ageing. Those are exactly what vendor calculators tend to forget.
How to use the calculator
Measure your load's real power (power meter, UPS display, PDU) and enter it in watts — not the nameplate rating of the PSUs.
Enter battery blocks, volts per block and capacity in Ah. The values are printed on the battery or in its datasheet (e.g. 2 × 12 V / 9 Ah).
Pick the battery type — the depth-of-discharge, efficiency and ageing sliders are set accordingly and can be adjusted by hand at any time.
For sizing, add the target runtime and the load's power factor: you get the required nominal energy, the required Ah figure and the UPS class in VA and watts.
Worked example: a rack drawing 380 W
A rack draws a measured 380 W. The UPS holds two 12 V / 9 Ah blocks. The calculator returns 216 Wh nominal and — with aged batteries — 78 Wh usable, i.e. 12 minutes of runtime; with fresh cells it would be 97 Wh and 15 minutes. To bridge half an hour you need roughly 528 Wh of nominal energy (≈ 22 Ah at 24 V) with aged cells, 422 Wh (≈ 18 Ah) with new ones. The matching device is a UPS rated at least 550 VA and 500 W real power plus reserve.
Frequent questions
Why does the vendor state different runtimes?
Vendors measure with a defined load (often 50 %) and a fresh battery set. As soon as your load is higher or the batteries have aged, runtime drops noticeably — hence the ageing factor in this calculator.
Which depth of discharge should I choose?
Lead/AGM batteries last longer when you discharge them only halfway (50 %). For rare, short outages 70–80 % is acceptable. LiFePO4 tolerates 80–90 % without major wear.
What is the power factor?
It describes the ratio of real to apparent power in your load: server PSUs with PFC sit at 0.9 to 1.0, older gear without PFC at 0.6. It matters for the VA rating of the UPS, not for runtime.
Does a bigger UPS give longer runtime?
No. Runtime depends on battery capacity, not on the VA class. A 3000 VA UPS with a small battery pack runs shorter than a smaller model with an external battery pack.
Does the calculator replace the vendor datasheet?
No — it is a planning figure. For commitments to customers or in a data centre the vendor datasheet applies. The calculator helps you check quotes and disprove rough estimates.
Sources & further reading
The formulas and assumptions follow common battery and UPS practice (Peukert equation, depth of discharge, inverter efficiency). More in the blog: backup strategies and backups with restic. To plan the full hardware chain, use the RAID calculator, the IOPS calculator and the capacity converter.
🔋 Continue with the IOPS calculator — or the download time.