UPS Sizing Calculator – VA, Watts and Battery Runtime

Enter the total power of the devices on the UPS – typically the router, switch, access points and NAS – and how long they should keep running. The calculator adds headroom, converts watts to the VA rating used by UPS makers and estimates the runtime with a given battery set. Lead-acid batteries deliver less of their capacity at high load, so treat the runtime as an estimate.

Formula

W(UPS) = load × (1 + headroom)
VA = W(UPS) / PF
Wh = load × t / (η × usable)
t(est) = V × Ah × n × η × usable / load

Calculations run locally in your browser. Results are for information only — for engineering, electrical or financial decisions, verify the assumptions and applicable standards.

Connected load (W): 60 · Required runtime (min): 15 · Headroom (%): 30 · UPS power factor: 0,6 · Inverter efficiency (%): 85 · Usable capacity (%): 60 · Battery voltage (V): 12 · Battery capacity (Ah): 9 · Number of batteries: 2 → Recommended UPS rating: 130 VA
Connected load (W): 250 · Required runtime (min): 20 · Headroom (%): 30 · UPS power factor: 0,6 · Inverter efficiency (%): 85 · Usable capacity (%): 60 · Battery voltage (V): 12 · Battery capacity (Ah): 9 · Number of batteries: 4 → Recommended UPS rating: 542 VA
Connected load (W): 40 · Required runtime (min): 60 · Headroom (%): 30 · UPS power factor: 0,6 · Inverter efficiency (%): 85 · Usable capacity (%): 60 · Battery voltage (V): 12 · Battery capacity (Ah): 9 · Number of batteries: 1 → Recommended UPS rating: 87 VA

What is the difference between VA and W on a UPS?

VA is apparent power, W is real power. They are linked by the power factor: W = VA × PF. Many small UPS units have a power factor of about 0,6, so a 1000 VA model delivers only about 600 W – always check both values.

How long will my UPS run?

Runtime is roughly battery energy × efficiency × usable capacity ÷ load. Two 12 V 9 Ah batteries store 216 Wh; at 120 W with 85 % efficiency and 60 % usable capacity that is about 55 minutes. Halving the load more than doubles the runtime.