I think very few phones have a discharge current sensor. That makes it tough.
Battery Monitor Widget watches the battery voltage drop for an interval of time and then estimates associated % of battery capacity used during that interval. Divides % by the time interval to get % per time. That takes time to get an accurate result...the tiny %capacity used in a few seconds is not measurable to any degree of accuracy.
"Power tutor" will give you a real time estimate of power usage by watching what your phone is doing and using some VERY gross assumptions. So it's not very accurate. And it didn't have a widget...you have to go into the app to see the usage.
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