Usage time since last full charge?

I was surprised to not be able to find any widgets or applications that show the usage time since the last full battery charge. I couldn't even find any requests for such an application or widget. The iPhone has this built in. Wouldn't it also be useful for a laptop? It would help you keep track of how much battery life you're getting while doing various activities. Does this exist somewhere and I'm just missing it? If not, this would be a very useful widget to have. It would be a great feature to add to the iStat widget.