To fairly compare the batteries of the N900 and E7, you need to put them in your normal situations and see how they fare. For example, I went out all day and night yesterday in NYC, partying in bars, uploading pics to facebook, using Maps to find our way, texting friends, calling friends, even taking video of our escapades! I left the house (ok, apartment, it's New York) around 11am and got home around 2am. I left with a freshly charged battery, and when I got home I was down to just 1 bar. But my E7 did a stellar job all day and night, so having just 1 bar at the end of it all shows it did really well for my needs.
On a similar day/evening with my N900, I could get through to the early part of the evening, but there would be no battery left at 9pm to video my friend talking to his KFC burger before we hit more bars. But I would habitually carry a spare N900 battery, and swap it out when it was running low. I can't do that with the sealed E7 one, but after yesterday, it's pretty clear that I don't need to.
Incidentally, my friends' collection of Samsungs, iPhones and Droids either all died before the night was up, or were the same as me, running very low. So I would factor that into a fair (ish) comparison also.

On a side note, my E7 performed excellently all day and night, no freezes, no restarts. I took about 20 pics (some with flash some without) video, I was on Maps, Opera Mobile, Texting, Calling, Chat but it was all on 3G, no wifi, so maybe that makes a difference, I don't know. All I do know is that my N900 would have let me down on much of what I used my E7 for yesterday, and it has made me love my E7 even more. It truly is the best phone I've ever owned, and going by ndi's superb Anna review, it looks like it's going to get even better (when the US finally gets it) and then Belle will just make it even more perfect. I loved my N900, but I enjoy using my E7 more.

@ndi Thanks for your Anna review, another excellent slice of info. I will try your advice on the 80cm-then-zoom photo for close-ups. I'd love to come to Romania and sit down with you for a long, nerdy E7 chat! Maybe one day. Let me know if you are ever headed to New York. I owe you at least a beer!