Since last year, Comcast's wireless gateways have by default broadcast a second signal that turns each customer's modem and router into a public Wi-Fi hotspot. It's all part of Comcast's plan to create a nationwide Wi-Fi network of more than 1 million hotspots that the cable company can sell access to.

Comcast deflected criticism by arguing that the hotspot's bandwidth is separate from the bandwidth subscribers pay for, so it won't reduce the customer's Internet speeds. But what about electricity? Alex Gizis, CEO of Speedify, which makes software that bonds Internet connections to combine bandwidth, decided to investigate.

"As a bandwidth-obsessed engineer, I wanted to understand exactly what Comcast is doing here," he wrote last week. "Despite their claims that these routers cost subscribers nothing extra, we actually measured the power consumption on the router they sent us and were surprised by the results."

Unlike the guest networks that Internet customers set up for visitors, for which the homeowner can choose a password, the Comcast hotspots can be logged into by anyone with a Comcast subscription or anyone who buys temporary Wi-Fi access passes from Comcast. That means random people passing by your home could use the hotspot to get on the Internet.

To test the effect of people using the hotspot, Gizis plugged the Comcast modem and router into a power strip that was being monitored by a "Kill A Watt" meter. After testing the devices while idle, "we then connected two Windows laptops to the Xfinity hotspot, one watching Netflix and the other downloading files," he wrote. "You could immediately see the difference in the power meter, as the devices jumped from 0.14 Amps when idle, up to 0.22 Amps when actually being used. To translate this into dollars and cents, we used the average cost of power here in the Mid-Atlantic, which is $0.162 per KWh."

It may be unlikely that a hotspot will be used by passersby constantly, but if it were, it would cost the Comcast subscriber "up to $22.80 per year for those of us here in Philadelphia, or $1.90 per month," according to Speedify. (Comcast disputes these results—see the update below.)

Gizis started a Change.org petition demanding that Comcast compensate customers by raising their Internet speeds.

Speedify isn't a neutral observer here. The company has previously boasted that its channel bonding service can help Comcast customers reclaim the extra bandwidth from the hotspots, for a monthly or yearly fee.

To disable the hotspot on an Xfinity gateway, Comcast customers can log in to their account, navigate to Users & Preferences, and then click on "Manage Xfinity WiFi." They can also call customer service directly.

UPDATE: Comcast spokesperson Joel Shadle told Ars that the Speedify test relied on Comcast's business equipment, rather than the equipment that's used for the residential hotspot program, and that the equipment was outdated.

"There shouldn't be any discernible difference in the amount of electricity you're using because your router is already plugged in to do your own wireless in your home," Shadle said. Any extra amount of electricity usage "would be nominal at most," he said.

While it's possible people could use the hotspot from outside a subscriber's home, "the signal doesn't really stretch too far... it's not like the signal stretches all the way out to the street," Shadle said.

Speedify added a note to its blog post, saying, "Comcast has reached out and indicated that they would like us to retest with newer equipment. We’ll update our results as soon as we receive this new hardware."

Speedify told Ars that it will conduct the next text with the "latest, regular consumer hardware."