Tools · Cameras

UPS runtime calculator

Enter what is on the UPS, the recorder, the PoE switch and the cameras, and its battery: this works out how long it keeps them recording, corrected for a fast drain, and the battery a target runtime needs.

Examples from our guides

What is on the UPSwatts

Recorder (NVR or DVR)

PoE switch (its own power)

Cameras, entered as

PoE cameras

Watts each (measured)

6 W per camera is under the 802.3af limit of 12.95 W at the camera (15.4 W at the switch port).

Modem and router

Other equipment (W)

The battery and the target

Battery, entered as

12 V blocks

Amp-hours each (Ah, on the label)

Runtime you want (hours)

Inverter efficiency (percent, typical 85)

Drain exponent (typical 1.19)

Aging factor (typical 1.25)

UPS watt rating (W, 0 to skip)

Efficiency 85 percent: an example figure for the inverter. A UPS carrying a small fraction of its rating is often less efficient.

Exponent 1.19: a Peukert exponent fitted to one 12 V 9 Ah sealed lead acid data sheet, 0.45 A for 20 hours and 5.56 A for 1 hour. Use your battery's own data if you have it.

Aging 1.25: IEEE 485 sizes lead acid batteries so that one worn down to 80 percent of its rated capacity still carries the load.

Planning runtime

2 h 12 min

to plan on at 117 W (3 h 8 min by the formula)

This 432 Wh battery falls short of 4 h.

An estimate from the inputs above; a site survey sets the real numbers.

4 × 12 V × 9 Ah. At the 4 h target: empty after 2 h 12 min.

Planning 2 h 12 min
Formula 3 h 8 min
Aged battery 1 h 42 min

Charge is drawn as a straight line from full to empty at a steady load. The planning line includes the fast-drain correction; the aged line is the same battery at 80 percent of its rating.

Load: 117 W; 30 W recorder + 15 W switch + 12 × 6 W cameras = 117 W
Battery energy: 432 Wh; 4 × 12 V × 9 Ah = 432 Wh
Formula runtime: 3.14 h; 432 Wh × 0.85 ÷ 117 W = 3.14 h
Aged battery, to plan on: 1 h 42 min
UPS watt rating

Battery for the target

For 4 h at 117 W: 712 Wh of battery, or 890 Wh with the 1.25 aging margin. That is 9 blocks of 12 V 9 Ah (972 Wh), or the same watt-hours in an external battery pack.

Need the PoE number? Work out the cameras' cold night draw first.

PoE budget calculator

The math with your numbers

Load30 W recorder + 15 W switch + 12 × 6 W cameras = 117 W
Battery energy4 × 12 V × 9 Ah = 432 Wh
Formula runtime: battery Wh × efficiency ÷ load432 Wh × 0.85 ÷ 117 W = 3.14 h
Planning runtime, corrected for a fast drain20 × (3.14 ÷ 20) to the power 1.19 = 2.21 h, about 2 h 12 min
Aged battery, at end of life432 Wh ÷ 1.25 = 346 Wh left at end of life: 2.51 h by the formula, 1.69 h to plan on
Battery for the 4 h target20 × (4 ÷ 20) to the power 1 ÷ 1.19 = 5.17 h formula time. 117 W × 5.17 h ÷ 0.85 = 712 Wh; × 1.25 for aging = 890 Wh.

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How to use the UPS runtime calculator

Enter the load. The recorder, the PoE switch on its own and the cameras, as a count and watts each or as the total PoE draw from the PoE budget calculator. Add the modem and router if you want remote viewing during an outage.
Enter the battery. The number of 12 V blocks and their amp-hours from the label, or the watt-hours if you know them.
Set the runtime you want. The calculator works out the battery that target needs, with and without the aging margin.
Read the planning runtime first. The formula runtime is the optimistic figure; the aged line is what a worn battery gives.
Check it against the UPS maker's runtime chart at your measured load, which is the better number for a given model.

How UPS runtime is worked out

A UPS holds a battery and an inverter. How long it carries the closet depends on two numbers: how much energy the battery holds, and how fast the equipment uses it. The calculator runs the same four steps as the runtime figure in our power outage guide.

load = recorder + switch + cameras + other

battery Wh = blocks × 12 V × amp-hours

formula hours = battery Wh × efficiency ÷ load

planning hours = 20 × (formula hours ÷ 20)^1.19

battery needed = load × 20 × (target ÷ 20)^{1 ÷ 1.19} ÷ efficiency, × 1.25

Lead acid amp-hour ratings are measured over a slow 20-hour discharge, and a fast drain gives less. The 1.19 exponent is fitted to one 12 V 9 Ah data sheet (0.45 A for 20 hours, 5.56 A for 1 hour). Past 20 hours the drain is slower than the rating, so the formula figure stands. The 1.25 aging factor follows IEEE 485, so a battery worn to 80 percent of its rating still covers the target.

Worked example: a walk-up with 12 cameras

The first example is the five-story walk-up from our power outage guide: a 30 W recorder, a 15 W PoE switch and 12 cameras at 6 W, a 117 W load, on a UPS with four 12 V 9 Ah batteries. The battery holds 4 × 12 × 9 = 432 Wh. The formula gives 432 × 0.85 ÷ 117 = 3.14 hours; corrected for the drain, 20 × (3.14 ÷ 20)^1.19 = 2.21 hours, about 2 h 12 min to plan on.

For a 4 hour target, the formula time needed is 20 × (4 ÷ 20)^{1 ÷ 1.19} = 5.17 hours, so the battery is 117 × 5.17 ÷ 0.85 = 712 Wh, or about 890 Wh with the aging margin. That is about twice the example battery, nine 12 V 9 Ah blocks, which is why long runtimes come from external battery packs or a generator.

Typical power draws on a UPS

Example power draws, typical
Equipment	Example draw
8-channel recorder, one drive	15 W
16-channel recorder, two drives	30 W
32-channel recorder, four drives	50 W
PoE switch, its own power	10 to 25 W
Fixed camera, infrared on	6 W
PTZ camera with a heater	20 W
Modem and router together	15 W

Examples for this kind of equipment, not data sheet figures. Many UPS units show the load in watts on their display; a measured number beats any table.

Common UPS sizing mistakes

Reading the VA rating as runtime. The battery sets the runtime; two units with the same rating can hold very different batteries.
The recorder on the UPS without the switch. The recorder keeps running with no cameras feeding it, so nothing records.
A surge-only outlet. Some UPS outlets have no battery behind them. Plug the recorder and the switch into the battery-backed side and label each plug.
Planning on the formula runtime. A fast drain gives less than the rating, most of all for short runtimes.
Forgetting the battery ages. Heat shortens battery life, and a battery a few years old gives less than either figure. Write the install date on it.

UPS runtime calculator FAQ

How do I calculate UPS runtime?

Multiply the battery watt-hours by the inverter efficiency and divide by the load in watts. That is the formula runtime, and lead acid batteries give less when drained fast, so correct it: planning hours = 20 × (formula hours ÷ 20) to the power 1.19. For 432 Wh at 85 percent and 117 W, that is 3.14 hours by the formula and about 2 hours 12 minutes to plan on.

How long will a UPS run an NVR and PoE cameras?

It depends on the load and the battery, not on the size of the box. The example system on this page, a 30 W recorder, a 15 W switch and 12 cameras at 6 W on a 432 Wh battery, plans at about 2 hours 12 minutes. The UPS maker's runtime chart at your measured load is the more reliable number.

What size UPS do I need for security cameras?

Start from the runtime you want. Add up the load in watts, work out the battery watt-hours that load needs for that runtime, then add 25 percent for battery aging. The UPS watt rating has to be above the load, and the battery, not the VA rating, sets the runtime.

Why is the real runtime shorter than the simple formula?

A lead acid battery's amp-hour rating is measured over a slow 20-hour discharge. One maker's 12 V 9 Ah battery gives about 62 percent of its rating when drained in one hour, so short runtimes come out well under the simple formula. A battery a few years old gives less again.

Does the PoE draw count toward the UPS load?

Yes. PoE cameras take their power from the switch, so the switch pulls its own power plus everything it sends over PoE. Use the measured draw on the coldest night, not the class reservations; the PoE budget calculator works it out and carries it here.

Read the full guideDo security cameras still record during a power outage?What keeps recording when the power goes, how to choose and connect a UPS, and what to check when the power comes back.

Glossary: NVR (network video recorder), PoE switch and network rack.

Sources and standards

IEEE Std 485. Sizing lead acid batteries for stationary use: the 1.25 aging factor, so a battery at 80 percent of rated capacity still carries the load.
12 V 9 Ah sealed lead acid battery data sheet. 9 Ah at the 20-hour rate (0.45 A) and 5.56 A for 1 hour: the source of the 1.19 exponent.
IEEE 802.3af and 802.3at. 12.95 W and 25.5 W at the camera, used as limits, not typical draws.
Seneca Learn, security footage and power outages. The example system and its example power figures.

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