Tools · Networking
PoE budget calculator
Enter a PoE switch and the cameras and devices on it: this works out whether its PoE budget powers every device on the worst night, counted by class or by measured draw, and the switch the list needs.
Examples from our guides
The switch
Example switches
Port standard
Each port gives up to 30 W at the port, 25.5 W at the device, Class 4 at most.
The switch counts
Conditions
Devices on the switchcount
Camera draws are examples measured at the port for this kind of camera, not data sheet figures. For any other PoE device, such as an intercom door station, a door controller or an access point, use Other device with its class and its worst-hour draw.
Why 20 percent: the standard sets no margin. About 20 percent is a common planning figure: room for a camera added next year, a heater on a cold night and some aging in the switch power supply.
Class power: IEEE 802.3 class maximums at the port. Class 1 4.0 W, Class 2 7.0 W, Class 3 15.4 W, Class 4 30 W, Class 5 to 8 45, 60, 75 and 90 W. Class 0 is a device that announces no class: 15.4 W.
Port too small: a device whose class is above what the port gives is held to the port maximum. An 802.3af port treats a Class 4 camera as Class 0: 15.4 W at the port, 12.95 W at the camera.
Over by 44.4 W
2 devices get no power
2 devices get no power. Counted by class, the devices need 164.4 W from a 120 W budget.
An estimate from the inputs above; a site survey sets the real numbers.
- Indoor dome, Class 2
- Bullet with IR, Class 3
- PTZ with heater, Class 4
- No power
- Port too small
- Ports used
- 13 of 16
- Reserved by class
- 164.4 W
- 6 × 7 W + 6 × 15.4 W + 1 × 30 W = 164.4 W
- Measured draw, cold night, heater on
- 99 W
- 6 × 4.5 W + 6 × 8 W + 1 × 24 W = 99 W
- Measured draw, cold night
- 99 W
At least 13 PoE ports, 802.3at (PoE+) or higher, and 205.5 W of PoE budget counted by class, which keeps 20 percent spare.
Cold night draw of every device: 99 W. That is the PoE part of the UPS load.
Carry it to the UPS runtime calculatorThe math with your numbers
- Ports13 devices, 16 PoE ports: every device has a port.
- Reserved by class (each device holds its class maximum at the port)6 × 7 W + 6 × 15.4 W + 1 × 30 W = 164.4 W
- Measured draw on a cold night with heaters on6 × 4.5 W + 6 × 8 W + 1 × 24 W = 99 W
- As the switch counts (by class), powered in port order119 W powered of a 120 W budget; 2 devices asking for 45.4 W get no power.
- Planning line120 W × (1 minus 20 percent) = 96 W
- Switch that fits every deviceReserved by class, every device: 164.4 W ÷ 0.80 = 205.5 W of PoE budget, 13 ports or more, 802.3at ports or higher.
- For the UPSCold night draw of every device: 99 W. Add the switch's own power in the UPS calculator.
How to use the PoE budget calculator
- Enter the switch. The number of PoE ports, the PoE budget in watts from its data sheet, and the port standard: 802.3af, 802.3at (PoE+) or 802.3bt.
- Set how it counts. Many managed switches can reserve by class or count measured draw. If you do not know which, use By class.
- Add the devices. The camera types carry example draws; for anything else, use Other device with its class and worst-hour draw.
- Pick the conditions. Cold night, with infrared and heaters on, is the worst hour of the year for outdoor cameras.
- Read the headroom, then the switch that fits, and carry the cold night draw to the UPS calculator.
How the PoE budget is worked out
A PoE switch has two limits at once: each port is limited by its standard (15.4 W on 802.3af, 30 W on 802.3at, 60 or 90 W on 802.3bt), and all the ports together by the PoE budget, which is set by the power supply and is usually far less than the port count times 30 W.
Counted by class, the switch reserves each device's class maximum the moment it powers up: 7.0 W for a Class 2 dome even if it draws 3.5 W. Counted by measured draw, it adds up what the devices actually pull, which leaves more room but moves with the weather. The calculator powers devices in port order until the budget runs out, as the budget figure in our PoE guide does.
needed = sum of (count × watts per device)
watts per device = class maximum, or measured draw
headroom = PoE budget minus needed
switch that fits = needed ÷ (1 minus spare margin)
Class maximums come from IEEE 802.3 Clause 33 and Clause 145. The 20 percent margin is a planning figure, not part of the standard.
Worked example: 13 cameras on a 120 W switch
The first example is the four-story mixed-use building in Jersey City from our PoE guide: 6 indoor domes, 6 bullets with infrared and 1 PTZ camera with a heater, on a 16-port PoE+ switch with a 120 W budget. Counted by class, the domes reserve 6 × 7 = 42 W and the first five bullets 5 × 15.4 = 77 W, which leaves 1 W. The sixth bullet and the PTZ never get power, though the whole list draws only 6 × 4.5 + 6 × 8 + 24 = 99 W on a cold night.
Counted by measured draw, every camera runs, but at 99 W the switch is at 82.5 percent of its budget with 21 W to spare, past the 80 percent planning line. Counted by class with 20 percent spare, the list needs 164.4 ÷ 0.8 = 205.5 W, which the 250 W example clears.
Typical PoE values
| Standard | At the switch port | At the device | Pairs |
|---|---|---|---|
| 802.3af (PoE) | 15.4 W | 12.95 W | 2 |
| 802.3at (PoE+) | 30 W | 25.5 W | 2 |
| 802.3bt Type 3 | 60 W | 51 W | 4 |
| 802.3bt Type 4 | 90 W | 71.3 W | 4 |
| Camera | Class (reserves) | Day | Night, IR on | Cold night |
|---|---|---|---|---|
| Indoor dome | 2 (7.0 W) | 3.5 W | 4.5 W | 4.5 W |
| Bullet with IR | 3 (15.4 W) | 5 W | 8 W | 8 W |
| PTZ with heater | 4 (30 W) | 14 W | 18 W | 24 W |
Camera draws are typical examples measured at the port for each kind of camera, not data sheet figures. The data sheet lists the class; the switch's PoE status page shows the real draw.
Common PoE budget mistakes
- Multiplying ports by 30 W. The budget is the power supply's number, and it is usually much smaller.
- Measuring on a summer afternoon. Draw rises at dusk when the infrared comes on, and again on the coldest nights when heaters run.
- Not checking how the switch counts. A switch that reserves by class can run out of budget with half its watts unused.
- A PTZ or heated camera on an 802.3af port. It gets 12.95 W at most and restarts when it moves or the heater comes on.
- No spare. A camera added next year or an aging power supply takes the last of the budget.
PoE budget calculator FAQ
How do I calculate a PoE budget?
Add up what each device needs at the switch port: its class maximum if the switch reserves power by class, or its measured worst-hour draw if the switch counts by draw. Compare the total with the PoE budget on the switch data sheet and keep about 20 percent spare.
How many PoE cameras can one switch power?
Divide the PoE budget by what each camera needs at its worst hour, then check that there are enough ports. Counted by class with nothing spare, a 120 W budget covers 17 Class 2 cameras at 7.0 W each but only 7 Class 3 cameras at 15.4 W, whatever the port count.
What is the PoE budget of a switch?
It is the total power the switch can send to all of its PoE ports at once, set by its power supply and listed in watts on the data sheet. It is usually far less than the port count times the port maximum. A 16-port PoE+ switch with a 120 W budget can power 16 cameras that draw 7 W each, not 16 that need 30 W each.
Should a PoE switch count power by class or by measured draw?
Counting by class holds each device's full class power, so nothing loses power when the heaters come on, but the budget fills fast. Counting by measured draw fits more devices on the same budget, but the total moves with the weather, so size it for the coldest night. Either way, set port priority so the most important cameras keep power first.
Does a PoE switch need a UPS?
Every PoE camera goes dark the moment its switch does, so a switch that powers cameras belongs on a UPS together with the recorder. The UPS load is the switch's own power plus the PoE it sends, using the measured draw rather than the class reservations. The UPS runtime calculator takes the cold night draw from this page.
Related guides and tools
Read the full guidePoE security camera installation: a technical guideHow a port detects and classifies a camera step by step, which cable to run, and what to do when a camera sits past 100 m.Glossary: PoE (Power over Ethernet), PoE switch and managed vs unmanaged switch.
Sources and standards
- IEEE 802.3 Clause 33 (802.3af, 802.3at). 15.4 W and 30 W at the port, 12.95 W and 25.5 W at the device, and the class maximums for Class 0 to 4.
- IEEE 802.3 Clause 145 (802.3bt). Type 3 and Type 4 over four pairs: 60 W and 90 W at the port, 51 W and 71.3 W at the device, Class 5 to 8.
- Seneca Learn, PoE security camera installation. The example building, camera draws and planning margin.
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