Power-over-Ethernet was one of the big themes of Cisco's product launches yesterday. The company now has Catalyst 3750 models with 48 all-PoE Gig Ethernet ports, plus a new 8700W power supply for the Cat 6500 which enables one chassis switch to power an incredible 420 devices over their Ethernet cables.

Isn't pumping 8.7kW into an equipment rack rather a lot? After all, we said to Cisco's head of product marketing Neil Walker, it is a bit like sticking a lighted patio heater in your wardrobe.

Not at all, he argues - in fact, it could even improve an organisation's energy usage and efficiency.

"A Catalyst 6500 is typically in a basement or data centre, where you can manage the air-con," he says. "8700W is not a huge amount for a data centre, compared with the heating effects of 400 power-bricks distributed around the office."

He adds that it could be more efficient than 400 main adapters too, especially when you consider that they will be plugged in and wasting energy all the time, even if the attached device is off.

By comparison, a PoE switch not only concentrates the heat problem in one place, but if it senses the power demanded by each device and adapts that port's output accordingly - as the Cisco devices do - it will only draw as much power as is actually needed and used.

Plus, not all of the devices connected to a PoE switch will actually be line-powered of course - typically it might only be half, or at most two-thirds.

Walker cites the example of Cisco customer Lufthansa Systems, which is using the new fan-less 3560 Compact Switch in the IP-based comms and entertainment system that it's installing on the Aida Diva, a cruise-liner under construction in Germany.

Of the four PoE Fast Ethernet ports allocated to each cabin, only two actually require power, for the IP phone and the air-con controls. The others, for IP-TV and passenger broadband, do not.

So how much power do PoE devices draw? Walker says it can vary a lot.

"It depends on the manufacturer," he says. "Some IP phones suck 14W, say, while others are more efficient and only draw 10W."

He notes that while a basic webcam might take under a Watt, as soon as you want one with pan-and-tilt motors, or with the built-in heater needed for outdoor use, the power drain rises dramatically. In fact, he says, while you can have a PoE camera with pan-and-tilt or a heater, you can't have it with both, as they'd push it over the 15.4W per port limit.

"You can have PoE printers too, but not PoE thermal printers as they'd draw too much," he adds.

It's been a relatively long road to power-over-Gigabit, partly because there's been few devices that needed both power and Gig Ethernet, but also because of the extra technical challenges, he says.

The chief challenge is that you can't do Gig PoE the same way as Fast Ethernet PoE, by pumping the power over spare copper pairs in the cable, because in Gig Ethernet there are no spare pairs - the signal uses them all.

Instead, the power has to be overlaid on the signal itself by adding a voltage differential. That in itself adds more complexity because now you have to think about what it does to the signal and to how far you can drive it.

But as more devices appear that can use the bandwidth of Gig Ethernet, from video-capable IP phones and high-resolution CCTV cameras onwards, the opportunity for Gig PoE is rising.

And as companies get better at building power-efficient PoE switches and as prices fall - Cisco's stackables may cost as much as £200 per port, but unmanaged Gig PoE switches can be had for as little as £30 a port - PoE begins to look unstoppable.