What this calculator works out
This calculator totals the power drawn by devices on a switch, applies a reserve to the switch's PoE budget, and reports whether there is spare capacity or a shortfall.
The budget is a switch-wide total, not a per-port figure. A switch with 24 PoE ports rarely has enough budget to run all 24 at full power.
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Before you connect devices
- Use each device's actual draw, not the PoE class maximum. A camera in class 4 may draw 12 W rather than the 25.5 W the class permits.
- The switch budget is on its specification and is shared across all ports.
- A reserve of 10 to 20% covers devices drawing more under load — cameras with heaters and infrared illuminators are the usual culprits.
- Cable length matters. Power is lost in the cable, and a long run delivers measurably less than the switch supplies.
A PoE camera's typical draw is measured with heaters and illuminators off. On a cold night with infrared active, the same camera can draw double. A budget calculated from summer daytime figures is a budget that fails in January, which is when the cameras matter most.
How this calculator works
Device load against a reserved budget:
Device load = devices × average watts each
Safe usable budget = switch PoE budget × (1 − reserve ÷ 100)
Spare or shortfall = safe budget − device loadExceeding the budget does not usually damage anything — most switches shed power to lower-priority ports instead, which means devices stop working rather than failing loudly. That silent failure mode is the argument for the reserve.
Worked example: eight devices
Using the default figures — 8 devices at 12 W each, a 120 W switch budget, 15% reserve:
- Device load: 96 W
- Safe usable budget: 102 W
- Spare capacity: 6 W
Six watts spare — half a device. That is a system with no room at all, and if those are outdoor cameras their winter draw could exceed the full 120 W budget, not just the reserved figure. Either a larger switch or a PoE injector for the highest-draw devices is the answer, and it is far cheaper to identify now than after a night of cameras dropping offline.
Common mistakes
- Using PoE class maximums as device draw. Most devices draw well below their class.
- Assuming every port can deliver full power. The budget is shared.
- Ignoring cold-weather draw. Heated cameras double their consumption.
- Forgetting cable losses. Long runs deliver less than the switch supplies.
- Leaving no reserve. The failure mode is devices silently dropping off.
Frequently asked questions
What are the PoE standards?
802.3af provides up to 15.4 W at the switch, 802.3at (PoE+) up to 30 W, and 802.3bt (PoE++) up to 60 or 90 W depending on type. Because of cable losses the power available at the device is lower than the figure at the switch — around 12.95 W for af, for example. Device specifications usually quote the device-end figure.
How far can PoE run?
100 metres of Ethernet cable is the standard maximum for data and applies to PoE too. Power delivery degrades over distance, and lower-grade or copper-clad-aluminium cable makes it substantially worse. For runs near the limit with high-power devices, using good quality solid copper cable is not optional.
What happens if I exceed the budget?
Most managed switches shed power from lower-priority ports, so some devices simply stop. Unmanaged switches may behave less predictably. Neither typically causes damage, but the failure is silent — devices go offline without an obvious cause, which is difficult to diagnose after the fact.
Can I use a PoE injector instead?
Yes, and it is a practical way to power one or two high-draw devices without replacing the switch. An injector supplies power to a single link independently of the switch budget. It adds a component and a power supply to the path, which is worth considering where reliability matters.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- Health and Safety Executive — electrical safety and safe cabling practice
- National Cyber Security Centre — securing network infrastructure and connected devices
Sources are checked at publication and can change — how I choose and check references.