For those who are familiar with the E71, it probably has the best battery life on the market. Uses 1500mah BP-4L.

I ran some tests to compare the E71 and n900 battery usage, both pretty much idle, almost no screen usage, few mins phone calls.

Results:
E71 = 60 hours to get to 30% left
N900 = 24 hours to get to 30% left

Since both are idle and no screen usage, the n900 less battery life is not because of screen or running apps. Either the GSM radio is more power hungry or Maemo needs more optimizations in its background processes or idle state. Or what "black magic" is the E71 doing?

Test case:
- N900 completely stock. No apps ever installed. Has gone through 4-5 recharge cycles before this test
- no 3G. GSM only
- no WIFI whatsoever
- no Widgets
- screen on lowest setting on both units
- N900 automatically connect to At&T internet 10 min interval
- Both running Nokia Messaging with gmail,yahoo,hotmail every 30 mins.
- Both constantly connected to gtalk+skype (E71 uses Fring). So the internet is constantly connected on both devices.
- max 5 mins screen usage (only to check battery + take 5 mins of calls)
- each day involves 40 mins underground with no reception

E71
-----
day 1: 09:30 -> 09:30 => 70% free
day 2: 09:30 -> 09:30 => 50% free
day 3: 09:30 -> 21:00 =>30% free

N900 (repeated 3 different days, same usage, same results)
------
day 1: 09:30 -> 09:30 => 28% free

Outside these tests, I notice if I use the device ontop of the above usage with (again with no 3G)
- 30-40 mins web surfing
- 30 mins bluetooth music
- 10 mins misc playing around with the device
The battery is more like 09:30 -> 00:00 => 25% free
With the current battery life, I'd be very wary of moving to Tmobile and using this device on 3G and lasting 09:30 to 00:00

You're comparing a 1500mAh against N900 1250mAh...and against a phone which has a lot more powerful hardware compare to E71...

N900 has 1320mah battery not 1250mah

FWIW, my girl has an e71x, and gets about 3 days per charge...its comparable to my Motorola Q. The 900 should compare to the Droid, and even w/ a slightly larger battery, the Droid can go 24-30 hours where I'm getting only 12-18 under similar conditions w/the 900.

I just started some observations w/my 900 w/3G "off", via the Cellular Disable app., and WiFi "on", logged into my SIP and Skype clients. Preliminary results showing improved life after 5 hours

Even with constant full bars of 2.5G or 3G, by switching manually, it seems like cellular power draw is heavy.

1320mah for the n900 and both phones are pretty much idle with no screen time. So the powerful hardware should not be factor as Nokia claim the N900 to have excellent idle power usage.

Look into bug 6615 - there is some problem with WiFi drinking battery. So, repeat your measurement after you unload wl12xx, mac80211 and crc7 drivers (use "rmmod" in root shell).

my battery last me over 36hours with one single charge.

Wifi on and off very often. Sometimes checking the web for more than 1hour each time.
Taking snapshots and videos,
Customs apps installed.
Plenty of short cuts on screens.
Web browsing, email checking.
Playing Music and Videos casually.
SMS none stop, and answer a few phone calls (20mins).
3G connection always (set to Dual).
Playing some games (Chest, Blocks), and third party games (SNES..etc)
Screen Brightness set to 2 bars.
Sending photos to other phones through bluetooth.

All that last me over 36hours.

So far it is doing pretty well. I notice that my idle time is very long. I left it 12hours IDLE but I did not see any fluctuating in battery power at all. Charged it and left it over 12hours and it only used 5-7% of juice.
With my calculation I think it would last me 170-240Hours if I leave it on Idle all day without using it.

But I already bought another Genuine BL-5J battery in case of emergency when i'm away from home or recording videos constantly

does this not seem a bit contradictory? or can you get internet on GSM only?

yes it's called GPRS or, if you're network supports it EDGE