Power Requirements

I'm using an arduino uno board with an ethernet shield. i plan to build a little stand alone weather station with a data logger, as well (hopefully) transmit data to a database.
i plan to measure; rainfall with a tipping bucket(reed switch type) barometric pressure, wind direction (using an accelerometer), wind speed using a rotary encoder, RH% humidity with the HH10D sensor, temperature. My question is how much power would i need to power this device - or where could i find out where to work out my power requirements. i have tried a little 9v battery - useless!
i'm thinking of using a 3.7 volt rechargeable 500mAH LiPo Battery (solar Charging) - will this be enough? or can i connect two or more of these in series to power the device? (or perhaps a large 12v gate motor battery stepped down?)
By the way, will the uno even have enough I/O pins for all these sensors?
guidance would be much appreciated.

Key will be power management, as you have already figured out.

The Atmel microprocessor on the Uno board will run fine on 3.7VDC, not sure how the Uno board will fare in aggregate since it's also looking for a source of power to make 3.3V, etc. My suggestion would be to use one of the many shields out there that charge LiPOs out there. Presumably, the shield designer will have taken care of important aspects, such as how to charge the Lipo without frying the Uno board. Sparkfun has some boards and a shield (with a known hardware bug), but I generally prefer Adafruit designs.

Key will be how make the processor use as little power as possible. In other words, investigate how to make it sleep, yet ideally still pick up things like a rain cup ticking over or a wind meter rotation. The Atmel ADC can continue to run while the main processor is asleep, so this may be one way to do it. I defer to others re: the programming and mechanics, however.

or can i connect two or more of these in series to power the device?

You don't need any more voltage than one of those provides, so you could go for them in parallel rather than series. Though then you have to deal with balance when they're charging.

Thanks, yes power management is key. If i wanted to experiment and test how many mAh i could get out of my 3.7v 500mAh LiPo battery with my Uno board and then add a sensor at a time to see the result or load, how would you suggest i go about it?

Well, you could simply attach the rig to a fully-charged battery pack and see how many hours you get out of each configuration using a high-quality voltage meter monitoring the battery. Once the battery voltage reaches a certain level, shut the test down and note how many minutes, hours, or days you got. Many high-quality digital multi-meters (DMM) also allow you to measure ampere draws, which could be a more direct way to measure the flow of power. In this case, the DMM would be inserted in series with the battery pack to measure the current draw - see your DMM manual on how to do this.

Ok, so got my little proto type weather station up and running and transmitting wirelessly via xbee. I'm getting about 5 hours of power from my little 3.7v 500mah battery. So if I connected 2 of these in parallel would that in effect double my power time to 10 hours?