Do I need to get a soldering iron with 30 watts or 60 for Arduino? What's the best? What can you recommend?
Arduino has nothing to do with that...
you'll find tons of information and opinions on line
here is a random one
The word for which you are looking is (are) "temperature-controlled". 
Hi @sierrra1
If it is for amateur use and small soldering on Arduino board and modules I recommend between 15W and 20W.
RV mineirin
I have one of these, it works great but it's output is only in °F and it drives me crazy.
Assuming you wish to solder at a minimum the headers and perhaps wires instead of the headers. You will need to consider the tip size. Ideally a chisel tip with a width of about 1/2 to 2/3 of the pad size. Maybe up to equal the pad size.
There are two versions probably , mine displays in celsius between 150 and 450
Hi,
What soldering iron is that?
Did it come with a death adapter so you can plug the USA silly plug into a decent GPO?
In my opinion, for what it is worth, 50 to 60W, TEMPERATURE CONTROLLED.
TEMPERATURE CONTROLLED is the important bit. (pun intended)
Tom... 
Tom....
Hi,
I can't see it there in your link.
I would have sent it back and told them if it can't give or offer real world data then put it in with the other bit of junk they have, the Weller Iron without a power fuse!!!
Have you opened it up and looked for a jumper or switch in the control PCB?
Tom..
Yeah it’s an old version (many years back) - possibly a Chinese copy 
(It’s a 95W unit - I would assume there is a fuse somewhere(
Hello
I have been working with a Weller soldering station for half a century.
And there are still soldering tips available as spare parts.
Tom...
oh well 
checked under mine, it's 230V/24V
This is most ironic.
I was actually in the process of listening to that YouTube video about release of the "magic smoke", when the computer died - as did a number of other things. I faintly noticed a burning smell but it was at that point, evanescent. I automatically went down a few stairs to the (hidden) electrical cupboard and confirmed a tripped RCD (in fact, it was the RCD and the sub breaker for the offending zone).
Trotted down further to the kitchen to see what appliance my wife was using, but neither kettle nor toaster were in use. She then complained of the smell, coming from downstairs and presumably the laundry (and yes, it was the laundry breaker that tripped). OK, opened the stairwell door and identified the acrid smoke.
Well, our trusty washing machine of 41 years has clearly "bit the dust" after gearbox work way back and multiple replacement valves. We had only just been discussing swapping in another as it would not balance when spinning more than three towels. So that is my new job this long weekend. 
But - the irony you see! 
Hi,
6V6GT or 6L6GT or 300B or EL84.
Do you get a matched pair when replacing them.
The new solid state washing machines have WiFi too, my WiFi scanner picks up two washing machines and two dryers, somewhere in my neighbourhood.
Tom..
That could lead to some interesting consequences!
Drôle!
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