I'd like to see some references for your claim that a partial cycle equals a full cycle. Considering that what the N900 (and almost everything else) does once the battery reaches full is let it discharge 2-5% and then top it up again, it should be burning through a large amount of "cycles" once it reaches full. (And the green light comes on independent of whether the battery is actually full at that time or not, and stays green until charger is disconnected)
but I think that shadowjk got a point here. If you let the phone charge overnight, in fact it is just charged to full in few hours, then discharged to 95%, then charged again to 100, etc...
power is cut off until battery drop to 95% or so.
So, that would mean You got 10-20 cycles every night...
Look ndi. N900 battery rounded up got 5Wh - You're right at that. It's capable to safe deliver for our needs 1315 mA constant current @ 3.8V (rounded from 3.3V-4.2V, doesn't matter for our calculations) for time of 1 hour. Also, it can deliver 657 mA constant current for 2 hours, etc. Every fluctuation of current change total time, but after C (current)*V (voltage)=5W or higher, battery is already at 3.3V and is discharged, aka "dead".
Also, if we treat serious Your calculations about 10% beign 180W, that would mean we got 1800W at 100% - real beast. That would allow us for powering up high-power driller for an hour, just using our little battery, no need for wall plug I got nothing against that idea, but for now we must be happy with 5W thing...