1) A former colleague of mine did a research project for a power company on one of those machines you have in your house that goes red if you are consuming lots of electricity. One participant started boiling water on the gas hob when she was making tea because that way she could keep the thing from going past amber.
2) A different colleague of mine uses an iPhone app which is designed to help you monitor your power consumption to see if his flatmate is home. If it is amber then the flatmate must be home.
Over to you.