Why is UPS runtime shorter at higher loads than the math suggests?

Lead-acid batteries deliver less total energy when discharged quickly (the Peukert effect), and inverter losses are fixed overhead. Doubling the load more than halves the runtime — which is why runtime charts aren't linear.

What size UPS do I need for my load?

Size the UPS so your load is no more than about 70–80% of its watt rating. A 300W load calls for at least a 1500VA/900W-class unit — bigger if you want longer runtime, since larger units ship with more battery.

UPS Runtime Calculator

Q: How long will a UPS run an NVR and cameras?

A typical 1500VA UPS (two 12V 9Ah batteries) carries a 150W load — an NVR plus a PoE switch with a few cameras — for roughly 25–40 minutes. For hours of runtime you need external battery packs or a dedicated DC battery system.

Estimate how long a UPS will carry your NVR, switches, servers or head-end equipment through an outage — and whether it meets your runtime target.

Load & batteries

Total load (watts)

NVR ≈ 30–80 W · PoE switch ≈ its PoE load + 20 W · small server ≈ 100–300 W · modem/router ≈ 15 W.

Battery voltage

Capacity per battery (Ah)

Number of batteries

Tower 1000–1500VA units typically hold two 12V 9Ah batteries.

Target runtime (minutes)

Inverter efficiency

Estimated runtime

—

at — load → —

Battery bank energy (nominal)—

Usable energy (efficiency + discharge derating)—

Meets your target?—

Battery Ah needed for target—

Minimum UPS size for this load—

Runtime estimates include the Peukert effect — lead-acid batteries deliver less energy under fast discharge.

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Rules of thumb for backup power on security systems

Put the network on the UPS too. An NVR on battery is useless if the PoE switch feeding the cameras went dark. Cameras, switch, NVR and the internet modem all need protected power for remote access during an outage.
Load at 70–80% max. A UPS running at its limit has minimal runtime and shortened battery life. Oversize the VA rating.
Batteries age. Sealed lead-acid loses roughly 20% capacity by year 3. If the runtime you need is exactly the runtime you calculated, you'll be short in two winters — build in margin and replace batteries on a 3–5 year cycle.
Hours, not minutes? For 4–24 h standby on alarm panels and access control, use the dedicated 12/24V battery calculators on this site — DC standby batteries are far more efficient than running an inverter.

Frequently asked questions

How long will a UPS run an NVR and cameras?

A typical 1500VA unit carries ~150W (NVR + small PoE switch) for 25–40 minutes. For hours of backup, use external battery packs or DC standby batteries.

Why isn't runtime linear with load?

The Peukert effect: lead-acid delivers less total energy at fast discharge rates, and inverter overhead is fixed. Doubling load more than halves runtime.

What size UPS do I need?

Keep your load under ~75% of the UPS watt rating; for runtime, more battery matters more than more VA.

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