Battery savings: Laptop screen uses 15% of battery power

Today I tried to see if I could work out the battery power savings from closing the laptop (a Lenovo IdeaPad U310 with an intel i5 processor, running Ubuntu 12.05 LTS Desktop). I’ve always been surprised when the laptops and netbooks we use as field servers seem to last longer than the estimated battery time remaining. The estimates are based on the power being drawn from the battery and to see the estimate you need to open the laptop. Therefore, the estimates include the power drawn by the screen which is switched off when the laptop closes. So, because we generally close the laptop once the server is running, the displayed estimates are not really applicable.

To try and get a better indication of the saving made by closing the laptop, I disabled the WiFi and unplugged the power supply, and then periodically opened the laptop and noted the actual time and estimated battery duration remaining before closing the laptop again. That way I could compare the actual time (at) elapsed and estimated time remaining (tr), and the relative difference in minutes (diff). Although the estimated time remaining did not include seconds, the difference still gave a good indication of the power saving.

actual time (at) relative at (rat) estimated time remaining (tr) relative tr (rtr) diff (rat – rtr) saving percentage (diff/rtr)
13:31:33 05:11:17 05:51:00 04:30:00 00:41:17 15.29%
15:37:49 03:05:01 04:01:00 02:40:00 00:25:01 15.64%
17:07:04 01:35:46 02:44:00 01:23:00 00:12:46 15.38%
18:42:50 00:00:00 01:21:00 00:00:00 00:00:00 n/a

Across the set of four measures taken, the percentage savings could be calculated for three of the measures and they consistently identified a relative power saving of around 15%. Although not an entirely accurate measure (due to the rounding of the estimated time remaining to hours and minutes), it gives us a reasonably useful rule of thumb for managing the battery life.

So, to conclude the estimated battery time remaining of 5 hours and 51 minutes at the start of the test could be extended by 15% by keeping the laptop lid closed, boosting the running time up to 6 hours 43 minutes.