A power outage in downtown San Francisco on Tuesday knocked out Web sites including Craigslist, Red Envelope, Yelp Technorati and ZDNet.

The San Francisco Chronicle reported more than 30,000 Pacific Gas & Electric customers lost power for about two hours after voltage fluctuations caused a transformer under Mission Street to go out.

ZDNet blogger Larry Dignan was disturbed about the lack of backup power to the sites and urges companies to regularly test their backup plans.

The Chronicle reports backup generators were fired up after about 45 minutes and said PG&E offered no explanation about why they weren't engaged immediately.

According to the San Jose Mercury News, the outage occurred on the same day a consumer advocacy group raised questions about PG&E's bid to be rewarded $151,899 for its reliability.