Power output at LT is the amount of watts you can churn out at LT, which is the higuest point of exertion that an athlete can endure w/o invoking the anaerobic energy system.

Quickest and simplest way to find out your LT is to warm-up for 10-15 minutes at 65% of MHR, then do a couple of intervals at max for 1', with 1' recovery, then go for a 30' time trial (key being starting slow and reaching a pace you can sustain for the whole duration of the interval).

After you are done, look at your average HR for the last 20' of that 30' interval, should be a very good approx. of your LT.

I'm sure others will chime in with different ways but that's what I do and seems to coincide with what a lot of coaches are doing.

If you are interested in something more power-specific, like FPT (Functional Power Treshold) then I'd recommend buying the book "Training and Racing with a Power Meter" by Hunter Allen and Andrew Coggan, excellent stuff, the next level of training a beginner / intermediate cyclist can take.