MileIQ consumes almost no power when stationary (as it quickly goes to "sleep"), and it will only minimally increase battery consumption when in transit. So, assuming typical battery consumption of 5% per hour, driving with MileIQ would only consume an additional 1% or less over that hour. To achieve this minimal battery usage, MileIQ uses smartly constructed formulas to detect drives rather than only using GPS (which requires more power) for location data.
We know how important battery life is, and rest assured that we have done everything possible to maximize location accuracy while minimizing battery. Additionally, we are always working to improve battery life as new technologies and battery saving methods are developed.
One helpful hint is to keep WiFi on at all times. It may seem counterintuitive, but even if you aren't near a known hotspot, WiFi increases location accuracy and minimizes battery/data consumption.
For more information take a look at battery best practices.
